By Brian Allison, local government reporter

Computer traffic modelling carried out by Dundee City Council clearly shows that peak-time congestion in the city centre would be substantially reduced if the tolls on the Tay road bridge were abolished.

As shown by the graphics, which are screenshots from the computer model outlining the situation at the same time of day, the congestion apparent with the tolls operating almost disappears when they are removed.

That conflicts with modelling on which the Scottish Executive based its claim that removing the tolls would make congestion worse.

But council transport officials are adamant that the model used in Dundee is more accurate than the system used by the Executive.

The Paramics (parallel micro simulation) model used in Dundee shows the road network in the city centre and the traffic movements on it at various times of the day, with different colours used to represent the various types of vehicles such as buses, private cars, and goods vehicles.

Information obtained by carrying out surveys of motorists using the city centre—origin and destination, route, journey times, etc—was fed into the computer.

The model uses the information to show what will happen on the road network under a variety of conditions.

One of those scenarios involves the traffic situation at the evening peak time when thousands of vehicles are heading out of the city across the bridge.

Congestion on the roads leading to the bridge is apparent and significant when it is assumed that vehicles will be stationary at the tolls for four seconds.

When that is lengthened to a five-second halt the congestion increases considerably, to the extent that major roads become virtually gridlocked.

In reality the stopping time for vehicles at the tolls is usually between four and five seconds.

It is not only motorists using the bridge who are affected by the congestion but also those travelling across the city centre who are held up by the tailbacks from the bridge.

Running the model with the tolls removed shows a dramatic reduction in congestion, not only on the bridge approach roads but throughout the city centre.

That scenario was borne out in reality earlier this year when the tolls were closed because of industrial action—and congestion was almost non-existent.