Update: See the end of this article for the latest data.
It’s now several months since the speed limit in most of north Cambridge was reduced form 30mph to 20mph. The City Council has recently been measuring traffic speeds to see just how much difference the change has made. They’ve kindly sent me the results of their speed survey, so I can exclusively reveal that the average change in traffic speeds in north Cambridge since the 20mph limit was introduced is…0.7 mph.
The before and after figures vary quite a lot from street to street. Here’s a graph showing the change in average traffic speed at each measuring point. Green represents a speed decrease; red a speed increase. Ready for some scrolling?
A note on the names – a small letter in brackets refers to the direction of traffic, whereas a capital letter refers to the part of the street. So “Stretten Avenue (S) (n)” refers to northbound traffic in the southern part of Stretten Avenue.
As you can see, more streets have experienced a speed decrease than an increase, but the typical speed decrease is a great deal less than the 10mph change in the speed limit. And some roads have experienced an increase in traffic speed, despite the reduction in the speed limit. The average change in the average speed, across the whole of the area, is just 0.7 mph. Whether this represents a good return on the many tens of thousands of pounds spent on the scheme will no doubt be hotly debated. We’ll have a better idea about this when before-and-after accident figures become available, but with such a small reduction in average speeds, it’s hard to imagine a very dramatic change.
Update: Here’s the raw data: north_area_speeds
Further update: The City Council have now sent me some slightly revised data, which you can find here. This separates the roads into those where the average speed was above or below 20mph before the limits were changed. You might expect the faster roads to be more affected by introduction of the 20mph limit, and indeed this is what the data shows. However, the drop in average speeds on the faster roads is still only 1.1 mph, compared to 0.4 mph on the slower roads.