This article describes the working of average speed enforcement systems in the UK and their beneficial effect on traffic flow in roadworks. Within a zone controlled by this system both speed and variability of speed were reduced, generating smoother traffic flow and more reliable vehicle throughput; and fewer people were killed or seriously injured. The enforcement system SPECS uses two linked video cameras which continuously capture images of passing vehicles. Automatic number plate recognition is used to calculate a vehicle's average speed through a set distance: if this is over a given limit, an offence record is noted. Where SPECS was first used, changes in driver behaviour were noticed very quickly. At a major roadworks site on the M1 motorway the smoother traffic flows in the SPECS zone created more reliable journey times between junctions. In other sites the estimated congestion delays were greatly reduced, as calculated against the effectiveness of spot speed checks. Spot checks create "bunching" behaviour as drivers slow down and speed up sharply.
Abstract