Speed reductions – costs and benefits
For example, the current advice (DfT Circular 01/2006), is that speed limits should be set at the average speed rather than, as previously, at the speed below which 85% of motorists travel (the 85th percentile). That implies (a) a universal speed reduction in the range 5 mph to 10 mph and (b) 50%, rather than 15%, of us will be travelling more slowly than we would otherwise choose.
A question not addressed by the authorities is the delay cost due to the lower speeds and the corresponding value of the accident reductions. It is that that we address here.
The value of time for the average vehicle is circa £12.80 per hour at 2009 prices. Hence reducingthe speed of 1,000 vehicles per day from 25 to 20 mph over 5 miles would cost £234,000 per year. The same for all cars and vans on urban roads would cost £12.6 billion and a 5 mph speed reduction on cars and vans on all roads would cost £17.1 billion annually.
Transport Research Laboratory notes 421 and 511 suggest a range of accident savings that are often summarised as a 5% reduction in casualties per 1 mph reduction in average speed. If that is applied to the 230,000 casualties in 2008, along with the average casualty cost of £53,000 then the saving following a 5 mph speed reduction would amount to £3 billion, far below the £17 billion delay costs implied by the speed reduction.
Since the values of time and casualty are supposed to reflect the way humans react when faced with the real world these numbers suggest that, rather than slowing traffic down, the policy should be to speed it up.
Speed cameras are credited with saving 100 lives per year. However, this claim ignores effects such as regression to the mean in the context of the practice whereby a camera is installed only if there have been four killed or seriously injured casualties (KSI) in three years within 500 metres of the site.
Instead of the long established nation-wide downward trend in deaths per vehicle-km of 7.1% per year accelerating under the impact of the cameras, that trend flattened off to 2.5% after 1995 as illustrated in Figure 1. That happened despite the cameras being supported by tens, if not hundreds of thousands, of speed humps and the endless traffic management schemes that impose huge economic costs upon the nation.
Had the previous trend continued there would have been 10,000 fewer deaths than actually occurred. Indeed, compared with the pre-1995 trend, there were 370 extra deaths for every doubling of fines. The correlation is remarkable (see Figure 2). Of course there is no obvious causal link but, had the matter been the reverse of the facts, doubtless the cameras would have been given the credit.
The June 2007 value for a fatality is £1.64 million. If that is increased by 10%, to allow for lesser casualties, the 10,000 extra deaths imply a casualty cost, laid at the door of present policies, of £18 billion for the period. Additionally, in excess of 13 million motorists were fined, most of whom were driving as well as could reasonably be expected. If those fines averaged £70 the amount taken was nearly £1 billion, let alone the cost of increased insurance premiums and the losses suffered by those who were banned. A reasonable conclusion is that the road safety policies pursued over the last 15 years have been a financial and human disaster.