'Caterpillar on skates' could help save Forth road bridge

It has been described as a giant caterpillar on roller skates, but a design for a "road ferry" across the Forth road bridge near Edinburgh could provide a solution to the world's traffic problems.

Professor Stephen Salter has produced a blueprint for a fleet of 60m-long "road ferries" that would be loaded with vehicles before crossing the bridge. The craft, he says, would cut congestion, improve flow and minimise wear and tear on surfaces.

The "ferries", which run on soft tyres, were designed to reduce the effects of traffic weight on the 41-year-old bridge and are capable of carrying at least three times the current level of peak-time traffic. Vehicles would be transported at a faster rate while reducing the strain on the suspension bridge's cables.

Professor Salter, of Edinburgh University, made his name after inventing the first touch-screen computers in the late 1960s and has pioneered wave-power technology. His latest invention, he says, would cost a fraction of the £500m needed to build a second crossing next to the Forth bridge, which is deteriorating due to corrosion caused by a higher than expected volume of traffic.

His £80m plan would cause the load to be spread across the width of two carriageways. He said: "Drivers would approach just as they do now but, instead of going straight across, they would drive on to a platform which could hold up to 30 cars or eight heavy-goods lorries. Once enough vehicles were on board the platform would move."

By spreading the weight over a seven-metre wide carriageway mounted on 240 reduced-pressure tyres, the academic believes he could substantially reduce stresses on the structure.

"A fleet of 18 road ferries with one leaving every 30 seconds could equal the present maximum suggested capacity of 3,600 cars per hour. A fleet of 36 ferries, with one arriving every 15 seconds, could double it. If we use number-plate recognition technology to collect tolls, delays would be kept to a minimum," said the 67-year-old professor of engineering design.

Once the ferries reached the other side, the vehicles would drive off and the whole mechanism would be lifted over the central reservation on to the opposite carriageway, ready for a return journey.

"This is a straight-line device only, and it would not be able to make any tight turns or steep hill-climbs," said the professor. "But for long motorway sections where people aren't doing many turn-offs, road ferries could be an ideal way to improve traffic flow, reduce wear and tear, and increase safety." He said safety would be improved because road ferries would have better braking, longer separation distances and better maintenance than most vehicles now on the roads.

The bridge operators do not agree the idea would solve corrosion problems. "This proposal tries to better distribute loads on the bridge, but we remain unconvinced it will work in practical terms," said a spokesman for the operators, the Forth Estuary Transport Authority.