Original Research Paper
Comprehensive framework for
speed-zone guidelines
K. Joseph Shrestha
a,b, Pramen P. Shrestha
a,* aDepartment of Civil and Environmental Engineering and Construction, University of Nevada Las Vegas, Las Vegas, NV 89154, USA
bDepartment of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA 50011, USA
a r t i c l e i n f o
Article history:Received 16 July 2015 Received in revised form 24 September 2015
Accepted 29 September 2015 Available online 16 July 2016 Keywords:
Speed zone guideline Speed limit guideline Realistic speed limit Traffic safety
Nevada Department of Transportation
a b s t r a c t
Speeding is often considered as one of the major reasons of traffic crashes. As a result, State Departments of Transportations (DOTs) receive requests from communities to decrease speed limits in towns along rural highways. Such requests, coupled with the lack of proper manuals, have forced state DOTs to reduce the speed limits. A proper speed-zone manual is required to establish consistent speed zones across a state. This paper reviewed the literature on speed, crashes, enforcement techniques, and speed-zone manuals. A nationwide survey was conducted to identify various factors affecting the decisions of speed zones establishing. This paper identified best practices based on the literature review and expert opinions, and proposed a comprehensive framework for a speed-zone manual. The Nevada DOT traffic engineers were also interviewed during this research to incorpo-rate their opinions in the framework. The framework provided in this paper has six phases. They are; 1) speed-zone identification; 2) speed-limit determination; 3) transition-zone detailed design; 4) speed-zone approval; 5) speed-limit enforcement; and 6) follow-up study. Relevant studies, and manuals corresponding to each phases were presented in this paper. This framework is useful for NDOT, other state DOTs within the U.S., and trans-portation agencies worldwide that haven't had speed zone manuals currently to develop a proper speed-zone manual.
© 2016 Periodical Offices of Chang'an University. Production and hosting by Elsevier B.V. on behalf of Owner. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
1.
Introduction
From 1981 to 2009, traffic crashes have been the main contributor of fatalities in the U.S. (Subramanian, 2012). The major reasons of crashes can be divided into four main
categories: vehicles, drivers, roadways, and roadside factors. The auto industry has invested millions of dollars to develop safety features for safer driving experiences (Toyota Motor Sales, 2013). Some of the safety features developed and implemented in modern vehicles include the anti-lock braking system (ABS), electronic stability control (ESC), * Corresponding author. Fax: þ1 702 895 3936.
E-mail addresses:[email protected](K. J. Shrestha),[email protected](P. P. Shrestha). Peer review under responsibility of Periodical Offices of Chang'an University.
Available online at
www.sciencedirect.com
ScienceDirect
journal homepage: www .e lsev ie r.com/locate/jtte
http://dx.doi.org/10.1016/j.jtte.2015.09.008
2095-7564/© 2016 Periodical Offices of Chang'an University. Production and hosting by Elsevier B.V. on behalf of Owner. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
adaptive cruise control (ACC), lane-departure warning, and rearview cameras. In addition, the US Department of Transportation (DOT) has been actively pursuing the concept of connected vehicles, which could reduce up to 82% of the crash scenarios among unimpaired drivers (United State Department of Transportation, 2014). However, the development of vehicle safety technologies alone is not enough for a safer road. Human factors, roadway design, and the roadside environment play important roles in controlling the number of crashes that occur each year.
Although the number of traffic fatalities in Nevada had decreased from 2006 to 2009, the number of fatal crashes per 100 million vehicle miles traveled (VMT) was higher than the national averages during the same period (Nevada Department of Transportation, 2012). During 2012, the percentages of fatal, injury and property damage (PDO) crashes occurring in Nevada were 0.45%, 36.15%, and 63.04% respectively.Fig. 1presents the crashes distribution with the location type (rural and urban). It shows that while the percentages of crashes in rural areas are about one tenth in total crashes, PDO crashes, and injury crashes, the percentage of fatal crashes in rural areas is more than one third of the total. This is despite the fact that people travel more on urban roads than on rural roads [Federal Highway Administration (FHWA), 2000].
Before 1974, interstate highways through Nevadae as well as Montanae did not have any speed limits (Khan et al., 2001). Other states had maximum speed limits up to 70 mph. Due to the 1973e1974 oil crisis, the federal government enacted a national maximum speed limit (NMSL) of 55 mph. In 1995, this law was repealed, allowing each state to determine and set their own speed limits.
Speeding is often considered as one of the major factors leading to crashes. As such, speed zoning is considered to be a cure-all for crashes as well as other traffic-related problems (Automobile Club of Southern California, 1998). While crash severities increase with speed, the probability of crashes is more dependent on differential speeds than on absolute speeds of the vehicles (Idaho Transportation Department, 1997). The requests to reduce speed limit received by the state DOTs indicate that the public have a perceived idea of safety issues, which do not reflect actual issues. Therefore, a proper set of guidelines is necessary to establish consistent speed zones with realistic speed limits as well as to defend against public requests to decrease speed limits arbitrarily.
A ‘speed zone’ can be defined as a stretch of highway or roadway where the speed limit is different from other seg-ments of the highway. However, Nevada Department of Transportation (NDOT) does not have such a guideline or manual. This paper presents the results of a study undertaken to prepare a framework for developing guidelines to establish speed zones in towns along rural highways of Nevada.
2.
Objective
NDOT has a procedure to conduct engineering analyses before establishing speed limits in towns along rural highways. However, there is no documented process for this procedure. Such lack of documentation, along with staff turnover, can lead to the loss of knowledge (DeLong, 2004).
The objective of the study is to prepare a framework for a speed-zone guideline for NDOT. This guideline would be based on current practices in NDOT, various studies related to speed and crashes, existing speed zone guidelines and man-uals, and expert opinions from state DOT traffic engineers. Once the guideline is prepared, it can aid NDOT officials in establishing and updating speed zones with consistency throughout the state. Although this study is conducted for NDOT, the framework prepared in this study is also useful for other state DOTs to develop their own speed zone guidelines.
3.
Methodology
The study was based on the review of existing studies, including existing speed zone manuals, a nationwide survey of state DOT traffic engineers, and structured interviews of NDOT traffic engineers.
3.1. Literature review
Speed-zone guidelines and manuals from various state DOTs were reviewed, including Alaska, Kentucky, Massachusetts, Missouri, Connecticut, and Georgia. A number of other studies were reviewed relating to factors affecting the operating speed, crashes and their severities, the determination of a realistic speed limit, and various speed-reduction techniques.
3.2. Nationwide survey
A survey was prepared and revised based on feedback from NDOT's Technical Advisory Panel (TAP). It was sent to all the state DOTs, excluding NDOT and its objective was to deter-mine the current best practices for establishing speed zones and to obtain expert opinions from traffic engineers regarding speed limits and crashes. A semi-mixed mode of the survey was used. Respondents were contacted by email and phone. The survey was sent out during the summer of 2012 by email, and follow-up contacts were made through email and phone. Representatives from 37 out of 49 state DOTs responded to the survey, which was a 76% response rate.
Structured phone interviews were conducted with NDOT traffic engineers. An interview guide was prepared and sent to traffic engineers before conducting the phone interview. The Fig. 1e Crash distribution in rural and urban areas
first objective of the interview was to document current NDOT practices for establishing and updating speed zones in towns along rural highways. The second objective of the interview was to get their expert opinions for the new guideline.
4.
Literature review: The basics of speed
This section summarizes findings from the literature reviews including speed-zone manuals from state DOTs, results of the survey, and interviews with NDOT traffic engineers.
4.1. Design speed, operating speed, and speed limits
Over time, the definition of design speed has changed from“… the maximum speed that can be maintained over a specific section of highway when conditions are so favorable that the design features of the highway govern…” (AASHTO, 1994) to “… a selected speed used to determine the various geometric design features of the roadway …” (AASHTO, 2011). The selected speed can be considered as an expected operating speed, which is defined as“… the speed at which drivers are observed operating their vehicles during free-flow conditions.” (AASHTO, 2011). There has been a clear shift of the definition from being the maximum drivable speed to an arbitrary number for designing the road geometry. Operating speed is the speed at which the drivers actually are driving on the road. Conservatively, the difference between the design speed and the operating speed can be considered as the effect of the factor of safety used in the calculations.
A speed limit acceptable to all parties (drivers, residents, legislators, and enforcement officers) is determined by taking favorable weather and prevailing traffic conditions into consideration (AASHTO, 1994). According to the Idaho Transportation Department (ITD), some transportation professions have cited the design speed as a limiting factor
for determining a maximum speed limit (Idaho
Transportation Department, 1997). However, determination of speed limits for realistic speed zones should not be associated with the design speeds of the road. The design speed is selected to determine the geometry of a roadway. However, a speed limit should be determined based on the prevailing speeds of freely-flowing vehicles. This is based on a fundamental concept that the majority of motorists drive at a reasonably safe and prudent speed for existing roadway and roadside conditions. This results in voluntary compliance of the posted speed limit.
For changes in speed limits, the Institute of Transportation Engineers (ITE, 1993) suggested that an unbiased engineering study was needed to examine the following conditions: roadside development, road and shoulder characteristics, pedestrian and bicycle activities, speed limits on adjoining road segments, crash experience or potential, and population density. Najjar et al. (2000)suggested that most motorists tended to drive at a speed that depended upon the roadway conditions rather than the speed limit. Hence, setting an unrealistically low speed limit is likely to result in more variations in speed and more crashes. Dudek and Ullman (1987)found that a reduction in speed limits had a
detrimental effect on driver compliance to speed limits, which was true for both local and non-local drivers.
Reduction in the differences between those three speed limits will reduce the variation of speed of vehicles in a traffic stream. The variation of vehicle speeds is considered more important in reducing the crashes than the absolute speed of the vehicles (Idaho Transportation Department, 1997). The 85th percentile (of operating) speed is the most important factor considered by state DOTs in determining the speed limits for speed zones.
An understanding of the operating speed is necessary to determine a realistic speed limit. As such, a number of studies have been conducted to identify the effect of various factors affecting the operating speeds (Cooper et al., 1980; Cruzado and Donnell, 2010; Esposito et al., 2011; European Transport Safety Council, 1995; Fildes et al., 1991; Fitzpatrick et al., 2001; Jarvis and Hoban, 1988; Tignor and Warren, 1990; van der Horst and Ridder, 2007; Warren, 1982; Wisconsin Trans-portation Information Center, 1999). The factors identified in these studies are presented inTable 1.
Table 1e Factors affecting the operating speed.
Road characteristics Lateral road characteristics
Lane width
Paved shoulder width Total number of lanes Presence of median Lateral clearance Horizontal curve
Presence of the curve Radius of the curve Curve length Deflection angle
Other longitudinal road characteristics Straight sectional length
Gradient
Length of the grade Alignment Sight distance Roadside environment
Roadside infrastructure Number of intersections Presence of road sign Presence of traffic signals Roadside development Built-up areas Land use Human factors Drive age Driver capability Public attitude Enforcement Vehicle factors Vehicle types Presence of bikes Presence of pedestrians Other factors Pavement conditions Posted speed limit Traffic volume
4.2. Effect of speed limits on crashes and crash severities
The relationship among speed limits, crashes, and crash se-verities have been widely studied and documented (Agent et al., 1998; Haselton et al., 2002; Kockelman and Bottom, 2006;
Malyshkina and Mannering, 2008; Raju et al., 1998; Renski et al., 1999; Thornton and Lyles, 1996; Transportation Research Board, 1984; Wisconsin Transportation Information Center, 1999; Zahabi et al., 2011). These studies have analyzed the crash patterns in various types of highway and roadway segments, Fig. 2e Framework for establishing and updating speed zones.
and compared the crash patterns under similar segments. The results of these studies have varied due to obvious differences in the characteristics of the roads, roadside environments, and human factors in each location. For example, some studies have found that the speed limits did not have a significant relationship with the frequency or severity of crashes, while others have found a significant relationship between speed limit and the severity of crashes. Others have found that a higher speed limit did not necessarily increase the frequency of crashes, but did affect the crash severity.
4.3. Speed zone guidelines, manuals, and statutes to determine speed limits
The guidelines used by various state DOTs were based on the rule of majority (Institute of Transportation Engineers, 2004). Generally, the 85th percentile speed was the agreed-upon measure of the prevailing speed when speed limits for speed
zones were determined (Alaska Department of
Transportation and Facilities, 2000; California Department of Transportation, 2012; Wisconsin Transportation Information Center, 1999). The amount of detail presented in the speed zone guidelines and manuals varied. Some state DOTs documented methods to take into account such factors as road characteristics, roadside characteristics, and crash histories for reducing speed limits. Other state DOTs provided the freedom to make necessary changes based on the traffic engineer's engineering judgment and experience. Some DOTs had concise guidelines that focused only on how to determine speed limits for speed zones. Other manuals provided much broader information, from procedures to selecting a site for speed zoning, to how speed zones would be approved.
Nevada Revised Statutes (NRS) includes three chapters related to traffic speed limits: Traffic Laws NRS-484, Traffic Laws Generally NRS-484A, and Rules of the Road NRS-484B (Nevada Legislature, 2011, 2013a, 2013b). The purpose of those chapters were to“… establish traffic laws which are uniform throughout the State of Nevada…” and to “… minimize the difference between the traffic laws of the State of Nevada and those of other states.” This statute allows the NDOT to pre-scribe and eliminate speed zones after necessary studies have been made. The three speed limits set by the NRS are:
A maximum speed limit of 75 mph A speed limit of 15 mph for school zones
A speed limit of 25 mph for school crossing zones In 2013, the maximum speed limit of 75 mph was increased to 85 mph (Vogel, 2013). Although the statute-enforced maximum speed limits are followed throughout Nevada, there is no guideline for consistent speed limits for speed zones throughout the state.
5.
Framework of speed zone guidelines
The framework developed for establishing and updating the speed zones is presented inFig. 2. The whole framework can be divided into six phases:
1. Speed-zone identification 2. Speed-limit determination 3. Speed-zone detailed design 4. Speed-zone approval 5. Speed-limit enforcement 6. Follow-up study
Based on the phone interviews of NDOT traffic engineers and literature reviews, the speed-zone study can be initiated for three main reasons:
1) Requests from the public or local jurisdictions.
2) A significant change in the driving environment, such as: a) Addition or elimination of driveways
b) Changes in the travel lanes number c) Significant residential development d) Significant commercial development
3) Review of speed zones after a certain year; typically, five to 10 years, as currently practiced by state DOTs.
5.1. Phase 1: Speed-zone identification
State DOT representatives mentioned 167 factors that influ-ence a decision to establish a speed zone in rural highways. Out of those, 143 factors were categorized into eight cate-gories. These factors and the response counts are presented in Table 2. The results from the survey indicated that the driving environment as well as requests from the public were major factors that influenced decisions when a speed zone was established. In addition, all the state DOTs but one who responded to the survey received requests to update speed-zone features, such as the speed limit.
At NDOT, the district first conducts informal speed studies to check the need for establishing a speed zone or changing the characteristics of a speed zone, such as a speed limit. If further study is deemed warranted, a request is sent to NDOT headquarters in Carson City. After that, the speed-study crew from the headquarters conducts a more extensive study. Be-sides spot-speed studies, when a new speed zone is estab-lished or an existing one is updated, factors such as crash data, geometric features of the road, and site maps showing roadside features and development are considered.
Table 2e Top factors influencing speed zone establishment.
Factors Response
count
Percentage
Prevalent traffic speed (usually 85th percentile)
34 92
Crash history 27 73
Road geometry 22 59
Roadside environment 22 59
Political and public influence 13 35
Pedestrian and bicycle 10 27
Access road count/density 9 24
5.2. Phase 2: Speed-limit determination
Determining a realistic speed limit is the most important aspect of establishing or updating speed zones. Speed Zoning Information: A Case of“Majority Rule”, published byInstitute of Transportation Engineers (2004) is the most popular publication among the state DOTs for setting up the speed limits. The rules state that the majority of people drive safely and in a reasonable manner, and laws are to protect people from unreasonable behaviors. As such, a realistic speed should be based on the driving behavior of the majority of drivers. Such realistic speed limits are easier to enforce because they result in voluntary compliance and more uniform speeds. It should be noted that the probability of crashes is more dependent on differential speeds than on absolute speeds of the vehicles (Idaho Transportation Department, 1997). The process of determining a realistic speed limit is presented inFig. 3.
Spot-speed data is the most important data to collect when the speed limit for a speed zone is determined. Typically, radar guns are used to record spot speeds that should be taken under normal conditions to ensure ideal free-flow conditions as close as possible. Some factors that need to be considered include good weather as well as a location for the spot-speed study that is far from intersections. In addition, the drivers should not be aware of the ongoing spot-speed study.
When the details of the speed studies are determined, there is a lack of standards for the number and locations of spot-speed studies, the number of vehicles and speed-zone sections, among other issues. For example, to get a random sample, spot-speed studies should be conducted at different times (e.g., a.m. and p.m.). However, none of the manuals examined for this paper recommends any such factor, possibly because of the extra time and resources required for such a random sampling. Thus, there is an opportunity for further research in this area. Some recommendations for
those parameters, based on state DOT manuals, are provided inTable 3.
Once the spot speed data is collected, prevailing speed parameters of the vehicles should be calculated, including the 85th percentile speed, the mean speed, the median speed, and the pace (upper limit of 10 mph pace). Along with the pre-vailing speed limit, test-run speed is another speed parameter considered by many state DOTs when the initial speed limit is determined. A study conducted by the FHWA showed that when the speed limit was set close to 85th percentile speed, the frequency of crashes was lower (Stuster et al., 1998). Survey results also show that the 85th percentile is the most important factor influencing a speed zone setting up, based on the opinions of state DOT traffic engineers, as shown in Fig. 4. However, it may be necessary to test the validity of the 85th percentile theory. Some state DOTs use objective
decisions based on a set of guidelines when each of these parameters is considered to determine the speed limits. Other state DOTs leave it to the discretion of the traffic engineers.
Although, it is widely believed by the traffic engineers that crash frequency is not related to the speed limits, many state DOTs do make recommendation to adjust the calculated speed limit for crashes. In addition, state DOTs have set lower speed limits based on public complaints and political pres-sure. NDOT traffic engineers stated that, sometimes, speed limits below the 85th percentile were set as a direct result of these public complaints. DOT representatives from 13 out of 23 states said that reducing the speed limit based on requests from the public did not solve the problem. One state DOT representative commented“The action of reducing the speed limit based on public request often results in an appeasement
Table 3e Recommendations for parameters during spot-speed studies, based on state DOT manuals.
Item Description
Number of spot-speed study locations One spot near the midpoint if the speed zone is less than or equal to one mile
One spot for every 0.25 miles if the speed zone is greater than one mile Additional locations if there is a major change in the driving environment
Location of spot-speed studies Avoid locations near intersections
Data collection time Avoid rush hour, weekends, holidays, and special events
Ideal weather
Number of vehicles At least 50 (preferably 100) free-flowing vehicles in each lane in each
direction OR
In low volume roads, at least 4 h of data
Initial speed limit based on 85th percentile speed
Mean speed
Upper limit of 10 mph pace Test run speed
Adjusted speed limit 5%e10% or 5-mph to 10-mph reductions, based on the crash histories
Some adjustment for features that are not readily visible to prudent drivers
Adjusted speed limit criteria Should not be less than the average speed
Should not be less than the median speed
Should not be more than the critical speed (95th percentile speed)
and perceived improvement. The majority of cases do not indicate compliance or improved operational or safety con-ditions.” This poses a serious question whether such speed reductions based on complaints should be continued. Although there is perceived safety in lowering the speed limit, this is nothing more than a misconception (Institute of Transportation Engineers, 2004). One DOT representative mentioned that in that state, there were efforts to increase the speed limit based on the 85th percentile speed because historically, the speed limits were set to values lower than the 85th percentile.
Various road and roadside environments that are readily visible to prudent driverse such as the number of lanes, the presence of a median, and highway alignmente should not be taken into account for speed limit calculations. California Vehicle Code 22358.5e Downward speed zoning (California Vehicle Code 22358.5, 2011) as well as the Alaska DOT (Alaska Department of Transportation and Facilities, 2000) both stated that such factors, if they were apparent to prudent drivers, should not be the only reason to establish speed zones with reduced speeds. The drivers already realize those factors, and that is reflected in the prevailing speed of their vehicles. However, if any road and roadside environment is not readily visible e such as lane drops, access points, presence or absence of sidewalks, or traffic control devicese then those features should be considered when the calculated speed limit is adjusted.
5.3. Phase 3: Transition-zone detailed design
Regarding design of transition zones, many speed zone guidelines lack the details or links to relevant guidelines. The FHWA's Manual of Uniform Traffic Control Devices does pro-vide guidelines for placing sign posts (FHWA, 2012). Typically, NDOT uses speed increments/decrements of 10 mph. A higher increment on a step up can be considered in a low-speed to high-speed transition zone, according to one of the NDOT traffic engineers. However, the values are arbitrary. Therefore, studies should be conducted to identify the effect of different speed increments or decrements on compliance and crashes.
Compared to European countries, relatively less effort has been made in the U.S. to study and use different forms of transition zones to reduce speed limits. Guidelines from the
National Cooperative Highway Research Program (NCHRP) provide limited details for designing transition zones (Torbic et al., 2012). Because studies vary from location to location, studies should be conducted in the desired locations in Nevada regarding the effect of various transition zone details, such as the length of transition zones and traffic calming devices. Table 4 presents some of the current practices across different state DOTs regarding transition zone properties.
5.4. Phase 4: Speed-zone approval
To approve an establishment of a new speed zone or update the existing speed zone in NDOT, a well-documented report for the site has to be submitted to the chief of the Traffic and Operation Division of NDOT. Based on the review of existing manuals, the following documentation should be included for the approval:
Photographs of the site, with a special focus on peculiar conditions, if any
Details of previous speed zones, if any
Reasons for conducting a speed study, including com-plaints or revision after a certain number of years Results of speed studies
Specifications of new speed zones, such as speed limit(s), as well as the starting point and ending point of each speed limit
Exceptions made with reasons stated
After the change in existing speed zone or new speed zone is recommended by the Traffic and Operation Division, it is approved by the Director of NDOT. NDOT traffic engineers are satisfied with the current chain of command for approving speed-zone changes.
5.5. Phase 5: Speed-limit enforcement
Speeding is one of the top 10 reasons of crashes, according to the nationwide survey conducted in this study (Fig. 5). In addition, 59% of the state DOT representatives perceived speeding as a problem in their state highways. Studies have shown that arbitrarily raising or lowering the speed limit does not have much effect on vehicle speed. Any arbitrary
Table 4e Transition zone properties as practices by various state DOTs.
Feature Criteria Source
Minimum length 0.5 mi Massachusetts Highway Department (2005)
Michigan Department of Transportation (2004)
2 mi Missouri Department of Transportation (2010)
Distance traveled in 25 s while driving in the posted speed limit
Alaska Department of Transportation and Public Facilities (2000)
Speed increment/ decrement
10e15 mph California Department of Transportation (2012)
10 mph (typical) Current NDOT practice
5, 10, and 15 mph Alaska Department of Transportation and Public Facilities (2000)
10 mph Kentucky Transportation Cabinet (2012)
Beginning location of a speed zone
At least 0.2 mi from major signalized at-grade intersections and 0.02 mi from other at-grade intersections
Georgia Department of Transportation (2012)
reduction in a speed limit has resulted in decreased compliance to the speed limit. If the speed limit is to be decreased, proper enforcement is required. In contrast, lack of proper enforcement may increase the differential speed of vehicles.
A European study found that differential speeds of vehicles increased the possibility of crashes (European Road Safety Observatory, 2007). The Alaska Department of Transportation and Public Facilities (2000) recommended that in order to make the speed limits more effective, the reduction of the speed limit should be coupled with police enforcement. Additionally, enforcement will make road users aware of the changes in the speed limit, if any.
The effectiveness of various speed reduction techniquese such as police enforcement, radar technologies, speed camera technologies, dynamic-speed display system, and various other traffic calming methods e have been studied by a number of researchers. These studies have shown that police enforcement has resulted in a consistency in the significant decrease of vehicle speed as well as significant decrease in the percentage of vehicles exceeding the speed limit as a result of police enforcement (Armour, 1986; Benekohal et al., 1992; Hauer et al., 1982; Stuster, 1995; Vaa, 1997). Further, repre-sentatives from state DOTs indicated that police enforcement was the most effective method for speed-limit compliance (the highest rating of 4.2 out of 5), followed by installation of proper speed zone sign posts, changing road characteristics, and driver education (Fig. 6).
Studies regarding the effectiveness of speed-enforcement cameras on the reduction of average speed and frequency of crashes have shown various results. For example,Elvik (1997) showed that the crashes decreased significantly when a speed-enforcement camera (photo radar) was used. On the other hand, Rogerson et al. (1994) found no significant reduction in the average speed of vehicles or the crash frequency. Other studies showed the effectiveness of using drone radars to reduce the average speed of vehicles
(Freedman et al., 1994). In terms of the average speed or the 85th percentile speed, studies showed a consistent and significant effect when dynamic displays were used to reduce speed (Arnold Jr. and Lantz Jr., 2007; Cruzado and Donnell, 2009; Sandberg et al., 2006). The results of comparison between some speed enforcement technologies and methodologies are presented inTable 5.
To enforce the speed limits in Nevada, NDOT coordinates with law-enforcing agencies whenever NDOT changes or recommends changes in speed limits. If a speed limit is set below the 85th percentile speed, then a commitment is required from the law enforcement agency to enforce the speed limit.
Although such enforcement methods as police enforce-ment are effective methods to increase compliance to speed limits, such methods are costly (Stuster, 1995). As such, traffic-calming devices would be alternatives that are more economical in the long run. Studies have found that various traffic-calming devices e such as a removable pedestrian island, pedestrian crossing devices, roundabouts, and traverse pavement markings e are effective in decreasing the average speed as well as increasing traffic compliance (Kamyab et al., 2003; Torbic et al., 2012). A proper study in the effectiveness of such traffic-calming devices in the locations of interest should be conducted as an economical alternative to other traffic enforcement methods. In addition, the speed-zone guidelines should provide guidelines regarding the use of these types of traffic calming devices rather than just focusing on determining the speed limit. Many state DOTs have prepared public-awareness pamphlets about speeding, speed limits, and crashes. These pamphlets could be followed by NDOT.
5.6. Phase 6: Follow-up study
The NDOT traffic engineers agreed that follow-up studies should be conducted to assess the effectiveness of a new Fig. 5e Average rating of crash reasons by state DOT representatives.
speed zone or the changes in the speed zone, depending upon each case. These studies should be conducted to measure the short-term and long-term effects of the speed zone or its modifications. Any changes in crash rates and severities should be identified in the speed-distribution pattern. Anal-ysis of the follow-up studies could be used to evaluate and modify the guideline. However, the majority of traffic engi-neers at NDOT that were surveyed thought that no follow-up studies currently are being conducted to determine the effects of a new speed zone in Nevada.
6.
Framework approval by NDOT traffic
engineers
The framework developed in this study was presented to NDOT traffic engineering experts who will be the users of the guideline. The experts noted that while many state DOTs had parts of this framework, NDOT lacked the comprehensive speed zone guidelines that could be developed based on the proposed framework. During the presentation, they approved the speed zone guideline framework. NDOT is planning to distribute this framework to their traffic engineers, so that
they can use it while setting the speed limit in towns along rural highways. Researchers and NDOT traffic engineers noted that the holistic six-phase-approach presented in this study would aid them in establishing realistic speed zones, which would reduce the public requests of speed limits reduction and increase the compliance of the speed limits. Once a guideline based on this framework is implemented, its effects on a realistic speed zone setting up and crashes reduction can be studied as an extension of the validation of the framework.
7.
Conclusions and recommendations
This study proposed a comprehensive framework of speed zone guideline in six phases, as shown inFig. 4, along with the important aspects of each phase. Relevant studies and manuals corresponding to each phases were presented in this paper. In addition, alternative approaches used by various state DOTs were presented for each phase to allow NDOT to choose the best one that suits them. Finally, arbitrary values adapted for establishing speed zones were identified, and topics for future study were recommended. Fig. 6e Average rating of the factors to control speeding traffic on rural highways.
Table 5e Comparison of speed enforcement technologies and methodologies.
Criteria for comparison Source Better technology Compared to
Detecting speeding vehicles in heavy traffic
Blackburn et al. (1989) Cross-the-road radar Down-the-road-radar Cost effectiveness to
reduce speeds
Bloch (1998) Display board without police Display board with police presence, photo radar Halo effect (speed limit
compliance for a longer duration)
Rather than relying on expensive police enforcement, a realistic speed limit, public awareness, and the use of transi-tion zone modificatransi-tions are important to increase speed compliance. A proper speed-zone manual should be devel-oped in Nevada, using the framework provided in this study. Once prepared, the manual can aid NDOT traffic engineers in setting up speed zones with realistic speed limits. Further-more, it could help defend against public requests to decrease speed limits arbitrarily. The manual should be updated based on follow-up studies of the speed zones as well as other new and relevant studies. The findings of this study can be utilized by other state DOTs and transportation agencies worldwide to prepare speed zone manuals for their states.
Acknowledgments
The authors would like to acknowledge Nevada Department of Transportation (NDOT) for funding and guidance for this study. The funding of this research was provided under Grant #P255-11-803 by NDOT. The views and opinions expressed in this paper are solely of the authors and do not reflect those of NDOT. In addition, we would like to thank Mrs. Julie Longo, Technical Writer at the Howard R. Hughes College of Engi-neering, University of Nevada Las Vegas, for editing this paper.
r e f e r e n c e s
Agent, K.R., Pigman, J.G., Weber, J.M., 1998. Evaluation of speed limits in Kentucky. Transportation Research Record 1640, 57e64.
Alaska Department of Transportation and Facilities, 2000. Policy and Procedure: Establishment of Speed Limits and Zones. Alaska Department of Transportation and Facilities, Juneau. American Association of State Highway and Transportation
Officials (AASHTO), 2011. A Policy on Geometric Design of Highways and Streets, sixth ed. AASHTO, Washington DC. American Association of State Highway Transportation Officials
(AASHTO), 1994. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington DC.
Armour, M., 1986. The effect of police presence on urban driving speeds. Institute of Transportation Engineers 56 (2), 40e45. Arnold Jr., E.D., Lantz Jr., K.E., 2007. Evaluation of Best Practices in
Traffic Operations and Safety: Phase I: Flashing LED Stop Sign and Optical Speed Bars. No. VTRC 07-R34. Virginia Transportation Research Council, Charlottesville.
Automobile Club of Southern California, 1998. Realistic Speed Zoning: Why & How. Automobile Club of Southern California, Los Angeles.
Benekohal, R.F., Resende, P.T.V., Orloski, R.L., 1992. Effects of Police Presence on Speed in a Highway Work Zone: Circulating Marked Police Car Experiment. No. FHWA/IL/UI-240. University of Illinois, Urbana-Champaign, Urbana. Blackburn, R.R., Moran, R., Glauz, W.D., 1989. Update of
Enforcement Technology and Speed Measurement Devices. No. DOT HS 807 584. Midwest Research Institute, Kansas City. Bloch, S.A., 1998. Comparative study of speed reduction effects of photo-radar and speed display boards. Transportation Research Record 1640, 27e36.
California Department of Transportation, 2012. California Manual on Uniform Traffic Control Devices: FHWA's MUTCD 2009
Edition as Amended for Use in California. California Department of Transportation, Sacramento.
California Vehicle Code 22358.5, 2011. Downward Speed Zoning. San Diego Ticket King, San Diego.
Connecticut Department of Transportation (ConnDOT), 2012. Speed Limit? ConnDOT, Newington.
Cooper, D.R.C., Jordan, P.G., Young, J.C., 1980. The Effect on Traffic Speeds of Resurfacing a Road. SR 571 Monograph, 22. Transport and Road Research Laboratory, Berkshire.
Cruzado, I., Donnell, E.T., 2010. Factors affecting driver speed choice along two-lane rural highway transition zones. Journal of Transportation Engineering 136 (8), 755e764. Cruzado, I., Donnell, E.T., 2009. Evaluating effectiveness of
dynamic speed display signs in transition zones of two-lane, rural highways in Pennsylvania. Transportation Research Record 2122, 1e8.
DeLong, D.W., 2004. Lost Knowledge: Confronting the Threat of an Aging Workforce. Oxford University Press, Oxford. Dudek, C.L., Ullman, G.L., 1987. Speed Zoning and Control. No.
334-2F. Texas Transportation Institute, Arlington.
Elvik, R., 1997. Effects on accidents of automatic speed enforcement in Norway. Transportation Research Record 1595, 14e19.
Esposito, T., Mauro, R., Russo, F., et al., 2011. Operating speed prediction models for sustainable road safety management. In: International Conference on Sustainable Design and Construction 2011, Integrating Sustainability Practices in the Construction Industry, Kansas City, 2011.
European Road Safety Observatory, 2007. Speed and Accident Risk. European Road Safety Observatory, Brussel.
European Transport Safety Council, 1995. Reducing Traffic Injuries Resulting from Excess and Inappropriate Speed. European Transport Safety Council, Belgium.
FHWA, 2012. Manual on Uniform Traffic Control Devices for Streets and Highways. FHWA, Washington DC.
FHWA, 2000. Speeding in Rural Areas. FHWA, Washington DC. Fildes, B.N., Rumbold, G., Leening, A., 1991. Speed Behaviour and
Drivers' Attitude to Speeding. HS-041 292. Victoria Road Safety and Traffic Authority, Hawthorne.
Fitzpatrick, K., Carlson, P., Brewer, M., et al., 2001. Design factors that affect driver speed on suburban streets. Transportation Research Record 1751, 18e25.
Freedman, M., Teed, N., Migletz, J., 1994. Effect of radar drone operation on speeds at high crash risk locations. Transportation Research Record 1994, 69e80.
Georgia Department of Transportation, 2012. Traffic Control-Speed Limits: Establishment of Speed Zones. Georgia Department of Transportation, Atlanta.
Haselton, C.B., Gibby, A.R., Ferrara, T.C., 2002. Methodologies used to analyze collision experience associated with speed limit changes on selected California highways. Transportation Research Record 1784, 65e72.
Hauer, E., Ahlin, F.J., Bowser, J.S., 1982. Speed enforcement and speed choice. Accident Analysis and Prevention 14 (4), 267e278.
Idaho Transportation Department, 1997. Speed Limit and Speed Zones: a Guide to Establishing Speed Zones in Idaho. Idaho Transportation Department, Boise.
Institute of Transportation Engineers, 1993. Technical Council Report Summary. A Proposed Recommended Practice: Speed Zone Guidelines. Institute of Transportation Engineers, Washington DC.
Institute of Transportation Engineers, 2004. Speed Zoning Information: a Case of “Majority Rule” (Within the United States). Institute of Transportation Engineers, Washington DC.
Jarvis, J.R., Hoban, C.J., 1988. The Development of a Speed Zoning Knowledge-based System. Monash University, Clayton.
Kamyab, A., Andrle, S., Kroeger, D., et al., 2003. Methods to reduce traffic speed in high-pedestrian rural areas. Transportation Research Record 1828, 31e37.
Kentucky Transportation Cabinet, 2012. Traffic Operations Guidance Manual. Kentucky Transportation Cabinet, Frankfort.
Khan, N.M., Sinha, K.C., McCarthy, P.S., 2001. An Analysis of Speed Limit Policies for Indiana. Purdue University/Indiana Department of Transportation JHRP, West Lafayette.
Kockelman, K.M., Bottom, J., 2006. Safety Impacts and Other Implications of Raised Speed Limits on High-Speed Roads. NCHRP Web Document Transportation Research Board, Washington DC.
Malyshkina, N.V., Mannering, F., 2008. Effect of increases in speed limits on severities of injuries in accidents. Transportation Research Record 2083, 122e127.
Massachusetts Highway Department, 2005. Procedures for Speed Zoning on State and Municipal Roadways. Massachusetts Department of Transportation, Bonston.
Michigan Department of Transportation (MDOT), 2004. Establishing Realistic Speed Limits. MDOT, Lansing.
Missouri Department of Transportation, 2010. Speed Limit Guidelinese Engineering Policy Guide. Missouri Department of Transportation, Kansas City.
Najjar, Y.M., Stokes, R.W., Russell, E.R., 2000. Setting speed limits on Kansas two-lane highways: Neuronet approach. Transportation Research Record 1708, 20e27.
Nevada Department of Transportation, 2012. Nevada Traffic Crashes 2010. Nevada Department of Transportation, Carson City.
Nevada Legislature, 2013a. Rules of the Road. Nevada Legislature, Carson City.
Nevada Legislature, 2013b. Traffic Law Generally. Nevada Legislature, Carson City.
Nevada Legislature, 2011. Traffic Laws. Nevada Legislature, Carson City.
Raju, S., Souleyrette, R., Maze, T.H., 1998. Impact of 65-mph speed limit on Iowa's rural interstate highways: integrated Bayesian forecasting and dynamic modeling approach. Transportation Research Record 1640, 47e56.
Renski, H., Khattak, A.J., Council, F.M., et al., 1999. Effect of speed limit increases on crash injury severity: analysis of single-vehicle crashes on North Carolina interstate highways. Transportation Research Record 1665, 100e108.
Rogerson, P.A., Newstead, S.V., Cameron, M.H., 1994. Evaluation of the Speed Camera Program in Victoria. 1990e1991, Phase 3: Localised Effects on Casualty Crashes and Crash Severity. Phase 4: General Effects on Speed No. 54. Monash University, Accident Research Centre, Clayton.
Sandberg, W., Schoenecker, T., Sebastian, K., et al., 2006. Long-term effectiveness of dynamic speed monitoring displays (DSMD) for speed management at speed limit transitions. In: Compendium of Technical Papers, Institute of Transportation Engineers Annual Meeting and Exhibit, Milwaukee, 2006.
Shinar, D., Stiebel, J., 1986. The effectiveness of stationary versus moving police vehicles on compliance with speed limit. Human Factors: The Journal of the Human Factors and Ergonomics Society 28 (3), 365e371.
Stuster, J.W., Coffman, Z., Warren, D., 1998. Synthesis of Safety Research Related to Speed and Speed Management. No. FHWA-RD-98e154. FHWA, Washington DC.
Stuster, J.W., 1995. Experimental Evaluation of Municipal Speed Enforcement Programs. No. DOT HS 808 325. National Highway Traffic Safety Administration, Washington DC. Subramanian, R., 2012. Motor Vehicle Traffic Crashes as a Leading
Cause of Death in the United States, 2008 and 2009. No. HS-811 620. National Highway Traffic Safety Administration, Washinton DC.
Thornton, M., Lyles, R.W., 1996. Freeway speed zones: safety and compliance issues. Transportation Research Record 1560, 65e72.
Tignor, S.C., Warren, D., 1990. Driver speed behavior on US streets and highways. In: Compendium of Technical Papers, Institute of Transportation Engineers Annual Meeting and Exhibit, Washington DC, 1990.
Torbic, D.J., Gilmore, D.K., Bauer, K.M., et al., 2012. Design Guidance for High-Speed to Low-Speed Transition Zones for Rural Highways. FHWA, Washington DC.
Toyota Motor Sales, 2013. Toyota's collaborative safety research center announces four new safety research projects. Available at: http://toyotanews.pressroom.toyota.com/releases/toyotaþ collaborativeþsafetyþresearchþcenterþfourþ
newþpartnersþdecemberþ2011.htm(accessed 20.05.15.). Transportation Research Board, 1984. A Decade of Experience. No.
204. Transportation Research Board, Washington DC. United State Department of Transportation, 2014. Connected
vehicle research. Available at: http://www.its.dot.gov/ connected_vehicle/connected_vehicle.htm(accessed 26.06.14.). Vaa, T., 1997. Increased police enforcement: effects on speed.
Accident Analysis and Prevention 29 (3), 373e385.
van der Horst, R., Ridder, S.D., 2007. Influence of roadside infrastructure on driving behavior: driving simulator study. Transportation Research Record 2018, 36e44.
Vogel, E., 2013. 85 mph speed limit approved by Nevada Senate. Available at: http://www.reviewjournal.com/news/nevada-legislature/85-mph-speed-limit-approved-nevada-senate (accessed 03.07.14.).
Warren, D.L., 1982. Synthesis of Safety Research Related to Traffic Control and Roadway Elements, vol. 2. No. FHWA-TS-82-233. FHWA, Washington DC.
Wisconsin Transportation Information Center, 1999. Setting Speed Limits on Local Roads. No. 21. Wisconsin Transportation Information Center, Madison.
Zahabi, S.A.H., Strauss, J., Manaugh, K., et al., 2011. Estimating potential effect of speed limits, built environment, and other factors on severity of pedestrian and cyclist injuries in crashes. Transportation Research Record 2247, 81e90.
Dr. Pramen P. Shrestha is an associate pro-fessor in Department of Civil and Environ-mental Engineering and Construction at University of Nevada Las Vegas. He joined the department in 2007. He has about 10 years of experience in constructing high-ways, bridges, and irrigation canals. He has conducted research on construction and project management, traffic safety, work zone safety, and sustainability of highways, buildings, and infrastructure projects funded by State, Federal, and private agencies of United States. He has received about $2 million grants for research work and has pub-lished over 65 peer reviewed papers related to this research work.
