A new traffic safety system at the Dartford crossing on the M25 is designed to automatically detect and identify vehicles that may pose a safety risk (e.g. because they are oversized or carrying dangerous loads), safely escort such vehicles through the tunnel, or re-direct them to a suitable route and to alert drivers in the event of an incident and stop all vehicles entering the tunnel.
PEL Services designed, supplied and commissioned a Bosch PA system, incorporating ambient noise sensing to automatically adjust sound levels to compensate for substantial variations in the levels of background traffic noise. The system has been set by PEL to maintain the sound level at 5dB above ambient levels, in real time, to ensure announcements are audible but not anti-socially loud. Peter Mapp, the Acoustic Consultant on the project, made dB and noise spectrum checks around the clock and in extremes of traffic and weather conditions in order to establish the dynamic range that would be faced by the system. He also devised the PA system concept and speaker layout.
PEL Services designed the system around a Bosch Praesideo digital PA system with master and slave network-controllers. Unlike traditional PA systems, Praesideo uses a network configuration rather than having all system elements connected to a central controller. By connecting the network ends together, a ‘redundant’ loop is created such that any break in the cable doesn’t affect operation – an important consideration in a safety system such as this.
Twelve Bosch LBC 3432-03 uni-directional projection speakers have been used with Praesideo at four locations; three speakers on each pole, at different heights to cater for the varying vehicle heights. The speakers have been configured by PEL in a contingent redundant arrangement, as an additional fail-safe. There are three amplifiers for each location including one on standby, and the entire system is connected to an uninterrupted power supply.
PEL’s Technical Director Vic Swain said: “In this project it is the designed intelligibility of the vital PA output, delivered in harsh windy conditions, directly to drivers of various vehicle-types, which makes the system work for drivers, while reducing congestion for many more passengers on the surrounding road network.”