We make some very rough and ready calculations based on yesterday's casualty and road traffic stats from the DfT...

Whenever cyclist casualty statistics are published, the question always arises of whether cycling is becoming safer or more dangerous. Changes in the number of cyclists killed or seriously injured cannot answer that - the statistics need to be framed in the context of any changes in cycling patterns. As well as publishing the Reported Road Casualties Great Britain for 2011 yesterday, the Department for Transport also published Road Traffic Estimates for the same year, enabling us to perform some very rough calculations to try and get an idea of what is happening.

We have to stress that the calculations, while not quite back-of-a-fag-packet, are very rough. We're not statisticians. Moreover, there are potential pitfalls in the DfT data regarding billion vehicle miles for bicycles: first, they're rounded to the nearest 100 million. So 3.1 billion could be 3.149 billion, or it could be 3.050 billion - big difference; secondly, data for levels of cycling are by their nature notoriously difficult to capture accurately, perhaps more so than for any other form of transport. This methodology note from the DfT isn't specific to pedal cycles, but it explains how the data are collected.

Another thing that needs to be pointed out is that while analysing the data in this way might give a very rough idea of what may be happening at national level, there is no way that it can represent what is happening at local level because there are too many variables in play - levels of cycling can vary wildly between different towns and cities even within the same county, or, in London, between neighbouring boroughs; traffic patterns, and drivers' attitudes towards or awareness of cyclists, can also differ, as can emergency response times and access to a hospital with a major trauma unit, for instance.

Following on from those comments about differences in levels of cycling by location is the fact that with the data being at national level, we simply cannot make any assumptions about the supposed 'Safety in Numbers' effect that increased numbers of cyclists are said to bring about. Clearly, some towns and cities have seen huge increases in cycling in recent years, others haven't. Any increases in cycling suggested by the road traffic estimates aren't going to be happening at a uniform rate across Great Britain, nor are any increases or decreases in casualty numbers.  

So with those caveats, here is our very rough overview of how the casualty figures and road traffic estimates anounced today relate to each other as far as bicycles are concerned.

Cyclists reported killed

                              bn miles              Rate per bn miles
                               cycled      Killed       cycled

2005-09 average                  2.8         130          46.4
2007                             2.6         136          52.3
2008                             2.8         115          41.1
2009                             3.0         104          34.7
2010                             3.0         111          37.0
2011                             3.1         107          34.5

2011 % change on 2005-09 avge   10.7       -17.7         -25.7
2011 % change on 2010            3.3        -3.6          -6.7

On this measure, cycling certainly looks like it is getting safer; the number of cyclists killed per billion miles cycled last year is a quarter down on the 2005-09 average, and there has also been a near 7 per cent drop on 2010.

Cyclists reported seriously injured  
                              bn miles   Seriously  Rate per bn miles
                               cycled     injured       cycled

2005-09 average                  2.8       2,398         856.4
2007                             2.6       2,428         933.8
2008                             2.8       2,450         875.0
2009                             3.0       2,606         868.7
2010                             3.0       2,606         868.7
2011                             3.1       3,085         995.2
2011 % change on 2005-09 avge   10.7        28.6          16.2
2011 % change on 2010            3.3        18.4          14.6  

Data here depict a very different picture. The increase in billions of kilometres ridden means that the percentage increases after taking that into account aren't as steep as they are based purely on the casualty numbers - but we are still looking at double-digit increases both from the 2004-09 average and, more worryingly still, from 2010 to last year. Also, while there can be swings from year to year in deaths due to the relatively low numbers involved, that's less likely in this instance, given the thousands of recorded incidents.

Cyclists reported killed or seriously injured  

                             bn miles               Rate per bn miles
                              cycled        KSI         cycled

2005-09 average                 2.8        2,528         902.9
2007                            2.6        2,564         986.2
2008                            2.8        2,565         916.1
2009                            3.0        2,710         903.3
2010                            3.0        2,717         905.7
2011                            3.1        3,192       1,029.7

2011 % change on 2005-09 avge  10.7         26.3          14.0
2011 % change on 2010           3.3         17.5          13.7

The average from 2005-09 was five cyclists killed for every 100 killed or seriously injured (KSI); in 2011, it is a little over three, which represents progress. But while the figures do suggest that a serious incident is slightly less likely now to result in the death of the cyclist, the fact remains that such incidents appear to be increasing at an alarming rate, up 14 per cent over the 2004-09 average, ad 13.7 per cent against 2010 once billions of kilometres cycled are factored in.

Cyclists reported slightly injured  

                             bn miles    Slightly   Rate per bn miles
                              cycled      injured       cycled

2005-09 average                 2.8       13,934       4,976.4
2007                            2.6       13,631       5,242.7
2008                            2.8       13,732       4,904.3
2009                            3.0       14,354       4,784.7
2010                            3.0       14,414       4,804.7
2011                            3.1       16,023       5,168.7

2011 % change on 2005-09 avge  10.7         15.0           3.9
2011 % change on 2010 3.3                   11.2           7.6

As with the fatality and serious injury data, only injuries that are actually reported to the police are recorded in the statistics (the definition is here) so if for instance you come off your bike, suffer some road rash and perhaps a sprain but visit your GP following an incident in which the police weren't involved, and it's unlikely to be recorded. It doesn't seem too much of a stretch to say that many more incidents involving cyclists suffering slight injury go unreported compared to those with car occupants, not least because of legal requirements to report incidents involving motor vehicles.

All cyclist casualties

                            bn miles                 Rate per bn miles
                             cycled         All         cycled
2005-09 average                2.8        16,463       5,879.6
2007                           2.6        16,195       6,228.8
2008                           2.8        16,297       5,820.4
2009                           3.0        17,064       5,688.0
2010                           3.0        17,185       5,728.3
2011                           3.1        19,215       6,198.4

2011 % change on 2005-09 avge 10.7          16.7           5.4
2011 % change on 2010          3.3          11.8           8.2

So here's the final set of figures, based on all cyclist casulaties - deaths, serious injuries, and slight injuries. The CTC claims that the rise this year is due to the current government failing to make road safety a sufficiently high priority. Granted our calculations are very rough and ready, but a more than 8 per cent rise in the rate of casualties per billion kilometres cycled between 2010, the year the coalition was formed, and 2011 suggests they have a point.

Born in Scotland, Simon moved to London aged seven and now lives in the Oxfordshire Cotswolds with his miniature schnauzer, Elodie. He fell in love with cycling one Saturday morning in 1994 while living in Italy when Milan-San Remo went past his front door. A daily cycle commuter in London back before riding to work started to boom, he's been news editor at road.cc since 2009. Handily for work, he speaks French and Italian. He doesn't get to ride his Colnago as often as he'd like, and freely admits he's much more adept at cooking than fettling with bikes.