SPIs on Speed

Introduction

A speed Safety Performance Indicator (speed SPI) is thus a useful tool for road design and managing purposes, and a decision-making support tool for policy purposes: it allows policy makers

related to road safety to take better decisions and to evaluate the impact of those made. Speed SPIs are not only handy for the policy makers interested on road safety, but also for

those interested in traffic management or environmental issues (e.g. higher speeds are usually associated with higher fuel consumption rates and increased pollutant emissions).

The speeds that are most relevant for safety purposes are spot speeds measured at various locations on the road network during periods when traffic can be considered free flowing, i.e. not during periods of congestion when speeds are severely restricted. The SPIs developed are the mean speed, the standard deviation, the 85th percentile speed and the percentage of drivers exceeding the speed limit. These indicators should be segregated by road type, vehicle type, period of day and period of the week, i.e. weekdays and weekends. For road types it is suggested to adopt the classification developed in the roads task. In the SafetyNet SPI Manual document procedures are described to obtain statistically valid results on a national basis, calculated from the sample of sites at which speeds are measured.

Read more about SPIs for speed in the report Safety Performance Indicators: Theory

Measurements methods

The SPI Manual makes several recommendations for measurement of speed SPI. Some of the main practical guidelines are presented below:

• Speed SPIs should be segregated by road type, vehicle type, period of day and period of the week (week-days and weekends).
• The selection of measurement sites should be based on a random sample of roads in a road network. This will allow generalisation of the results to all traffic in that particular road network.
• As a rule of thumb, 30 locations per surveyed road type should constitute an accurate sample if the locations are carefully chosen.
• In order to ensure reliability and comparability of speed data, the locations at which speed measurements are carried must be chosen carefully. All places where vehicles are likely to stop, accelerate or brake, should be avoided. Therefore, the locations should meet the following criteria:

• Ideally, the sampling procedure should comprise a selection from a database consisting of a list of uniform road segments, with their geographic coordinates and their characteristics such as:

• Usually, road network databases are already divided in segments of relatively small length (typically one segment per portion of road between two intersections). In this case, the sample of measuring locations should be selected via a simple random sampling with probabilities of inclusions that are proportional to the length of the segments (e.g. the likelihood of a segment of 2 km to be included in the sample is twice higher as the likelihood of a 1 km segment).
• If the initial database is not constituted of sufficiently small segments of roads, one should split the initial segments into smaller ones and repeat the operation described above. A simple random sample with unequal probabilities and without replacement is a common type of sampling that can be applied in common statistical software.
• Once the road segments have been sampled, it is necessary to conduct field work before the measurements take place. On each segment a profile must be found that corresponds to the suitability criteria listed in section 4.4.2.1.2, i.e. allows an unobtrusive measurement of speeds and does not compromise safety of the measuring device operators.

Recommendations

Before speed data can be used to support any policy decisions, they should be representative, reliable, valid and precise enough. Therefore, there should be clarity on how

this quality can be produced and maintained. It is recommended to consider all the separate steps in the process of production of speed data and to set up standards or recommendations for the execution of specific steps in this process. The SPI Manual provides several standards and recommendations.