Monitoring and evaluation of sign only 20 mph speed limit

The monitoring and evaluation of any speed limit / speed management intervention is vital to determine whether it works, to adapt it if necessary, and to provide evidence for continuing support at the level of decision makers, key stakeholders, and the public.

Monitoring and evaluating will not only provide feedback on the effectiveness but will also help to determine whether a speed limit / speed management intervention is appropriate, whether there are any problems with its implementation and support, and whether there are any ongoing issues that need to be resolved before any further intervention is implemented.

It is important to plan for evaluation early in the design process to allow a baseline to be created.

For the monitoring and evaluation of the speed limit it is recommended to:

  1. Determine the aim and outcomes of the evaluation and consider other data that can be monitored such as levels of walking and cycling, air quality, decrease in traffic flow etc.
  2. Conduct a “before implementation” speed analysis to create a baseline.
  3. Run the speed analysis throughout the entire day over a seven-day period across all selected sites.
  4. Conduct at least two “post implementation” speed analysis to be collected at approximately 3 months apart, or as appropriate.
  5. Ensure consistency in measurement by monitoring the same sites to gain an equivalent comparison.
  6. Write and disseminate a monitoring and evaluation report for committee or public.
  7. Use results to plan or promote interventions or speed management measures.
  8. Monitor the speed of the newly introduced speed management intervention and repeat as above.

The suggested data to be collected:

  1. Traffic volume
  2. Traffic classification (Car/Van/HGV)
  3. 85th percentile speed
  4. mean speed.
  5. Using 3 speed bins of vehicles travelling at or below 0-25 mph, between 25–30 mph, above 30 mph

Measurement for action

0-25 mph - Average speeds at or below 25 mph will be assumed to be at a level where no speed management interventions are required – The speeds should be continued to be monitored for any deviations in future.

26-30 mph - Average speeds between 25-30 mph will indicate that softer speed management measures should initially be considered or a reduction/shift in it extents. Once implemented, monitored again and refined further is necessary.

The measures used will depending on the road environment, for instance:

  • Gateway features (such as red surfacing, “dragon’s teeth,” countdown signing)
  • Adding repeater signs or increasing the number of repeater signs - Carriageway roundels may be used; however, these can be difficult to remove if the 20mph limit is not made permanent.
  • Additional road markings to emphasise road features or reduce carriageway widths, such as middle lane hatching, white line cycle lanes.
  • Vehicle activated signs.
  • Consider your wider delivery plans – do they include active travel measures which can be added to reduce width or road.
  • Liaising with Police Scotland to consider if enforcement on a local level can be deployed.

Above 30 mph - Average speeds of above 30 mph will indicate that speed management measures are required to change the road environment to achieve better speed compliance. Consider using softer features initially, such as road markings or a reduction/change in the extent of the speed limit. Any change should be monitored before moving on to more physical engineering measures which will help to evidence the reason for change.

As all roads have been pre assessed as being appropriate for a speed limit of 20 mph, if not reaching a desired level of speed compliance, it is expected that feasible speed management measures should be exhausted wherever possible, or a reduction/change in the speed limit extents, before any consideration to return them, either in full or partially, to the previous speed limit of 30mph unless other evidence suggests the road is not appropriate.

Longer Term Monitoring and Evaluation

Both the collision history and speed analysis are important factors to monitor over a longer period.

For collision history, to gain an appropriate level of data, an evaluation should not be conducted until at least 1 year of post installation data is available. It is desirable to have 3 years of collision data to provide a larger sample size and a more realistic indication of outcomes.

For speed analysis, although the initial speeds will be monitored as early as possible and evaluated to give early indications on the levels of compliance, the recommended period for a speed analysis after a major engineering change (e.g. a new speed limit or road design element) is 1 year. Waiting a full year will allow motorists to get acclimatised to the new treatment and environment and will allow it to be encountered in all types of weather conditions.

Communicating results

Once an evaluation is complete, it is important to provide feedback to key stakeholders and the public, even if results were not particularly good.

While a speed management intervention may have succeeded in achieving its objectives, it is still helpful to examine and discuss what worked well and why. If the intervention has not been successful, it is important to share this with others so that weaknesses or relevant issues are considered in similar interventions, including whether to introduce such interventions in the future.

Findings should be analysed and it should be considered whether they demonstrate any tangible benefits, problems to be rectified or elements to be abandoned. Moreover, the evaluation could discover unexpected side-effects of the interventions, both positive and negative. These should be considered in the further development of interventions.