| October 2010
Wp Ref Website/TOPIC 9 JUCTION DELAYSTOPIC 9: Traffic management and its costs versus casualty savings
Over the past decade the traffic engineers have restricted capacity at the most critical points in the road network, namely the junctions. They have done that by a series of minor, almost insignificant, measures e.g.
- Arranging matters so that all the traffic lights show red long after green would be sensible at least somewhere.
- Setting stop lines back by two or three car lengths at signal controlled junctions. That reduces the number of vehicles that can exit when the traffic lights turn green so generating queues when none need exist.
- Channelisation schemes that allocate a particular lane to each turning movement. The result is congestion for the major movements while lanes for the minor movements stand empty. Click to see pictures
- Road markings and traffic islands that restrict the number of lanes at stop lines to the number on the approach roads thereby ensuring that perhaps only half the capacity of the intervening the links can be use.
- Banning turns; even left and straight ahead turns are not immune from that. The consequences are substantial detours and overloading at other junctions.
- The installations of thousands of signal controlled pedestrian crossings that show red long after a loan pedestrian may have crossed.
- Bus lanes that often carry as little as one vehicle every 10 minutes.
There has also been a progressive lowering of speed limits so extending journey times. For example, the DfT Circular 01/2006 advises local authorities to set see limits at the median speed rather than, as hitherto, at the 85th percentile. The effect of that would be to reduce all speed limits by 5 mph and to ensure that half the population, instead of 15%,.would be driving at a speed lower than they would otherwise choose.
The calculations in Appendix 1 provide indicative costs associated with those measures along with the cash values of the casualty savings associated with speed reductions. Here is the summary where the price base is 2008 and where the costs exclude the fuel and other operating costs imposed on vehicles held stationary at junctions or in congested conditions.
Delay: One minute per thousand vehicles/day at a junction costs £83,700 per year. Two minutes added to all vehicle trips would cost £12 billion annually.
Diversions:Adding 1 km to 1000 journeys per day, where the speed is 40 kph (25 mph), costs £189,000 per year. Adding 1 km at 40 kph to all vehicle trips would cost £13.5 bn annually. Since most journeys start or end in urban areas and since it is there that most diversions occur, 40 kph (25 mph) is reasonable.
Speed: Reducing the speed of 1,000 cars per day from 25 to 20 mph over 5 miles would cost £234,000 per year. The same speed reduction imposed on all cars and vans on urban roads would cost £12.0 bn per year and a 5 mph speed reduction on cars and vans on all roads would impose time costs of £16.5 bn.
Casualty savings: The cash value of the casualties saved by reducing speeds by 5 mph is only £3 billion, far below the £16.5 bn cost of the time delay.
Comment: The polices pursued these last 15 years have imposed delay costs on the nation that are far in excess of the value of any possible casualty saving. Since the values used by the DfT reflect the way people behave when faced with some small risk, the implication is that, rather than slowing us down, they should be speeding us up.
Worse still, the polices pursued these last 15 years appear to have sabotaged the long established downward trend in road deaths rather than accelerating that trend, see. http://www.transport-watch.co.uk/transport-speed-cameras.htm
Note on sources:
Calculations in Appendix 1 depend on:
- National data dealing with the value of time and vehicle operating costs as published in Unit 3.5 6 of the Department of Transport's Transport Appraisal Guidance, Web TAG at http://www.dft.gov.uk/webtag/documents/expert/unit3.5.6.php
- The 2008 edition of Transport Statistics Great Britain, the TSGB
- The 2006 National Travel Survey.
|
