SHM: A New Measure of Traffic Safety

The approaches currently in use for measuring and assessing the safe operation of vehicles in traffic lack vital characteristics that regulators and the public will require.

February 22, 2023

Measuring and assessing the safe operation of vehicles in traffic is essential to the deployment of automated mobility, as well as improving traffic safety for human-operated vehicles. The approaches currently in use lack vital characteristics that regulators and the public will require.

Measurements of traffic safety and risk go back at least a half-century, to John Hayward’s introduction of the key Time-to-Collision (TTC) metric in 1972. Since then, additional traffic safety measures have been proposed and implemented, including leading and trailing indicators. However, none give a satisfactory, complete answer to the crucial question the industry wants and needs to answer: How safe is safe enough? 

We at Streetscope have introduced a novel, leading measure of collision hazards in traffic (SHMTM , for Streetscope Collision Hazard Measure) that overcomes the problems and inadequacies of previous measurements. SHM is quantitative, objective, continuous, and general.

Trailing (or lagging) traffic safety indicators are important benchmarks, but they pose a problem when assessing the risk and safety of new systems, either roadways, traffic controls, or vehicles. They require the accumulation of collision statistics; to put it bluntly, they need collisions to occur. Traffic collisions incur property damage, injuries, and death, and the ethical problems of inflicting pain and loss on society (in determining the safety of an AV system) are unacceptable.

When assessing new systems, particularly automated vehicles, we must instead use leading indicators of traffic risk and safety. Some leading road safety indicators already exist, but unfortunately they, too, suffer from limitations and shortcomings.

A new general measure is needed. 

Notably, a general quantitative measure of traffic safety and risk must treat each traffic object (vehicle, pedestrian, bicyclist, etc.) as a “black box.” What does this mean? Only the external behavior of the object can be used in computing the measure. Incidentally, this is identical to the conditions of an on-road driving test, where the evaluator simply observes the actions (behaviors) of the vehicle as it is being driven and is not engaged in dialogue with the driver about what he or she sees, nor what considerations the driver is making to control actions. 

SHM, our Streetscope Hazard Safety Measure, is such a “black box” observational measure. It provides an objective, quantifiable, repeatable and, importantly, independent way to assess both driver (automated or not) and environment. Only when such a system is in place can we say that safe is, indeed, safe enough.


Photo by William Warby on Unsplash

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