Myth: Trams cause more casualty crashes on streets where they run

Myth: Trams cause more casualty crashes on streets where they run
Fact: This is a classic case where ‘correlation does not imply causation’. Streets with trams do have more crashes on average than streets without, but this has little to do with the trams.

In an article in The Age on 14 February 1999, road lobbyist John Cox claimed that the presence of trams made roads unsafe. The evidence he put forward was that roads with trams have on average 52 serious casualty incidents per 100 million vehicle-kilometres, compared with 32 incidents for roads without trams.

This urban myth had new life breathed into it more recently, when Cox’s finding was included in a tram-bashing paper by some RMIT researchers.

We have no reason to doubt Cox’s figures: only the interpretation put on them. This is an instance of a classic logical fallacy, that of noticing a statistical correlation between A and B, and jumping to the conclusion that A is the cause of B. For example, it is a known fact that crime rates tend to go up whenever sales of ice cream do. The fallacy then is to argue that ice cream causes crime.

In our example, the real reason is that there is an underlying factor leading to both higher crime and higher ice cream sales, namely fine, warm weather. So it is with trams and casualty crashes.

If one considers all the roads in Melbourne on which trams do not run, one observes a wide variety: there are freeways, single and dual-carriageway arterial roads, collectors and local streets, with speed limits varying from 40kph to 100kph and traffic volumes varying from 100 to 100,000 per day. Of course, there is also enormous variation in the crash statistics for each of these roads.

On the other hand, almost all the roads on which trams do run fall into just one category. With few exceptions they are arterial roads with 60kph speed limits, high volumes of both car and non-car traffic, numerous intersections, and high levels of adjoining commercial activity. In other words, they are precisely the category of road where common sense would lead one to expect a high rate of collisions, whether the tram tracks are there or not. And indeed, this is borne out in the statistics.

So when anti-tram lobbyists point to crash statistics for roads with and without trams, what they are really comparing is the rate of crashes on selected inner-urban arterial roads versus all other roads. The underlying factor here is the combination of high traffic volumes, high speed limits, high pedestrian activity and highly ‘conflicting’ routes that both increases the likelihood of casualty crashes and characterises most Melbourne tram routes.

It is unlikely that the crash statistics on these roads could be reduced other than by reducing the volume of traffic, or holding it to a consistent low speed limit. The presence of trams on many of these streets actually has a beneficial effect, by carrying people who would otherwise be in cars. Removing trams from inner-urban streets would have the effect only of increasing the number of cars on these streets, and with it the level of road trauma.

Last Modified: 10 August 2004

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