traffic displacement: myth or reality?

Some people think that when Mill Road bridge was restricted, levels of traffic on surrounding roads got worse (they often mention Cherry Hinton Road and Coldhams Lane). And that intuitively makes sense doesn’t it? I mean, all that traffic has got to go somewhere, right? And if displaced traffic is clogging up other roads, then maybe we shouldn’t do anything to restrict traffic on Mill Road until a larger scheme is in place to manage the fallout…

Fortunately we don’t have to rely on our intuitions and anecdotal evidence. Back in 2019 the far-sighted folk at Cambridgeshire Insight installed traffic sensors on Mill Road and the surrounding streets. These sensors have been busily counting vehicle numbers ever since.

And the evidence is clear – the 2019 & 2020/21 periods of bridge restrictions had no sustained impact on traffic levels on those roads. What we do see is that, when restrictions on Mill Road traffic are introduced (or removed), there is an initial increase (or reduction) in traffic on surrounding roads, but within a few weeks levels return to normal.

To be very clear: we’re not arguing that nothing needs to be done about traffic levels on surrounding roads. But using the idea of displaced traffic to argue against early action on Mill Road is simply not justified by the evidence.

We’ve collated the sensor data into weekly motor vehicle volumes on Mill Road, Coldhams Lane and Cherry Hinton Road. This chart shows these numbers over the period from June 2019 when the sensors went in, to late October 2021.

Our detailed observations are:

  • there is no evidence for *sustained* increase in traffic levels on Coldhams Lane or Cherry Hinton Rd as a result of the bridge restrictions in 2019, or 2020-21
  • when the 2019 restrictions were first introduced, traffic levels on both roads increased by about 20%; but after a few weeks, and before the bridge was reopened to traffic, levels had returned to normal
  • following lockdown 3 in early 2021, traffic levels on both roads gradually increased to peak in early April 2021 at levels above normal; but again within a few weeks levels had returned to the typical levels seen over periods with no restrictions on Mill Road
  • since the bridge restrictions were removed in early August 2021, there has been no sustained reduction in traffic levels on Coldhams Lane

We have asked Cambridgeshire Insight for clarification on the post-May 2021 Cherry Hinton Road data, where levels look unusually low.

Cambridgeshire Insight Sensor Locations