Over the past 10 years we have introduced a large number of passively safe signs on the road network. TMS has analysed national single vehicle loss of control collisions over this period to look for trends in the data. Given the increase in the number of passively safe signs, one might reasonably expect to see a reduction in the number of collisions with signs, and also a reduction in severity.
As expected, injury collisions with signs reduced - by 29%, with KSI collisions down by 34%. Clearly, a lot of this reduction could be due to factors like improved safety of vehicles, so we used tree collisions as comparative data over the same time period. Collisions with trees went down by 23% overall, and KSIs with trees reduced by 31% over the same period. So there does appear to be a reduction over expected levels within the signs collisions.
We then looked at signs collisions as a proportion of all objects struck off carriageway. In 2002 signs represented 7.7% of all objects struck, this increased to 8.2% in 2011 (KSI up from 6.9% to 7.8%). One possible explanation for this is that if there are more unprotected signs out there, there are more signs to hit that there were before. But the slightly bigger increase in the KSI proportion is counter intuitive.
What really surprised us was a comparison of the 2011 severity index with ten years previous. The severity index is KSI/total collisions expressed as a percentage. If we just look at non built up roads and motorways (where most of the passively safe signs and high speed collisions are) then the severity index has remained the same over the 10 years at 23%. We would have expected this to have reduced, if more of the signs that were being hit were passively safe.
The national statistics are only a small part of the picture - we need to do in-depth studies to investigate this issue more. Collecting data from the HA Areas, combining collision stats, maintenance records and asset records could tell us more about the success - or otherwise - of passively safe devices.
Please refer to the Safety Studies section of our website for more details of our experience