Is speed reduction claim reliable?
IN response to the public annoyance over the speed cushions in Hollycroft, Leicestershire County Council has claimed a reduction in average speeds from 34 mph to 27 mph as justification for the money it has spent (HT report Feb 15, 2017).
Whether this is adequate or not is open to debate; what is more important however is whether or not this comparison is valid and even if it is, does it actually tell us anything? No it is not and no it doesn’t.
1) The pre-installation data was collected over a reasonable period, but it was over a year before work started and nothing was done to check it was still representative. The “post” data by contrast was collected over just a two hour period one Friday morning covering the rush-hour when traffic is heaviest and therefore slowest. It is ridiculous therefore to suggest the comparison is anywhere near like-for-like. More reliable and representative are the results of MVAS (Movable Vehicle Activated Signs) data collected by the local Neighbourhood Watch team soon after the installation covering over 39,000 movements. It showed: a) 40% did not comply with the speed limit; b) speeds consistently over 50 mph and as high as 81 mph.
2) Average speeds are meaningless. In a 30 mph limit, 100 vehicles at 40 -plus 100 vehicles at 20 average to 30, suggesting all is well. The reality is that 50% break the law by a significant margin. The only relevant measure is the number/proportion that exceeds the legal limit, the target for which HAS to be zero. LCC made no attempt to look at this data and worse still discounted it (see below).
3) The people who collected the data are allowed to ignore readings they feel are irrelevant, but there is no standard definition of what this means so there is plenty of opportunity for bias and inconsistency.
4) Hard to believe but after all this THE FASTEST 15% of speed readings are removed. The averages therefore cover only the slowest 85%. LCC blames this on an edict from the DfT, but it has completely misunderstood the guidelines DfT provides. Common sense alone would dictate that you don’t kick out the data you are most interested in.
Behind these false statistics there may well be a reduction in average speeds, but this is very easy to explain. In general terms, conscientious drivers who have always driven within the speed limit now drive a bit slower.
This is evidenced by the amount of data recorded at 20 mph and below – this NEVER occurred precushions unless there was congestion. The more “savvy” drivers who have always driven above the limit realise that if you straddle the cushions evenly you can continue to do so. The impact of the cushions has therefore been the exact opposite of what was intended.
There is also a claim of no injuries since the calming was installed – great news! But given this distortion over speeds who can expect us to believe these numbers haven’t been “massaged” too? For example, what about the accident reported in the HT that took place on July 1, 2015? What is most disingenuous is that on the one hand LCC says it supports the Government campaign on the importance of sticking to 30 mph limits, but then implies that modest excesses are acceptable – not once does it refer to legal speeds, only “sensible” speeds. John Newman