Chapter 5 Contemporary Assessment Methodologies
5.4 The United States
As with the UK and Ireland, the Federal Highways Administration in the United States feels that a rational speed limit is one that is safe, appropriate, protects the public and can be enforced. They believe a knowledge based expert system (Appendix L User Guide for USLIMITS, March 2012, Federal Highway Administration) can provide assistance to those setting speed limits for specific conditions on road sections.
The expert system described in the user guide employs a decision algorithm to advise the user of the appropriate maximum speed limit for the specific road section of interest. The expert system is accessed through the internet and provides recommended speed limits for speed zones on all types of roadways from rural two-lane roads to urban freeway segments. The types of speed limits not addressed by the system include statutory limits, such as maximum limits (set by State legislatures for interstates and other roadways), temporary or part-time speed limits, speed limits posted in work and school zones and variable speed limits that are raised or lowered based on traffic, weather, and other conditions.
The system is outlined in full in documentation available at the Federal Highway Administration website (https://safety.fhwa.dot.gov/uslimits/). The system is web based. User inputs for each route type are as follows.
Limited Access Freeway
▪ Operating speed: 85th percentile speed and 50th percentile speed
▪ Section length
▪ Presence/absence of adverse alignment ▪ Current statutory limit for the type of road ▪ Terrain (level, rolling, mountainous)
▪ Is this section transitioning to a non-limited access highway? ▪ Number of interchanges within this section
▪ Crash statistics
Road Section in Undeveloped Areas
▪ Operating speed: 85th percentile speed and 50th percentile speed ▪ Section length
▪ Annual average daily traffic
▪ Presence/absence of adverse alignment ▪ Current statutory limit for this type of road
▪ Is this section transitioning to a road section in a developed area? ▪ Roadside rating
▪ Divided/undivided section ▪ Number of through lanes ▪ Crash statistics
Road Sections in Developed Areas
▪ Operating speed: 85th percentile speed and 50th percentile speed ▪ Section length
▪ Annual average daily traffic
▪ Presence/absence of adverse alignment ▪ Current statutory limit for this type of road ▪ Whether it is a one-way street?
▪ Number of through lanes ▪ Area type
▪ Number of driveways within the section ▪ Number of traffic signals within the section ▪ Presence/usage of on-street parking
▪ Extent of pedestrian/bike activity ▪ Crash statistics
Collision data is required, and this data is analysed by a developed crash module. Two types of speed limit are recommended. SL_1 is the suggested speed limit based on analysis without collision data and SL_2 is the limit suggested with collision data analysed. The system appears to be complex, but apparently all encompassing. The desired outcome is as per the various speed assessment frameworks – a safe suitable speed limit is suggested for the route or section under assessment. Appendix K is available on the FHA website and contains flowcharts that illustrate decision rules. It is apparent from Appendix K that, in areas of adverse alignment, the use of Advisory Speed Limits is also employed. This provision is not available in Ireland yet.
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5.5
Summary
Contemporary assessment methods highlighted in this Chapter show that many jurisdictions focus on setting speed limits based on various factors, for instance setting the speed limit at or close to the 85th percentile, or other methods that put a
lot of emphasis on computational interrelationships between various factors (environmental, road environs, human factors, etc.) requiring vast amounts of data gathering. They assess what a speed limit should be and apply it and afterwards
For example, the Institute of Transportation Engineers (ITE) in the United States (Methods and Practices for Setting Speed Limits: An Informational Report, Institute of Transportation Engineers, 2012) outlines the four main methods commonly used to set speed limits;
The Engineering Approach where the speed limit is set based on either the 85th
Percentile or the Design Speed of the road and then adjusted based on traffic and infrastructure conditions.
Expert system approach: Speed limits set by computers using knowledge and inference processes to simulate expert knowledge and behaviour.
Optimisation: Setting speed limits by considering journey times, operating costs, collisions and environmental impacts when assessing optimal speed limits.
Injury minimisation/safe systems approach: Speed limits are set according to the crash types that are likely to occur, the impact forces that result, and the human body’s tolerance to withstand these forces – see also page 49 (Anders Lie) and Chapter 7 (Safe Systems Approach) of this thesis and Ayoola, Oke et al., A mathematical model to set speed limits for vehicles on the highway, 2006.
For example, in Illinois, the procedure considers access, pedestrian traffic, kerbside parking, and safety performance, along with existing speed profiles to recommend a speed limit with specific numerical adjustments (factors) specified for the items listed whereas the Northwestern Speed Zoning technique is similar to Illinois but they also consider the median/protection measures, lane widths, alignment etc by applying factors for different features. These approaches are outlined in the 2012 ITE report referenced above.
While operating speed model approaches to speed limit determination have been published, for both urban and rural scenarios and for single and dual carriageway type roads, they all collect speed and develop mathematical formulae based on certain parameters to arrive at a point where the model informs what the speed limit should be. Those parameters that are factored into the speed profile are those that are commonly accepted as being influencers on safe speeds and the selection of a driving speed by a driver, i.e.;
▪ Vehicle mechanical condition and characteristics; ▪ Driving ability/capabilities;
▪ Traffic volume: vehicles, pedestrians, cyclists; ▪ Weather and visibility;
▪ The roadway features; - Road function - Road Width
- Horizontal and vertical alignment - Sight distances
- Roadside development etc
The methodology proposed in this thesis (Chapter 6) attempts to develop a process of speed limit assessment based on what is happening in real life and allowing the observed driver behaviour (Safe Profile Velocity-Vsp) be ‘the model’ instead of
taking operating speeds and further manipulating them with complex mathematical formulae for the items above because, it is proposed, this has already occurred and the items above have been accounted for when actual driver
This chapter has highlighted the different methodologies that exist with respect to assessing and setting speed limits in many scenarios in the UK, Ireland and the United States. Some advantages and disadvantages of each are tabulated below.
Adapted from MASTER Framework Zealand (XLIMITS-VLIMITS) Adapted from Australia-New
United Kingdom Ireland United States
Advantage Disadvantage Advantage Disadvantage Advantage Disadvantage Tries to distinguish between different road classes In terms of lower tier roads, it may not be cost effective Tries to distinguish between different road classes (but contradicts itself) Speed limits to be lower on lower tier/access function roads (high density of
bends - much of the 'strategic' national road network is like
this)
Splits assessments into
3 categories of road, akin to our
motorways, urban roads, rural
roads. Appears to be a complicated set of interrelationships to be developed should a similar system be employed here Sensible starting
point for upper and lower tier
speed limits
No confidence in it from the Local Authorities and very
little understanding of it Federally supported online system Greater amount of inputs and data to
be collected
Tier based on collision rates which
could result in a conflict with the strategic function
Can have two options for a speed limit based on the availability of collision data
Having the option to proceed without collision data could result in collision data being routinely
ignored and an inappropriate speed
limit chosen
They appear complex and time consuming. The need for a simplified system becomes apparent when resources are limited, particularly when part of a major review and overhaul of an existing system. The UK and Ireland’s systems (derived ultimately from the MASTER Framework developed in Finland), in this author’s opinion, represent the past. They do not properly or exclusively consider all physical aspects of the road and actual driver behaviour.
They are based on a class/function/tiered system. Lower tier roads do exist in Ireland (likely in the UK as well) that have a higher AADT than an upper tier road. One example would be the R445 in Kildare that in some parts has an AADT in the region of 21,000 compared to the N56 in Donegal that has an AADT in the region of 11,000.
The Irish Speed Assessment Framework sprawls across several headings that offers little in the way of practical rules and advice and offers philosophies instead of hard and fast rules and frequently refers to “….a two-tiered hierarchical approach”. It is not easy to read or follow in a coherent fashion and would come as no surprise if it were to be discovered that it had not been used in the last nine years. It is for that reason that the methodology in the following chapter is being proposed as a more effective and simplified way of assessing speed limits in Ireland and one that would be more likely to be used by Local Authorities.
In this author’s opinion, Ireland may have been using (to what extent remains unquantified) a Speed Assessment Framework that was not fully suitable for use in its jurisdiction. The US system, while online and browser based, requires a large amount of data gathering with complex unseen interrelationships and would be difficult to either replicate or adapt for use in setting speed limits on Irish roads.
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