Sir, - Conor Sexton (April 28th) cites the "unsynchronised" sequence of traffic signals he encountered on his journey on Easter Sunday afternoon from Christ Church Cathedral to O'Connell Bridge as an example of traffic infrastructure in the city not working. Too many motorists who encounter more than one red signal assume, like Mr Sexton, that the signals are incorrectly calibrated. However, this is frequently not the case.
The signals on the route from Christ Church Cathedral to O'Connell Bridge are controlled by the Corporation's computer-based, adaptive, urban traffic control system (SCATS). This system operates in real-time, adjusting signal timings, phasings, cycle lengths and signal co-ordination in response to variations in traffic demand and system capacity. Detectors in the street at each major junction collect the necessary information on traffic volumes.
The Christ Church to O'Connell Bridge route has 10 signal sets covering junctions and pedestrian crossings. It is one of the most important routes in the city because of the volumes of general traffic and buses it carries and the high concentration of pedestrians in the area.
In calibrating traffic signals Dublin Corporation attempts to strike a balance between competing traffic demands while ensuring appropriate provision for pedestrians and buses. The Corporation also strives to keep cycle lengths as short as possible in off-peak periods when traffic volumes are generally low. (A cycle length is the time between the start of two successive green phases on the same approach.)
We believe that the advantages offered by low cycle lengths, in terms of shorter waiting times, more than offset the disadvantages associated with the increased probability of traffic being stopped and the reduction in overall network efficiency. There are also road safety benefits in that motorists are less inclined to accelerate through an amber phase if they know that the red phase will be relatively short.
In order to improve the lot of pedestrians the "green man" time at all pedestrian crossings in the city centre has recently been increased from six to 12 seconds. The doubling of pedestrian green times, combined with reduced cycle lengths, has resulted in a very significant reductions in average pedestrian waiting times especially at off-peak periods.
The net impact of all these factors is that traffic travelling from Christ Church to O'Connell Bridge, during off-peak periods, experiences more stops than previously, but the average delay at each stop is usually quite short.
The signals on the route were operating correctly on the Sunday in question. Traffic volumes were relatively low and the average cycle length was 60 seconds. I am satisfied that the calibration of the traffic signals ensured a reasonable allocation of the available capacity between the various competing demands. However, I understand that the north side of O'Connell Street was partially closed to traffic between 12.05 and 12.38 and again between 13.50 and 14.30 and this caused traffic congestion which may have affected Mr Sexton's journey.
The Corporation is very conscious of the critical role of signalling in overall traffic management in the city and we are currently undertaking a major programme of investment in this area. Motorists will, however, have to get used to the fact that traffic signalling policy is no longer exclusively focused on maximising vehicular flows on main routes without due regard to other traffic demands and the needs of pedestrians and buses. - Yours, etc.,
Owen P. Keegan,
Director of Traffic, Dublin Corporation, Civic Offices, Dublin 8.