Highways Agency - Safe roads, Reliable journeys, Informed travellers

The Dartford - Thurrock River Crossing

Background - Maintaining the Service

The Dartford River Crossing is open 24 hours a day, 365 days a year and has operated continuously since 1963.

Although the nature of the work undertaken by staff remains essentially the same as when the first tunnel opened, annual throughput of vehicles has risen dramatically, from 5 million in the first year to approximately 55 million.

Colour television cameras positioned along the route are linked to 54 monitors in the Operations Centre. These provide the duty Traffic Controller with an instant and continuous view of traffic conditions on both approach roads, in the tunnels and on the bridge. Some of the cameras are shared with both Kent and Essex Police Forces. Also, direct-line phones link the Crossing with the emergency services in both counties, and contact is maintained with Police Control Centres around the M25.

A radio system allows communication with staff manning patrol Land Rovers or breakdown and maintenance vehicles and those on foot. The internal telephone system also allows communication throughout the Crossing and includes emergency phones at regular intervals in the tunnels and on the bridge.

This communications network enables a rapid response to any incident and ensures that maximum standards of safety are maintained at all times.

Monitoring systems measure vehicle exhaust emissions within the enclosed environment of the tunnels to ensure the correct level of ventilation is maintained. On the bridge, sensors buried within the road deck measure surface and air temperatures; anemometers at mid-span and on the pylons measure wind speed and direction. The computer systems feed this information to the Operations Centre and then by telephone links to the London Weather Centre. By return, forecasts for ice, and high winds are displayed on the screens in the Operations Centre allowing the Controller to respond to situations in a timely and informed manner.

The traffic management system for the Crossing is one of the most complex and concentrated in Europe. Based on the National Motorway Control System, substantially enhanced, powerful computers drive an array of matrix and variable message signs located on a series of gantries across the approaches to both the tunnels and the bridge. These signs allow speed limits and fog warnings to be displayed, lanes on the bridge or in the tunnels to be closed for operational or maintenance reasons and traffic stopped in one or both directions in an emergency.

In high wind situations the system allows a progressive series of measures to be applied to restrict the speed and position of vehicles on the bridge, and at very high wind speeds to close the bridge and send all the traffic through the tunnels.

All the control systems for the Crossing and all the engineering functions of both bridge and tunnels ultimately depend upon the security of the electricity supplies needed to operate them. High voltage supplies are fed to each side of the Crossing and are backed up with standby generators, which will automatically start up within 20 seconds of a failure of the public supply. Essential equipment and control systems are further protected by a battery backed uninterruptable power supply system.

At the entrances and exits to the tunnels, variable photocell operated transition lighting ensures that drivers make the change from daylight to artificial light and vice versa, safely. Within the tunnels, light fittings are fed alternately from the Kent and Essex electricity supplies. This is arranged so that if either fails the tunnels will still retain half their lighting and remain evenly lit along their lengths.

Lighting on the bridge had to be designed with aircraft and shipping in mind, as well as road traffic. Street lighting continuously illuminates the road deck at night. In addition aircraft warning lights are located on the pylons and shipping navigation lights mark the channel, to avoid the possibility of collision with the structure.

The aerodynamics and ventilation of a major sub-estuarial road tunnel is critical both to ensuring user comfort during peak traffic times or in an emergency situation following a fire within the tunnel.

Fresh air is generated by axial flow fans situated in buildings at ground level on both sides of the river, and supplied to the under road ducts which extend between the two ventilation shafts. From this ducting, the fresh air enters the traffic space through adjustable grilles situated just above road deck level, with each fresh air fan supplying half the tunnel. Exhaust fans are also situated in the same buildings, above the crown of the tunnel.

If a fire did take place, jet fans located in the tunnel roof provide a high longitudinal forward airflow to prevent smoke and fumes spreading back to affect users behind the incident.

Pumping systems ensure that any water seeping into the tunnels and rainwater running off the bridge approach viaducts is diverted into the River Thames.