ALISON Thewliss (Letters, February 14) is, I'm afraid, mistaken in her belief that removing the tolls on the Forth Road Bridge would allow traffic heading on to the bridge from Edinburgh to flow more freely and thus lessen congestion and pollution.

Each of the seven toll booths can process 520 vehicles per hour. The capacity of the bridge is 1800 vehicles per hour per lane. The throughput of the toll plaza (3640) is therefore slightly above the carrying capacity of the two-lane road on the bridge (3600) into which these seven toll booths feed their traffic.

Queues either way on to and off the bridge are a function of the volume of traffic and the road space available.

Southbound, the queues coming off the bridge will hopefully be considerably eased by the new M9/M8 link road currently under construction to replace the overloaded A8000.

Given, however, that only two lanes of traffic can be accommodated on the Forth Road Bridge itself in either direction, the tailbacks each way on to the bridge to which Ms Thewliss refers can only be shortened by actually reducing the numbers of vehicles presenting at the bridge at period periods - precisely the reason why Feta, with support in principle from the Scottish Executive, proposed variable tolls to encourage car-sharing and the use of public transport at these times.

Councillor Lawrence Marshall, Convener, Forth Estuary Transport Authority, South Queensferry, Edinburgh.

ALISON Thewliss highlights the tailbacks on the Forth Road Bridge each evening. Clearly the toll booths must contribute to congestion in so far as they cause vehicles to form queues and then stop and start as the toll is paid. But these booths and the tolls are not in themselves the only reason for congestion. In the morning there are reports of long tailbacks affecting those travelling in southerly direction, that is in the direction in which one does not pay a toll and where one is not obstructed by toll booths.

Congestion on the Forth Road Bridge will occur when the flow of cars reaching the bridge exceeds the flow across the bridge. If tolls are removed then those on the north side who are willing to park at Inverkeithing and travel on to Edinburgh by train might be tempted to cross the bridge.

The modest toll must have some moderating effect on the number of cars and other vehicles which cross the bridge. The removal of tolls will therefore increase the congestion at certain times in the day.

Sandy Gemmill, 10 Mertoun Place, Edinburgh