Transport Watch UK Focusing on UK's Traffic & Traffic Systems

9, Traffic management and its costs versus casualty savings

Updated June 2016

This updates and replaces the previous note produced for 2009.

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..

  1. Arranging matters so that all the traffic lights show red long after green would be sensible at least somewhere.
  2. 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.
  3. 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.See pictures
  4. 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.
  5. Banning turns; even left and straight ahead turns are not immune from that. The consequences are substantial detours and overloading at other junctions.
  6. The installations of thousands of signal controlled pedestrian crossings that show red long after a loan pedestrian may have crossed.
  7. Bus lanes that often carry as little as one vehicle every 10 minutes.

There has also been a progressive lowering of speed limits, extending journey times.

The spread sheet enables the effects to be examined.  Here are some results, where the financial data is at the 2016 price base and where the traffic flows are those arising in 2015 and where the tacit assumption is that vehicles drive at the speed limit.


Lowering the speed limit to 60 mph would impose time plus vehicle operating losses of £1,455 million pa and achieve casualty savings worth £171 million, providing a net loss of roughly £1,280 million pa.  The losses in time and vehicle operating costs are over 8 times the casualty savings.

Increasing the speed limit to 80 mph for cars and vans alone would reduce time and operating costs by £662 million but increase casualty costs by £269 million, a net gain of £393 million.  The ratio of gains to losses is 2.5

Increasing the speed limit on goods vehicles, buses and coaches to 70 mph would increase time and operating costs by £454 million and casualty costs by £29 million – supposing the equalisation of speeds did not actually reduce accidents due to reducing overtaking.

Rural A:

Lowering the speed limit to 50 mph would impose time plus vehicle operating losses of £3,730 million whilst saving casualties worth 928 million yielding a net cost of roughly £2,800 million.  The loses are four times the value of causality savings

Increasing the speed limit of goods vehicles, buses and coaches to 60 mph, matching cars and vans, would provide time and operating cost savings worth £261 million and casualty costs of £159 million, providing a net saving of £102 million although reduced overtaking may bring casualty savings not captured here.  The saving in time and vehicle costs is 1.6 times the casualty savings.

Urban A-roads:

Lowering the speed limit on urban roads would not affect all speeds because often traffic conditions prevent vehicles driving at the speed limit.  The values in the text below, take no account of this reality, except where stated.

Lowering all speeds from 30 mph 20 mph would impose time plus vehicle operating costs £15.4 billion whilst saving casualties worth £1.15 billion a net loss of roughly £14 billion.  If the change affected only 20% of urban driving then these numbers should be divided by five. Providing a net loss of £2.8 billion,  The losses in time and vehicle costs are over 13 times the casualty saving.

Delay: A one minute delay per thousand vehicles per day at an urban junction costs £103,400 per year. Two minutes added to all vehicle trips by junctions costs £22.8 billion annually.

Diversions: Diverting 1,000 vehicles per day by one km imposes time costs plus operating costs plus casualty costs of £263,400 per year, if speeds are 20 mph, and £198,000 per year if speeds are 30 mph.

If every journey in the country were so diverted the annual costs would amount to £29 billion at 20 mph and £22 billion at 30 mph.


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.

Note for the spread sheet

Table 1a provide the values of time and the coefficients enabling vehicle operating costs to be calculated.  The price base is 2010 but the costs are those forecast for 2016.  Table 1b provides the GDP deflators which enable values in table 1a to be inflated to the 2016 price base. All these values are from the WEG TAG data book

Table 1c, opposite table 1a, provides the traffic composition for 2015.  Below that table is a more detailed table. It provided traffic composition data for 2010.  That table enables the Goods vehicles in table 1b to be split between the two classes, OGV1 and OGV2, see table 1d.  The source for all that data is Transport Statistics Great Britain.

Tables 2a, 3a and 4a have the same pattern.  As set up, the tables calculate the costs for driving at the speed limit and at 10 mph below that limit.  In all cases other speed changes can be tested by changing the values in the “speed change” rows.  Likewise, the effect of other traffic composition data can be rapidly tested.

Tables 3b, 3b and 4b provide the casualty saving or changes, consequential on the speed changes.  Those casualty changes are set to 5% the per 1 mph speed reduction, following  TRL reports 421 and 511.

Tables 2c, 3c and 4c sum the effects in the previous tables.

Tables 4d and 4e provide the cost of delay and diversion in urban areas.  They are self explanatory and contain the source references.


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