Using Speed Models Appropriately

presented by Bill Frith and Fergus Tate, WSP

Thursday 18 September 2025

According to Ministry of Transport statistics excessive speed is a prime contributor to road trauma. In 2022 there were 104 fatal crashes, 459 serious injury crashes, and 1,405 minor injury crashes where travelling too fast for the conditions was a contributing factor. In these crashes, there were 114 deaths, 572 serious injuries, and 2,043 minor injuries.
There is a need to be able to assess the crash impact of speed changes. These changes may be upward or downward and may result from road safety measures, other unrelated factors, or a combination of both. In many cases such analysis requires models that link speed changes to crash changes (speed-crash models).
Speed crash models are well established in the literature but there is little in the way of simple guidance for practitioners on which models are appropriate to use in what contexts. For instance, different models may apply to different road types. Different models may also apply when the speed distribution changes in different ways. The speed distribution may be truncated, as in the case of a successful deployment of mandatory intelligent speed adaptation. In the case of successful Police enforcement, the distribution may be subject to an overall downward move and some truncation accompanied by a reduction in the width of the distribution. These impacts may, of course happen in the reverse direction where speeds are allowed to increase. When choosing a model in a particular situation the principles on which the model is based are important.
This presentation is intended to provide practitioners with some simple guidance on the type of speed crash model they might use when assessing the crash impact of a speed change, both before and after the event.
 

Bill Frith, WSP Research, has managed many large-scale research projects and reviews relating to road safety, road transport and policy development. He has written and spoken extensively as an advocate and technical expert on road safety and other areas of land transport. Bill has been involved in speed management over much of his career.

Dr Fergus Tate, WSP, is a Chartered Professional Engineer with over 40 years’ experience ranging from highway construction and maintenance management, professional services consultancy, academic and applied research projects, to policy development and performance monitoring. A skilled safety professional Fergus has an extensive understanding of safety issues, crash and conflict data and safety auditing including the safe system assessment framework and its variants.
     

Link to cloud recording Passcode: 60Ft8A#s

Link to research paper here

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