Chart of the day: Unmet demand for cycling in Southwark

Over the last five years, Southwark council has carried out surveys into the travel habits and preferences of pupils in 89 of its 111 schools. The survey records the mode of transport the pupils currently use to get to school, and asks them what their preferred mode would be. The chart below shows the results for bus, walking and cycling, with current rate on the left and preferred rate on the right.

Around 20% of pupils would like to cycle to school, but only 3% actually do. 

Source: Southwark draft transport plan

Urban myths and the misuse of urban data

Following up my post about the mismeasurement of urbanisation in Egypt, I would highly recommend anyone interested in this area to read David Satterthwaite's article on 'Urban myths and the mis-use of urban data' (pdf).

Satterthwaite's article covers a lot of ground, including alarmism over 'out of control' urbanisation in Africa, the extreme paucity of census data in some areas, and problems in measuring city size and therefore various indicators of city performance. But I'm going to focus on his discussion of how the comparability of the UN's urbanisation statistics is undermined by the use of different definitions of urbanisation at country level. For example:

China’s level of urbanization in 1999 could have been 24 per cent, 31 per cent or 73 per cent, depending on which of three official definitions of urban populations was used. India appears to be a predominantly rural nation, but most of India’s rural population lives in settlements with between 500 and 5,000 inhabitants, which are considered as villages and therefore classified as rural; many more live in settlements with more than 5,000 inhabitants, which are still classified as rural. If these were classified as ‘urban’ (as they would be by some national urban definitions), India would suddenly have a predominantly urban population. 

And then there's Egypt:

in 1996, 18 per cent of Egypt’s population lived in settlements with between 10,000 and 20,000 inhabitants that had many urban characteristics, including significant non-agricultural economies and occupational structures. These  were not classified as urban areas – although they would have been in most other nations. If they were considered urban, this would mean that Egypt was much more urbanized, causing major changes to urban growth rates.

Remember that Egypt's central government had an incentive to systematically under-estimate its urban population, as granting city status to an area meant allocating it more funding and representation in parliament.  

The lesson here is that people should be careful about using data on cross-country comparisons, they should be extra careful when it comes to data on topics like urbanisation where there is no standard definition, and they should probably be extra extra careful about data that is produced by governments like Egypt's. After all, if your argument is that the Egyptian government is dysfunctional, then shouldn't you be at least slightly sceptical about the data that government produces?

Game theory and the risks of cycling vs driving

I've been reading Richard Tay's short article from 2002 on "The Prisoner's Dilemma and Vehicle Safety". Tay uses game theory to examine the implications of different fatality rates for drivers of small cars and four-wheel drives (4WD) in Australia. He finds that even though smaller cars are safer in that a collision between two small cars is less likely to result in fatality than one between two 4WDs, drivers will tend to prefer 4WD because in a collision between a small car and a 4WD the driver of the small car has a much greater risk of death. In the language of game theory it is a classic prisoners' dilemma, set out in the box below (the relative risk rates are taken from Australian data).

I thought I'd try and repeat the exercise for the relative risks of cycling and driving in the UK. I got data on the number of miles travelled and the number deaths or serious injuries (KSI, standing for 'killed or seriously injured') from accidents involving bikes and/or cars from the Department for Transport's 2009 road casualties report (tables 1a and 23c respectively), summarised below.

Using the KSI rates we can produce a new version of Tay's table as follows:

There is a similar outcome to the one Tay found, but the relative risks are much more extreme. In a collision between a bicycle and a car, the bike rider is roughly 100 times more likely to be killed or seriously injured than the car driver (1988 divided by 17). But the risk of a fatality is twice as high for a collision between two cars as for one between two bikes*.

None of this is particularly surprising, I suppose - I think everyone knows that if you're going to be in a road collision, it's better to be in a car than on a bike. But it does show how everyone making that calculation means that as a society we choose a sub-optimal equilibrium - a world in which the vast majority of people drive and the overall risk of death or serious injury is higher than it would be in a hypothetical world in which the vast majority of people rode bikes. That hypothetical world may not be a reasonable prospect, but we could surely do with getting a bit closer to it. Since the relative risks in a bike/car collision are probably not going to change very much, significantly reducing the rate of collisions has to be the aim.

*Incidentally, I used KSIs rather than just deaths as there were zero deaths resulting from bike-to-bike collisions in 2009. 


[Update, 5th May - I've corrected an error in the table, but not one that changes the conclusions]

20mph zones and congestion in London

Cyclists in the City report that the City of London is considering introducing a 20mph zone. This could be transformative if it goes ahead, and not just because of the positive impact on road safety. A 20mph zone would probably also lead to less congestion, because road collisions are responsible for around 28% of congestion in London, and 20mph zones reduce collisions by an average of 37% (p. 50 here). There is little point ever going above 20mph in the City anyway, yet at the moment many drivers do accelerate sharply whenever there is a bit of open road ahead of them. Cutting out that kind of behaviour would be a small but significant step to a saner city.