Traffic Efficiency Guidelines
for
Temporary Traffic Management
Version 3.1 Final
July 2013
Contents
1 Glossary of Terms ... 3 2 Setting the Scene ... 4 2.1 COPTTM TMP Principle ... 4 3 Traffic Volumes ... 5 3.1 Hourly Profiles, Peak Periods, Directions and Traffic Counts ... 5 3.2 Rules of Thumb to Use When Limited or No Traffic Count Data are Available ... 5 3.3 Traffic Count Sources ... 5 3.3.1 Local road traffic counts ... 6 3.3.2 State Highway traffic counts ... 6 4 Merge Capacities ... 7 4.1 Typical Merge Capacity ... 7 4.2 Factors Influencing Merge Capacity ... 8 4.3 Multi‐lane Roads ... 8 4.4 Flow Profile ... 8 5 Speed‐Flow‐Density Relationships ... 9 5.1 Volumes Above 1500 vph/lane ... 10 5.1.1 Speeds Below 15 km/hr ... 10 5.1.2 Speeds greater than 15 km/hr with less than 1500 vph/lane ... 10 6 Stop / Go Capacity ... 11 7 Road Closures and Detours ... 12 8 Worksites Adjacent to an Intersection ... 13 8.1.1 Worksites close to an intersection ... 13 8.1.2 Impact of a Worksite on Traffic Signals ... 13 8.1.3 Impact of Traffic Signals on a Worksite ... 13 Appendices A Useful Equations 14 B Speed Flow Capacity 151 Glossary of Terms
AADT Annual Average Daily Traffic
COPTTM Code of Practice for Temporary Traffic Management
HPMV High Productivity Motor Vehicle
km/hr kilometres per hour
NZTA New Zealand Transport Agency
RCA Road Controlling Authority
SIDRA Signalised & Unsignalised Intersection Design and Research Aid
STMS Site Traffic Management Supervisor
TMP Traffic Management Plan
TSL Temporary Speed Limit
TTM Temporary Traffic Management
2 Setting the Scene
Road works have the potential to disrupt traffic flow and result in delays to road users. To ensure network efficiency, queuing at road works should be minimised, to prevent motorist frustration and improve safety.
These guidelines are aimed at the designers of TMPs, to help them to minimise network impacts when designing Temporary Traffic Management Plans.
These guidelines provide information that can be used to help optimise worksites to ensure minimal disruption to a network. The base expectation is that existing traffic movements will be maintained wherever possible.
These guidelines are intended to add to COPTTM Section C16.
2.1 COPTTM TMP Principle
Temporary Traffic Management (TTM) shall be carried out to avoid, or at least
minimise, inconvenience and delay to road users whilst still providing safe conditions for both the road users and those carrying out the work activity.
Traffic volume capacity predictions included in this document may need to be reduced if:
The road is rough or unsealed,
The horizontal geometry restricts speeds to less than 40 km/hr, The proportion of heavy vehicles exceeds 12%.
3 Traffic Volumes
3.1 Hourly Profiles, Peak Periods, Directions and Traffic Counts
The weekday morning peak period is typically between 7am and 9am. The evening peak period is typically from 4pm until 6pm, but may run until 7pm in larger cities like Auckland, Wellington and Christchurch. Typically, one direction has more traffic volume than the other during these peak periods. There are shoulder periods each side of the peak periods when traffic volumes are building or reducing. The inter-peak period is typically between 9am and 4pm though there is often an increase in flow between 2pm and 4pm (Fig. 1).
Weekends and public holidays often have a single peak, but this usually has a longer duration which typically lasts from 10am until 4pm.
FIGURE 1. Typical Daily Traffic Volume Profile.
3.2 Rules of Thumb to Use When Limited or No Traffic Count Data
are Available
The morning or evening peak periods (1 hour flows) can be assumed to be 10% of the AADT flow.
Counting traffic volumes for 15 minutes in a typical morning peak, and multiplying by 40 will provide a rough approximation of the AADT flow. For weekend traffic counts, it can be assumed that the maximum hourly flow
is equivalent to the weekday inter-peak (2-4pm) traffic flow. However, on some major routes the weekend maximum hourly traffic flow can be equivalent to the morning peak traffic flow.
3.3 Traffic Count Sources
Historical traffic count data provides guidance when deciding on the proposed TTM methodology, but do not guarantee that the methodology and deployment
to deployment of the TMP, to confirm that the impact on the network will be as expected.
Traffic volumes on a public holiday or during school holidays differ from those found on typical working or weekend days. Traffic volumes on holiday start and end days are often higher than on days during the holiday period. These differences occur in both the peak traffic volumes and the daily profile. Care must be taken when
designing TMP’s for these days. It is recommended that TMP designers contact their local RCA Traffic Engineer for guidance.
There is an NZTA State Highway Traffic Monitoring System which contains detailed directional counts by quarter hour. In addition some sites are counted 365 days a year, which can be useful when looking for annual trends. There are times of the year which generally experience lower traffic volumes than others e.g. school holidays.
3.3.1 Local road traffic counts
RCA’s hold their traffic count data in different ways and on different systems. Links to data for some of the major RCA’s are given below. Please contact your local RCA for more data.
Auckland http://www.aucklandtransport.govt.nz/improving-transport/maintenance/Road/Pages/Traffic-Counts.aspx Hamilton http://www.hamilton.co.nz/our-services/transport/maintainingimproving/Pages/Traffic-Counts.aspx Tauranga To be located Wellington To be located Christchurch http://www.ccc.govt.nz/cityleisure/projectstoimprovechristchurch/transport/trafficc ount/index.aspx
In Christchurch, given the changes to traffic patterns around the city in the post-earthquake environment, actual traffic counts may differ from the historical data. Use count data from a 2011 or 2012 count – even if you need to use an adjacent site.
Dunedin
To be located
3.3.2 State Highway traffic counts
4 Merge Capacities
To minimise delays and queuing in merge situations, the actual volume of traffic at a site must be taken into account.
4.1 Typical Merge Capacity
The most straight-forward merge (two lanes into one) typically operates efficiently at volumes of up to 1300 vehicles per hour (vph). Higher traffic volumes lead to
significant slowing, queuing and potential ‘tail-backs’ (flow breakdown) extending back along the approach, which can have significant negative effects on safety and efficiency in other parts of the roading network.
It is recommended that when traffic volumes in one direction are in excess of 1300 vph, a minimum of two lanes should be maintained (Fig. 2)
FIGURE 2. Example of Flow Breakdown.
Figure 2 shows that as traffic volumes exceed 1300 vph, vehicle speeds rapidly decrease. Vehicle speeds only recover once traffic volumes fall below 1300 vph. A figure of 1300 vph should be used as a guideline for the volume at which the merge capacity breaks down. The TMP designer must ascertain what is appropriate for the site, depending on the site-specific factors present. It is recommended that TMP designers contact their local RCA Traffic Engineer for guidance.
4.2 Factors Influencing Merge Capacity
Site-specific factors affect the actual capacity of a merge operation. Important factors include:
Geometry Sight distance Merge lengths
Upstream / downstream operations Site complexity
Flow profile
Environmental factors.
If all of these factors are at optimal levels, merge operations can achieve a capacity of 1600 vph, but this is rare.
In most situations, some factors are not optimised and the merge capacity is consequently lower. A good example is a curved alignment, with minimum merge lengths and relatively close upstream or downstream priority-controlled
intersections. In this situation, the environment is more demanding on road users and the merge capacity is likely to be quite a lot lower than 1300 vph.
4.3 Multilane Roads
For multi-lane roads, multiples of the guideline 1300 vph capacity can be used to predict if impacts on traffic flow will occur. For example, when merging from three lanes into two, a merge operation can typically operate efficiently up to 2600 vph. A typical merge capacity figure of 1300 vph is generally reliable in a range of speed environments. However, the TMP designer must ascertain the appropriate volume at each site. It is recommended that TMP designers contact their local RCA Traffic Engineer for guidance if uncertain.
4.4 Flow Profile
The flow profile must be considered when deciding the start and end times for a TMP. Generally, setting out a merge at a worksite should only commence once the flows are below the estimated merge capacity. Any consequent delays and queuing will be shortlived, as the approach volume is reducing. This means that site set-up should typically occur after the peak periods.
It can be dangerous to maintain a merge operation as traffic volumes are climbing towards the estimated merge capacity (e.g. as the morning peak period is
beginning). If delays and queuing begin before the TTM is uplifted, queues can continue throughout the day, until after the evening peak period. The impact on network safety and efficiency can be significant, and may result in intense media, political and stakeholder pressure on the RCA and contractors involved.
The components of a TMP that require merge operations (e.g. merge taper; mobile operations that close a lane), must be fully uplifted well before traffic volumes climb to the merge capacity. The estimated merge capacity (and consequent TMP start and end times) can be verified on site and refined if necessary. The STMS should count traffic volumes during the closure to identify when traffic volumes start to increase
5 SpeedFlowDensity Relationships
To minimise delays when reducing the speed limit, the volume of traffic per lane must be taken into account. Figure 3 has been developed to help
determine if the flow of traffic will break down at a site with a proposed decrease in speed, which may cause significant queuing and delays.
FIGURE 3. Relationship Between Vehicle Speed and Flow Breakdown. (Refer to
Appendix A for the detailed calculation equation)
The shaded area below the solid line in Figure 3 shows the speed at which a total breakdown in flow will occur:
Veritcal solid line - At high speeds, the maximum lane capacity is
1800vph/lane, based on a 2 second spacing between vehicles (the 2 second rule).
Sloping solid line - Drivers need a minimum spacing between vehicles of around 20m (front bumper to front bumper) for traffic to flow smoothly, if the spacing drops below this then traffic flow will break down. At
1800vph/lane, the spacing drops below 20m when the speed drops below 36km/h. As the speed is reduced below 36km/h, the vehicle throughput has to reduce as well to maintain the 20m spacing necessary for smooth traffic flow.
Horizonal solid line – Experience has shown that at speeds lower than 15km/h the traffic flow breaks down.
COPTTM Section C4.1.4 states that a Temporary Speed Limit must be:
At least 20 km/hr less than the existing permanently gazetted speed limit In multiples of 10 km/hr
Appropriate to the condition of the road Not lower than 20 km/hr.
It is recommended that Temporary Speed Limits be increased or removed whenever a worksite is not being worked on, if site conditions allow.
5.1 Volumes Above 1500 vph/lane
While the capacity of a single lane in theory is 1800 vph, in reality traffic flows can breakdown at much lower traffic volumes, or may not break down till as high as 2200 vph/lane. Site-specific factors such as side-friction (resulting in “rubber necking”), intersections (roads or motorway ramps), and the number of sideways shifts required along the worksite can all affect the actual capacity of a site. COPTTM Section C16.2.4 indicates that interrupted traffic flows and queuing are likely to occur at about 1500 vph, and caution is recommended whenever traffic flows are approaching or exceeding 1500 vph. The TMP designer must ascertain what is appropriate for the site, depending on the site-specific factors present (e.g. the requirement for an ‘anti-gawking’ screen etc). It is recommended that TMP designers contact their local RCA Traffic Engineer for guidance if uncertain.
5.1.1 Speeds Below 15 km/hr
Experience with Rolling Blocks has shown that it is not possible for a line of queued vehicles to travel at speeds lower than 15 km/hr without the traffic flow breaking down. Therefore, the solid line in Figure 3 has been levelled off at this speed. However, to avoid flow breakdown, it is recommended that a minimum speed of 20 km/hr or more should be maintained.
5.1.2 Speeds greater than 15 km/hr with less than 1500 vph/lane
This is the traffic speed and volume conditions occur at most sites. There is no need for any special consideration should these conditions exist.
6 Stop / Go Capacity
COPTTM (Section C10.2) states that Stop / Go operations shall not be used where two-way traffic flow can be maintained past a worksite.
To minimise delays under Stop / Go control, the optimum ‘Go’ time should be determined and applied to deliver maximum capacity. Figure 4 has been developed to help determine the optimum Go time.
The following assumptions have been made in developing this chart:
the speed through road works that require manual traffic control is 30 km/hr (this equates to 500 meters in one minute)
the departure rate of a vehicle in a queue is 1.8 sec/vehicle
all of the vehicles that arrive whilst the ‘Stop’ control is operating are able to depart in the following ‘Go’ time.
FIGURE 4. Determination of Stop / Go Times. (Refer to Appendix A for the detailed
calculation equation)
Manual Traffic Controllers using Stop / Go paddles are considered to be more responsive to changes in traffic patterns than portable traffic lights or other options. If it is intended to use Stop / Go operations at a worksite near a set of traffic signals, please refer to Section 8.
7 Road Closures and Detours
Road closures resulting in a detour need to be managed efficiently and effectively. The following factors should be considered:
Can an alternative methodology be used to maintain traffic movements? For example:
staging the works differently
keeping a single lane operational through the worksite detouring only a single direction of traffic
night work that can take advantage of reduced traffic volumes The diverted traffic volumes need to be determined.
The proposed detour routes need to be assessed:
This may be able to be completed on an intersection by intersection basis, or network modelling may be required
Can the normal volume plus the detoured volume be accommodated at each intersection on the detour route?
Is the proposed detour route sensible? (It should match motorists’ expected detour route.) Would you want/expect to drive the detour?
Check for the presence of HPMV’s and Over Dimension vehicles. These can be either heavy (greater than 44 tonne) or longer / wider / higher than normal, making turning manoeuvres more difficult.
These types of vehicles are generally restricted to a permitted route from Point A to Point B, so detouring them will require permit changes which is often a very substantial task, but can be done.
HPMV operators often have a perception that their vehicles are overweight and therefore cannot travel where there are weight restrictions on bridges. However, this may be permissible. Please consult your local RCA for further information.
If any detours are required, the TMP designer must contact the local RCA Traffic Engineer to agree the appropriateness of the detour and if any modelling and analysis are required.
8 Worksites Adjacent to an Intersection
8.1.1 Worksites close to an intersection
Particular care needs to be taken with any site that will be located near an intersection, especially if there is a roundabout or traffic signals. The guidelines outlined in this document must be used to ascertain if queuing will occur through the intersection. If this is the case, the TMP designer must consider appropriate mitigation for the intersection. It is recommended that the TMP designer contact their local RCA Traffic Engineer for guidance.
8.1.2 Impact of a Worksite on Traffic Signals
If a worksite is within 250 metres of an intersection with traffic signals, and it will change the operation of the intersection, then the TMP may disrupt traffic flow, cause additional queuing, and possibly have wider network effects. The following factors should be carefully considered in designing the TMP:
changing the turning movements allowed from any lane closing lanes
shortening lanes
placing plant and equipment near an intersection where they may be detected by the traffic signal detection loops (e.g. a temporary fence, sign, container or any vehicle).
Please contact your local RCA to agree the appropriate level of analysis required (which may include modelling) to determine the efficiency impacts of your proposed TMP. Software such as SIDRA are useful tools for micro-modelling traffic flow through intersections.
Adjustments to the operation of an intersection (e.g. green times, signal phases) may need to be designed into the TMP to maintain network efficiency. The RCA welcomes requests to change the operation of a set of traffic signals if it improves the
efficiency of the network. However, the operation of traffic signals is complex and not all requests may be considered appropriate or be implemented.
8.1.3 Impact of Traffic Signals on a Worksite
If a worksite is greater than 250 meters away from an intersection with traffic signals, it is unlikely that the worksite will affect the traffic signals. However the traffic signals may have an impact on the operation of the worksite.
The effect on a worksite from traffic signals is most important when the site uses a Stop / Go operation, because the traffic signals will cause traffic to arrive at the worksite in groups (platoons). The Stop / Go green times will therefore need to be closely linked to the traffic signal’s green times.
Appendix A: Useful Equations
Speed-Flow-Density Relationship Equation
The Speed-Flow-Density relationship equation is shown below.
)
/
_(
)
/
_(
1000
)
/
_(
)
/
_(
vehicle
metres
spacing
km
metres
hour
km
speed
hour
vehicles
Flow
In the range of speeds 15-36kph a minimum spacing of 20m should be assumed.
This equation does not apply at less than 15 kph
For greater than 36kph, the spacing should be calculated from a 2 second gap which requires the following equation (two seconds at 100 kph is 55.6m.)
)
/
_(sec
3600
)
_(sec
)
/
_(
1000
)
/
_(
)
/
_(
hour
onds
onds
spacing
km
metres
hour
km
speed
vehicle
metres
spacing
Optimum Go Time Equation
The hourly traffic flow in each direction through a manually stop-go controlled site can be approximated by taking the number of vehicles passing through the site on a single green phase, and dividing by the total cycle time.
Considering vehicles travelling up the page:
1. Go Time 2. All Red 3. Stop Time 4. All Red
tg trr tr trr
Number of vehicles in a cycle = DepartureRate x tg
All Red, trr = Length of Site, L
Speed through site, V Traffic Flow =
Number of vehicles moving in each cycle
Total cycle time DepartureRate x t
g
tg +L/V + tr + L/V If we assume Stop Time t
r and Go Time tg are the same, then we can rearrange the
equation for Go Time, tg.
Go Time, tg = 2 x TrafficFlow x L/V DepartureRate – 2 x TrafficFlow NOTE UNITS: TrafficFlow - vehicles/second Length, L - metres
Speed through site, V - metres/second (use 8.33m/s, which is 30km/h) DepartureRate - vehicles/seconds (use 0.56)
Queue Length at Manually Controlled Stop-Go Sites
Considering vehicles travelling up the page:
1. Go Time 2. All Red 3. Stop Time 4. All Red
The queue that has been building up while the STOP sign has been showing begins to clear out as soon as the GO sign is shown (vehicles 1,2 and 3 in the diagram). However it doesn't clear immediately. Each vehicle takes off a short time after the one before it takes off (assumed to be 1.8 seconds). While vehicles are leaving the queue from the front, other vehicles are arriving and joining the queue at the back (vehicle 4). The queue is only completely cleared once the last vehicle (vehicle 4) leaves, allowing arriving vehicles to proceed through without stopping (vehicle 5). The maximum queue length is the distance to the back of the last stopped vehicle (vehicle 4). This length is equal to the number of vehicles arriving in one complete cycle, multiplied by the average length of each vehicle.
Likely maximum number of
vehicles queuing = TrafficFlow x tg + trr + tr + trr
= Flow x 2(tg + trr) (assume tg = tr) = Flow x 2(t
g + L/V)
Average length of vehicle = LengthLight x %Light + LengthHeavy x %Heavy = 7m x 90% + 28m x 10%
= 9.1m
Maximum queue length = Number of vehicles x average length of vehicle = Flow x 2(tg + L/V) x 9.1m NOTE UNITS: TrafficFlow - vehicles/second Length, L - metres
Speed through site, V - metres/second (use 8.33m/s, which is 30km/h) Go time tg -seconds
(because trr = L/V as
