Hammersmith and Fulham Council is proposing putting all roads under a 20 mph speed limit.

What would be the impact on bus journey times?

Transport for London tells me:

The potential impacts on bus passenger journey times of wide-scale introduction of 20 mph zones will be lowest during the daytime (where average speeds are slowest, at around 9mph). 80% of daily weekday journeys occur between 0700-1900, where the additional journey time on the average journey (with a length of 3.5km) will be of the order of magnitude of 5-10 seconds. Evening and night times have higher average speeds (around 12-14mph), with additional journey time on the average journey of the order of magnitude of up to 45-60 seconds.

So if you were going by bus in the evening the average journey is a couple of miles and takes 10 minutes. The 20mph zone would mean it would take 10% longer.

That might not sound much but given the busy leads we leave would be a source of frustration. There is also the damaging impact on air quality of having the buses crawling along more slowly.

It could also have a perverse impact of making bus travel less attractive compared to driving. This is because TfL claim that they would ensure bus drivers would follow the new requirement:

“Bus operators must ensure their drivers adhere to speed limits and schedule services accordingly.”

By contrast the police have made very clear that they would not enforce the new 20 mph limit. So in practice the new limit would be ignored by private motorists but, according to TfL, followed by bus drivers.

Therefore someone wishing to get from A to B as quickly as possible would be less likely to take the bus and more likely to drive.