Annex E: Speed Assessment Framework - New Approach to Speed Limit Setting for Single Carriageway Roads in Rural Areas
Speed limits should be considered as only one part of rural safety management. The first priority where accident rates are high should be to seek cost effective improvements to reduce these rates, targeting the accident types that are over-represented.
If high rates persist, despite these measures, then lower speed limits may also be considered. But speed limits on their own without supporting physical measures, driver information and publicity or other measures will not necessarily change driver behaviour and therefore result in substantial numbers of drivers continuing to travel at unacceptable speeds. This may lead to significant enforcement cost. So every effort should be made to achieve an appropriate balance between speeds, speed limits road design and other measures. This balance may be delivered by introducing one or more speed management measures in conjunction with the new speed limits, and/or as part of an overall route safety strategy.
An Assessment Framework has been developed by TRL to help decision-makers weigh up, in a more transparent way, the advantages and disadvantages of each speed limit option and reach a well-founded conclusion for these roads.
The basis for the Speed Assessment Framework procedure is:
- a firm theoretical basis for choosing speed limits for road functions taking account of safety, mobility and environmental factors,
- roads classified into two tiers based on road function,
- closer integration of speed limit choice with more general rural road safety management measures,
- driver choice of desired speed to be reflected by mean speed,
- local flexibility of choice within a consistent overall procedure.
The assessment framework combines safety and mobility costs to show how the overall total cost and the balance between the component costs change if different choices of speed limit are made. For a particular road type, total cost is similar over a relatively wide speed range, with mobility benefits being exchanged for safety benefits as speed decreases.
A simple two-tier functional hierarchy should be used, with roads having either primarily a through traffic function (upper tier) or a local access function (lower tier). Both need to be provided safely. Mobility benefits will be more important for the upper tier than for the lower tier roads, whilst environmental benefits are likely to be of greater importance for the lower tier roads.
There may be many roads below A and B classification which serve a mixed through traffic and access function. Where that traffic function is currently being achieved without a high accident rate, these roads should be judged against the criteria for upper tier roads. If however, for all or parts of these roads, there is a substantial potential risk to vulnerable road users, these sections should be assessed against the criteria for lower tier roads.
Decisions on speed limits should take account of other accident reduction measures that might be applied. To help in this process a technical guide on Accident Analysis on Rural Roads (downloadable from the TRL website www.trl.co.uk) has been developed, which provides information on typical accident rates and typical proportions of different accident types, on different types of rural road. These can be used to judge whether other site or route specific measures might be appropriate, which would reduce either speeds or accidents along the route.
Mean speed should be used for the assessment. For the majority of roads there is a consistent relation between mean speed and 85 th percentile speed. Where this is not the case, it will usually indicate that drivers have difficulty in deciding the appropriate speed for the road, suggesting that a better match between road design and speed limit is required.
The aim should be to align the speed limit to the prevailing conditions, and all vehicles moving at speeds as close to the posted speed limit as possible. An important step in the procedure is to gain agreement with the police that the mean speed of drivers on the road with any new speed limit is acceptable.
The aim of the framework approach is to achieve a consistent application of speed limit policy throughout the country. But local issues in relation to particular routes can be reflected in the functional tier to which the road is assigned, and also through final decisions on acceptable mean speeds for each limit, on the importance given to local environmental factors, and on the choice of additional measures that could change the appropriate speed limit regime recommended.
Within routes, separate assessments should be made for each section of road of 600 metres or more for which a separate speed limit might be considered appropriate. When this is completed, the final choice of appropriate speed limit for individual sections might need to be adjusted to provide consistency over the route as a whole.
A flow chart for the decisions to be made for selecting speed limits for rural single carriageway roads is given in Figure 1. It includes the following steps:
1. consider if the level of development requires special treatment,
2. consider which functional tier is appropriate for the road,
3. measure the current mean speed and accident rate (as all injury accidents per 100 million vehicle km),
4. check the accident rates against the thresholds set out in Section 6.1,
5. if the accident rate is high, check the proportion of different accident types against the investigatory thresholds recommended in Accident Analysis on Rural Roads and consider whether site or route treatment is appropriate before deciding speed limit,
6. if a speed limit lower than the current one is indicated, estimate the mean speed and accident rate and the influence on social factors that would result from implementing the new limit,
7. check that these values are acceptable; if not consider whether further measures are necessary to bring speed and accident rates into balance.
For mean speeds to be acceptable, they should be no higher than the posted limit after it has been implemented. Research shows that for a typical distribution of vehicle speeds on single carriageway rural roads, the 85 th percentile speed is about 6 mph above the mean speed for roads with a 50 mph limit, and about 8 mph above mean speeds on roads with a 60 mph limit. Setting acceptable mean speeds at or below the limit is therefore consistent with current enforcement policy.
The choice of speed limits within each tier should take account of the following:
- whether the accident rate is below the appropriate threshold of injury accidents per 100 million vehicle kilometres,
- whether there is substantial development,
- whether the road forms part of a route for vulnerable road users.
The bands of appropriate accident rates by speed and speed limit are illustrated in Figures 2 and 3. If walking, cycling, horse riding or environmental factors are particularly important on the road section, consideration should be given to using the lower limit even if the accident rate is below the threshold shown.
The influence of development should be taken into account through the following factors:
- if the road section qualifies for village status, the advice in Traffic Advisory Leaflet 1/04 should be followed,
- if the section does not meet the definition in Traffic Advisory Leaflet 1/04 for a village, but the level of development is at least half the density implied (over a minimum of 600 metres), a speed limit of 40 mph should be considered.
Other factors that would strengthen the case for a 40 mph limit are a high incidence of bends or junctions or a high accident rate and specific development in terms of schools, public houses and vulnerable road users.
Figure 1 Flow chart for choice of speed limit
Figure 2: Speed limit zones in terms of mean speed and accident rate for upper tier roads
Figure 3: Speed limit zones in terms of mean speed and accident rate for lower tier roads