Wednesday, April 29, 2009

20's plenty

A follow on from my entry about 20mph statistics came in the form of an email from Jamie Shaw informing me as to where the figures came from. Sadly as I had to point out as another comment, this just refers me to more people quoting the results of data without presenting the data itself.

To illustrate why this is important consider one way the data could have been arrived at.

I could examine all pedestrian-vehicle incidents and filter out all those where excessive speed was a contributing factor, then take the subset of fatalities and create a report showing the ratio according to the speed limit at the point the incident occurred. Would this be enough? No.

Firstly you are assuming that anyone not travelling at 'excessive speed' will be travelling at the speed limit.

Secondly excessive speed includes that which is below the speed limit, but not suitable for the conditions.

Thirdly you're not taking into consideration the make and model of the vehicle - consider being hit at 20mph by a scooter compared to a articulated lorry.

Fourthly even though percentages are 'neutral' what's the proportion of both length of road and traffic capacity between 20 and 40 zones, the number of incidents, and as per the third point the type of traffic it carries.

Using ½mv² an average car weighs 1500kg, a heavy-goods vehicle is anything over 3500kg. So at 20mph (8.9408m/s) we get 59,953 joules and 139,891 joules. At 40mph (17.8816m/s) we get 239,813 joules and 559,565 joules - ouch.

Oddly enough if 20mph zones are highly populated with cars and 40mph with both cars and heavy-goods vehicles I would expect more fatalities in the 40 zones.

Fifthly if I use the figures in this PDF regarding terminal velocity that states that a fall with 5,725 joules is "much more than enough to kill a person" then by rights every collision above should be fatal except "it depends on how he or she lands", which of course is another factor that doesn't get shown in my report.

So if in my hypothetical study I quoted figures I could say that in an collision you have a 2.5% chance of dying while in a 20mph zone, but a 90% chance in a 40mph.

Would my study prove that travelling at 20mph is better than 40mph, I hope you're saying no because all this would show is that being in a 20mph zone is safer than a 40mph zone and that's it.

For anyone asking "Well so what if it's not a direct link it still shows 20 is better than 40" the point is that without knowing the cause simply slapping a 20mph on a 40mph won't necessarily do anything in the same way that giving wasps big pretty wings won't turn them into butterflies; they're still going to sting you.


Dan H said...

Have you tried looking at the statistics for places where a default county-wide 20mph has been applied? You could compare the accident trend since the change with the trend for the rest of the country at the same time.

Mind you, I tried getting accident data for Cambridgeshire a few weeks ago and it was not that easy, so don't try it until you have some days free, e.g. on a bank holiday weekend :-)

FlipC said...

Currently only figures for 2007 are available it seems it seems there's a lag - odd as you'd think everything would be computerised by now.

Anyway Cambridgeshire had 18 fatal pedestrian casualties in 2007. Of the 288 pedestrian casualties 59 occurred in a ≤ 40 mph zone within Cambridge itself, but it's not split further.

Will be interesting to see if that figure changes significantly.

Dan H said...

Cambridge doesn't yet have a default-20 policy: the recent policy change I mentioned the other week was to allow 20 zones in residential areas within the city. The towns with 20mph policies are listed on the 20s Plenty website.

As for computerisation, our police have lately been using "it would take to long for someone to go through all the records" as an excuse for not answering FoI queries, under the get-out clause that queries too expensive to answer don't count. I guess they'd like to keep it that way, so they don't want to add a search facility to their computerised record-keeping.

FlipC said...

Oh I thought they were planning a county-wide 20mph, I did only skim the news though.

Heh isn't amazing how 'government' computer systems seem so backwards at times. A bit like the revelation that the FBI database only allowed a search for a single term at a time.