Case Study 2

Early estimates of 2 million vehicles a year proved to be inaccurate and it became clear that a second tunnel was needed. The second tunnel took eight years to build and cost Ł45m. It opened in 1980. In 1986, the M25 was completed, which caused another major increase in traffic. Forecasts predicted a throughput of 40m vehicles per year and it became clear that even greater capacity was needed.

Dartford River Crossing Limited (a private company) successfully bid to build a new bridge and to open and operate the new crossing. The new bridge was opened by the Queen in 1991 after costing Ł86m to build.

The company now operates both the bridge and the tunnels. The company is proud of the fact that the new bridge was built on time and within budget. It believes that private sector involvement shortened the overall time of construction.

Under the terms of its agreement with the government, the company collects tolls as a means of recovering the costs of constructing the bridge and the costs of operating the crossing. Toll charges are strictly controlled by the government to ensure that they do not exceed the rate of inflation. The company estimates that, within 20 years of the concession period starting, all costs will have been recovered and all three Dartford crossings will be handed back to the government, debt-free.

The crossing has to be open 24 hours a day, 365 days a year. The Traffic Control Operations Centre overlooks the toll plaza and monitors a sophisticated array of equipment, geared to ensure the efficient and safe flow of traffic through the tunnels and across the bridge.

Colour TV cameras positioned along the roadway are linked to 37 monitors in the control centre, giving the duty traffic controller an instant and continuous view of traffic conditions in the tunnel, on the bridge and on both approach roads. Cameras are shared with Kent and Essex police and there are direct phone links with the fire and ambulance services and with the police control centres around the M25.

Staff manning Land Rovers and maintenance vehicles, and others on foot, patrol the crossing. They are linked by an internal telephone system. The company aim to ensure that any incident is dealt with safely and efficiently.

Pylons

84 metre high steel pylons are located above 53 metre high concrete piers giving the bridge a total height of 137 metres. From these the road deck is suspended by cables.

Cables

1,500 tonnes of galvanised steel were used, comprising 112 separate cables. At the time of construction, the designed loading was the heaviest ever specified for a bridge anywhere in the world.

Tunnel Cleaning

The tunnels undergo a regular programme of water jet spray cleaning and have to be washed weekly during the winter months.

Snow Clearance

Dartford River Crossing has contingencies for keeping the traffic flowing in all weather conditions including heavy snow and ice.

Patrol Vehicles

Each patrol vehicle travels an average of 40,000 miles per year within the concession area.

Main Span

The main span of the bridge is 450 meters, with the road deck 65 meters above the river providing at least 57.5 meters clearance over the navigational channel.

Concrete

145,000 cubic meters of reinforced concrete were used in the bridge substructure and approach viaducts.

Air Change

Four ventilation buildings are situated directly above the tunnels. Axial flow fans replace polluted air which is extracted through large chimneys.

Despite the three crossings, there are still delays because the increase in capacity has caused an increase in traffic wanting to use the crossing. Dartford River Crossing has to accommodate 40 million vehicles per year. Therefore, the company has invested Ł2.5m in a new toll collection system to make throughput of traffic quicker.

The DART-Tag is Europe's most advanced toll system. Certain lanes approaching the crossing are reserved exclusively for DART-Tag users. Motorists have to pay in advance (e.g. by direct debit) for their tolls but the DART-Tag itself is free. DART-Tag improves the flow of traffic because the vehicles using the tag pass through the toll booths faster than those who have to stop and pay. The system is known as AVI (Automatic Vehicle Identification).

DART-Tag is a microwave transponder tag about the same size as a tax disc. It is interrogated by an antenna each time the vehicle approaches. If the driver's account is in credit, the barrier opens automatically when the driver approaches. Traffic lights at the barrier tell the motorist how many credits he has in his account and the motorist can determine when warning lights should appear (e.g. when he has 10 credits left). A green light means the account is healthy, amber means it is running low, red means the barrier will not open.

The DART-Tag saves the company money (by reducing the number of staff needed) and it improves the flow of traffic, so the company wants to encourage people to use them. It has removed the facility whereby drivers could buy pre-paid books of tickets and it has given a 7.5% discount for DART-Tag users. DART-Tag users may use any lane and some lanes are reserved only for drivers with DART-Tags. 20% of vehicles passing through the crossing now use the DART-Tag.

Facilities still have to be available for non-DART-Tag users. Automatic Cash Machines (ACMs) allow money to be thrown in a basket. When the required amount has been thrown in, the barrier will open. Electromagnetic sensors determine the size and weight of the coins. Foreign or rogue coins are identified and removed (to prevent the machine jamming). The ACM keeps records of each vehicle passing through the toll booth and these are fed to a central computer.

MTTs (Manual Toll Terminals) have reduced in number. These booths are manually operated and an attendant will provide customers with change and keep records of vehicles which pass through and also of transactions made (for auditing purposes at the end of a shift).

There are 14 lanes on the North Plaza and 13 lanes on the South Plaza. On each plaza, there are automatic lanes, manual lanes and manual/automatic lanes. DART-Tag users can use any lane. One lane in each direction is reserved solely for DART-Tags.

3. What are the advantages of the IT systems over the manual systems they replaced?