If you’re aiming to block rat-running and make your residential streets safe, motor vehicle speed is potentially your one measurement to target. Basing your project on reducing the volume of traffic might sound attractive, but is harder to define as an end goal.

Why would you pick a single measurement? In the book How to Save the World, Katie Patrick encourages us to visualise the alternative future, concentrate on behaviour change, target a single measurement that defines success, and from that identify the most likely measures that might affect number. By having the constraint of one single target measure, it frees you to brainstorm a bunch of ideas that might get your community to there, and to design experiments and measures that might be most effective.

I live in Mayorstone in Limerick. It’s a housing estate that was built around 1955, for a very different dream than today’s reality. It has a couple of thousand people drive through it on weekdays. The speed reached by drivers may seem “reasonable”, but is deadly, and takes away the right of our kids, the older of our community, and everyone in between to be in their neighbour. A traffic engineer asked me directly: “What do you want, is it reduced speed, or reduced volumes?” Having already thought this through, I had my answer ready: “Reduced speed.”

Both speed and volume of motor traffic are easily measured. Limerick Council temporarily installed these traffic detection devices in Mayorstone. They allow for classification of vehicles, and analysis of speed and volume.

There’s a very good reason for preventing boxes of metal being moved at speed through a residential area: the inescapable laws of physics mean that your body (or your child’s body) can’t withstand the damage caused by one of these machines in motion striking it. The lower the speed of that machine, the higher a chance you have to survive the impact.

These laws of physics give your rat-running reduction project a clear target: to minimise the speed of high-mass vehicles. You can choose what your target is. My target clear: that 85% of drivers keep at or under 30km/h. This “85th percentile” is the parameter used by Limerick’s urban traffic calming policy (PDF). The 30km/h speed is not an ideal, because you still have a 10% chance of being killed when struck by a car at that speed (see the EU Commission’s details). A 15km/h speed limit would be even more safe, but 30km/h is more more realistic and is catered for in Irish law.

“But the number of vehicles is our real problem, if thousands of people are driving through each day.” Yes, volume, or lots of people driving through your area certainly brings its own problems. For each additional person driving through, the people using the space have a chance of being hit. More motor vehicles mean it’s harder to cross the road (sometimes we’re stuck for a while on one side of the street, trying to cross with our kids). Each car brings added pollution to the air in the street. Each car also makes noise.

“But targetting a speed goal won’t cut off rat-running.” That is true. This raises the question of what do you really want to achieve with your project? By all means visualise the fantastic alternative reality of an inviting green street where your community strengthens with each chance daily encounter. Your vision is a legitimate one, in its own right. Visions are an extraordinary driving force to get you from “now” to “future”. Yet only stating “I want my street to be nice” may prove ineffective as a strategy for real behavioural change.

If you have experience of reducing rat-running, or are involved in such a community project, please share your experiences. e@eoinoc.net and this Twitter thread.