Saturday, 10 August 2013


A recent front page story in the local newspaper made the unsurprising point that lowering speed limits reduces the number of fatalities. The Centre for Automotive Research at Adelaide University (those clever people who also published a study on protective headbands for motorists) now suggest that speed limits should be cut further to 40km/h - or so the article claims.

It was met with the usual predictable howls of protest on talkback radio and in the comments section of AdelaideNow but they included some original gags about going back to horses and carts and people carrying red flags while walking in front of cars. Hilarious stuff.

Reducing motor vehicle speeds undeniably reduces fatalities and serious injuries. The difference between 40km/h and 60km/h is significant:

Having said that, reducing speed is but one way of reducing conflict and serious injuries. It works perfectly in residential streets. Unley Council has a 40km/h speed limit on most of its side roads. That policy along with blocking off many of them to through traffic has worked very well.

At the same time, we need arterial roads that can move traffic. I would love to see 40% and 50% modal shares for bikes and similarly high shares for public transport but we need to move cars too. Simply reducing the speed limit is all very well but governments can create problems for themselves when the speed limit is difficult to enforce (as it would be) and difficult to justify (which it might be given some of the comments).

In those situations, there are other ways of reducing risk. Risk can be minimised through design. For example, by separating road users whotravel at different speeds and avoiding right hand unsignalised turns and other traffic movements with conflict built into them.

How you design the road depends of course on its intended use. A higher speed arterial road is designed and treated very differently from a narrower road that leads only to where people live. A guide to potential treatments is found in the Austroads cycling guide and is similar to others:

The question is which comes first. I get the feeling sometimes that traffic is allowed to congregate on certain roads and the amount of traffic then determines what treatment, if any, is applied. I'm probably wrong but it is often difficult to tell.

Far better is to designate the street in the first place. Decide what its purpose will be then you can apply the appropriate treatments, including blocking it, narrowing it, assigning the appropriate speed limit and adding the appropriate bus, pedestrian and cycling infrastructure. Designated arterial roads should be treated with appropriate speed limits, separation of users and, if necessary, multiple lanes. By contrast, other types of road should have speed limits and physical means of lowering speeds and blocking through traffic and of course feeding traffic on to the closest designated arterial road.

The mistake we make is to treat most of our roads in the same way. That is perhaps illustrated in calls for a blanket 40 km/h speed limit. I should add that I doubt that would happen anyway. We now have a default 50 km/h speed limit but many of our arterial roads remain signposted with a higher limit.

It has been said many times but you can see this in the morning rush hour. Main roads are full but many side streets have lines of cars running parallel to the main roads - they generaly seem to be parents on the school run.

So what so we do and why would it be better?

Step one is to designate the use and purpose of the road. The categories are not just main road and side street. Main arterial roads are pretty clear. They are the yellow ones in the street directory and on Google Maps. But there are other roads that while quieter than the main arterial road still serve an important feeder role. Galway Avenue next to the ABC Building on North East Road is an obvious example.

Once you have designated your main arterials, you then work on the feeder routes inside neighbourhoods and the residential streets that should not be able to be used as through routes.

There is a very useful post on the excellent Bicycle Dutch website that deals with this. It discusses a neighbourhood in Utrecht that was built in the 1960s. What makes it particularly relevant to us is that before it was retrofitted, its streets were just like ours:

In the original 1960s street grid for this area, motor traffic was able to use all streets to get from one end to the other. Some streets were even wider than others and served as through street. Most streets were purely residential, but all streets had the same speed limit of 50km/h (31mph).

So what did they do (and what should we do):

To channel the traffic flow better, the city designated a so-called ‘neighbourhood ring’. This is the street that is designed to give quick access from the city’s arterial roads to the purely residential streets. The latter type of streets have all become 30km/h (19mph) streets. This means that no 30km/h street has a direct access to an arterial road, but that traffic is forced to use the neighbourhood ring to get to the main arteries via only very few access points.

Read the whole post here. It's short and very clear.

Second question - why is this better? Well, among other things, it is obviously safer; it encourages alternatives to the car, which is also good for car users; rat-running and neighbourhood hoon driving are all but eliminated; neighbourhoods are quieter; effort can be put into making arterial roads work properly.

And so on.

Finally, definitely watch the four minute video:

Note how once the speed limit is lowered (in an appropriate environment) it is safe for different transport types to mix. It shows how segregation without separation works as part of a wider network.

1 comment:

  1. Interesting. Worth bearing in mind that at peak times, average traffic speeds are much lower than the limit, and that reducing the limit mightn't influence average speeds that much... Residential ring road idea sounds good :-)