Variable speed limit enforcement-the M25 controlled motorway pilot scheme

In August 1995, the pilot was launched of a `Controlled Motorway´ on the southwest quadrant of the M25 (in the UK). The concept behind the pilot is that by controlling the speed of traffic, at times of heavy flow, flow breakdown can be delayed or prevented, resulting in a reduction in accidents, an increase in “driver comfort” and possibly an increase in throughput over the controlled section. The development of the MIDAS (Motorway Incident Detection and Automatic Signalling) system and its initial deployment on the M25 provided the basic infrastructure upon which an automatic speed control system could be built. The key feature which differentiates a `Controlled Motorway´ from a conventional motorway is the use of mandatory speed limits. To ensure the high levels of driver compliance with the speed limits, the pilot includes an enforcement system which automatically detects and records vehicles exceeding the speed limit. The enforcement system was required to operate on all lanes of a four lane motorway and to enforce speed limits which vary by time of day or according to traffic conditions. To ensure that the enforcement system was always aware of the current speed limit, the signals were designed to confirm to the enforcement system, via a high integrity interface, the speed limit displayed to drivers. The M25 pilot scheme also makes use of automatic camera enforcement, using technology similar to that already proven for speed and `red light running´ enforcement on all-purpose roads