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Humps Have No Effect   


The following article was published in September 2006: 

Speed humps were installed in Manor Park Road, Chislehurst in the London Borough of Bromley in 1999. The traffic calming scheme for this road (and its continuation, Watts Lane) was the reason for the formation of local pressure group BBRAG in 1998 who opposed the installation of the speed humps because they said the cost of £40,000 was a waste of money and would have been better spent elsewhere. They also questioned whether the humps would prevent the kinds of accidents that took place in the road, which were relatively few anyway.  Well it is now seven years since the scheme was installed so what are the subsequent statistics for road accidents?  

Taking information from reports produced by Bromley Council staff back in 1998, and another similar report obtained recently, the figures are:  

For the three years prior to August 1998: 14 injuries. For the seven years prior to May 2006: 27 injuries. There were no fatalities in either period. In other words, there were 4.7 injuries per year before the installation of the humps, and 3.8 injuries per year after the installation of the humps.


Assuming that the reporting of injuries has been consistent (which is certainly questionable from other reports recently published), then there are two issues here:

1. Is this difference statistically significant against a control group (so that other changes in accident trends can be excluded)?

2. Is the difference likely to be accounted for by reduced traffic volumes due to traffic diversion?  


The 19% apparent reduction is only slightly better than the 14% reduction in road traffic accidents in the same period in the Borough of Bromley as a whole (and this was almost the last speed hump scheme installed in Bromley). The 5% difference is certainly not statistically significant.  

In practice that 5% improvement is almost certainly the result of traffic diversion as usually there would be more than 10% diversion of traffic as a result of speed humps (there is an easy diversionary route in this case). In conclusion therefore, this speed hump scheme has not cut accidents and it was a waste of money as BBRAG originally claimed.  

A full analysis of the data can be seen in the following pdf document: Speed-Humps-Have-No-Effect.  

Of course what is not mentioned in the report is the pain and suffering these humps have inflicted on residents over the last few years, the damage to vehicles, the extra road maintenance cost, the delays to emergency service vehicles and other adverse effects which we have repeatedly pointed out.


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