This post uses freely available County Council traffic sensor data to confirm that Mill Road bridge restrictions have no impact on traffic levels on boundary roads. This data has been publicly available for some years – we accessed the data recently to do this analysis.
The boundary roads included are Hills Rd, Coleridge Rd, Coldhams Lane & Cherry Hinton Rd.
Build roads, get cars…
No-one ever has a problem imagining that, when we add a lane to the M25 (or indeed to an LA freeway), it will fill up with traffic. Because we’ve all seen that happen:
And yet some people struggle to believe that the reverse could also be true. By reducing road capacity, we can reduce traffic.
Analysis
Following our 2021 analysis, we decided to look at the more recent data available from the County sensors. Traffic counts can be downloaded up to the end of 2022, and they show exactly what happened to traffic levels after bridge restrictions were removed in early August 2021:
- Mill Road traffic flows as recorded by the sensors at both ends of the road increased significantly. We would expect to see this.
- But what would we expect to see if the bridge restrictions had generated additional traffic on boundary roads? After removal of the restrictions, traffic levels should have reduced on those roads. But that’s not what the numbers show. The traffic levels on all the boundary roads stayed broadly the same after the restrictions were removed.
Compare the period after bridge restrictions were removed to the rest of the chart, where traffic levels by street broadly move in parallel. The highlighted period (red dotted line on the chart below) shows a very clear discrepancy between Mill Road and all the other streets. Mill Road traffic increases, but boundary road traffic (allowing for the August holiday period) stays constant.
Tap / click on the chart for a larger image:
The removal of the restrictions is what creates this discrepancy. To be clear, allowing through traffic back over Mill Road bridge generated additional motor vehicle journeys. A bit like adding a lane to the M25.
NB: every year there is a dip in levels on most roads during August (summer holiday period). We see a similar effect over Christmas / New Year. Ignore these atypical periods when looking for normal levels in the chart.
Conclusion
The conclusion is clear. Mill Road bridge restrictions substantially reduce traffic on Mill Road, and have no impact on boundary roads.
It would also be reasonable to assume that pollution increased during the period following removal of the bridge restrictions. Each vehicle journey creates pollution. So if there is an increase in the number of vehicles on the roads, then there will be a corresponding increase in pollution and emissions.
You can view our analysis of the sensor numbers on this spreadsheet (this is a large 45Mb download).