The A9 between Perth and Inverness is a route of strategic importance. It carries a substantial amount of traffic between the two towns, including a high proportion of heavy goods vehicles (HGVs). However, it is mainly single carriageway, restricting HGVs in excess of 7.5 tonnes maximum laden weight to a maximum legal speed limit of 40mph, and overtaking is severely restricted. Average speed cameras are soon to be introduced along the route, and consideration is being given to raising the speed limit applicable to these HGVs to 50mph on the single carriageway sections.
AECOM investigated the effects of the installation of the speed cameras on traffic behaviour, and provided simulated speed distributions. Scenarios included the situation in which 40mph is retained as the speed limit applicable to the relevant HGVs following the installation of the cameras, and the situation in which this limit is increased to 50mph on the A9 only.
The numbers of injury accidents in these scenarios were modelled based on established relationships between average vehicle speeds and injury accident numbers. Consideration was given to the effects on safety of reducing overtaking and reducing the speed differentials between different vehicle types. However, robust evidence was not identified that would have enabled such effects to be modelled reliably.
This accident modelling indicated that the introduction of average speed cameras is likely to reduce the future number of injury accidents. The reduction in the number of accidents is likely to be greater if the speed limit for HGVs in excess of 7.5 tonnes maximum laden weight is retained at 40mph rather than increased to 50mph.

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