The 25mph Conundrum

Many people ask what to do if the average speed on a road is 25mph or above. Can you set a 20mph limit? Lets consider what I call "The 25mph Conundrum".

Setting speed limits based on what drivers think is the right speed is a flawed approach. This is considered in a recent report from NACTO (National Association of City Transportation Officials). It concludes that "Relying on a percentile-based system focused on current driver behavior, rather than a defined safety target to set speed limits, significantly limits cities’ ability to reduce traffic deaths.


So lets go through the logic of solving this conundrum for UK situations.


  • If we set a speed limit does this endorse driving at or just below that limits?
  • Is that a good thing?
  • Should we expect drivers to use the speed limit as the correct reference point for their behaviour?

And if we do then we must accept that driver speed is highly dependent upon the speed limit set. And there will be a spectrum of compliance. Some would say that where the current limit is 30mph then an average of 25-29mph could be expected.

Guidance on setting speed limits says that we MUST take full account of the needs of road users. Hence we cannot rely upon the previous speed  of drivers, especially if they feel that up to 30mph is endorsed by the government, as a guide. We need to independently assess what is correct limit. And if we do so, it is difficult to come to any other conclusion that national 30mph speed limit is wrong and it needs a 20mph limit.

So what is the effect of setting a speed limit of 20mph where previous speeds were 25mph or above?

Well firstly be clear that there is nothing in the DfT guidance that restricts the creation of a 20mph zone or limit on such a road. Perhaps it is obvious that highest compliance will be for roads where the previous average speeds were less than 25mph. And indeed that is reflected in the guidance.

But what of the roads where the average with a 30mph limit is 25mph or above. Well research and evidence shows that these show the highest actual reductions in speed. In Portsmouth it was found to be a 6-7mph reduction. And even on free-flowing roads DfT research shows speeds on such roads without houses were 6mph lower for every class of vehicle than on similar 30mph roads.

Hence whilst you may get an average speed down from say 29mph to 23mph it will still not be compliant for most drivers.

So what are your practical options for a road with a current average speed of 29mph. :-

  1. Leave the speed limit at 30mph
  2. Implement a 20mph limit covering a wide area
  3. Implement a 20mph limit with physical calming

The issues are :-

  1. You will endorse 30mph and leave vulnerable road users in danger. No cost, but expect an outcome of 29mph. This will not follow DfT guidelines.
  2. You will get an outcome of an average speed of 23mph. It will cost about £1.1k per km.
  3. You will get an outcome of 20mph but will cost £60k per km.

Both 2 and 3 are within DfT guidance, but there are additional consequences of 3.

  • You will teach drivers to only slow down where there is physical calming
  • You will increase emissions if cars speed up and slow down between bumps/chicanes.
  • Reducing from 6mph from 29mph to 23mph will cost £1.1k per km
  • Reducing an extra 3mph will cost 50 times more at £60k per km

Also remember that when you have set a 20mph limit you can do other low cost interventions such as :-

  • Put a cycle lane in on the road. Increase the visibility of vulnerable road users.
  • Use paint for some road narrowing, including removal of central white line in road.
  • Add planters and other furniture to the side of the road. Make it look like a “people place”
  • Stagger on-street parking along the road.
  • Paint additional 20mph roundels on the road.
  • Use additional banners on lamp posts to get the message across about making it a better place through lower speeds.
  • Use Speed Indicator Devices.
  • Use enforcement.

All of the above are possible in combinations that are appropriate for the road.

So the outcome is that traffic authorities when considering the speed limit MUST take full account of the needs of vulnerable road users. In doing so they cannot decide to leave a 30mph limit purely on the basis of currently the 30mph speed limit endorses driving at a speed that results in an average above 24mph.

They could put in a 20mph limit or zone with physical calming, but this would be poor value-for-money.

The best and most cost-effective alternative is for the road to be part of a wide-area 20mph limit or zone with no additional physical calming. And deployed with light touch driver engagement and inducement to reduce driver speeds.

Showing 2 reactions

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  • Alex Hosking
    commented 2022-07-30 01:51:13 +0100
    Have you seen the video on the 85th percentile speed by traffic engineer and Strong Towns founder Charles Marohn?
    In it he claims that much of the non-compliance isn’t down to wilful disregard for the law, it’s more down to how our brains work and you have to engineer roads correctly if you really want to bring down speeds.
  • Rod King
    published this page in Blogs 2021-03-10 19:38:42 +0000