20mph Pilot Scheme

20mph Pilot Scheme - update

We carried out a large number of speed surveys across the pilot streets and areas before we implemented the scheme in October 2019. We will re-survey a sample of these streets in January, after the holiday period, to measure the impact of the initial layouts. We have had feedback about the size of the 20mph “repeater” signs in some of the new 20mph streets, and we have also received suggestions that on some roads it would be useful to add painted “20” roundels to the road surface to reinforce the signs that we already have.

We intend to add some signs and road markings in some streets where we have received feedback after the January surveys, and we will then carry out a further round of traffic surveys across the pilot schemes to measure the impact of the revised layouts.

Some people have suggested that we use speed cameras to enforce the speed limits, but unfortunately the Council does not have the legal powers to use speed cameras. However, we are working with the police on their Community Roadwatch scheme. See the TfL Community Roadwatch page for further information.

Finally, we will then review the evidence and make a decision on whether to make the pilot schemes permanent later in 2020.

We have no plans to consider any additional traffic calming measures in any of the streets or areas until we have monitored the pilot schemes, measured the speeds, and decided on whether or not to make the trial 20mph limits permanent.

If you have any comments on the pilot streets or areas, or would like to suggest other streets or areas for consideration for 20 mph speed limits in future, please email 20mph@rbkc.gov.uk

Map of streets and area

Key Decision Report



What is the context and background behind this policy?Exhibition Road is currently the only borough-managed road with a 20mph speed limit in Kensington and Chelsea. Most central London boroughs have made 20mph the default speed limit on their roads. Westminster City Council introduced 20mph limits on several roads in 2017.

Traditionally, lower speed limits were seen primarily as a road safety tool. Collisions at 30mph are much more likely to result in serious injury than collisions at 20mph. However, in recent years, the main arguments in support of 20mph limits have been around the idea of 'liveability', and of lower traffic speeds creating streets in which it is more pleasant to walk and cycle.

Both national and regional guidance support the use of 20mph limits in areas with large numbers of pedestrians or cyclists. Department for Transport (DFT) guidance mandates the introduction of 20mph speed limits on suitable streets. The Mayor’s Transport Strategy (MTS) advocates the implementation of 20 mph limits or zones ‘to create better routes to schools and local attractions’.

Do 20mph limits actually work?The MTS notes that lowering speeds reduces road danger because a person is five times less likely to be fatally injured if hit at 20mph than at 30mph. As a general rule for every one mph reduction in average speed, collision frequency reduces by around five per cent. However, there are mixed views on whether 20mph limits that are not supported by physical traffic calming actually reduce speeds. Typically, 20mph speed limits without additional traffic calming measures result in modest reductions of around 1-2mph in the 85th percentile speed.1 Most studies look at average and 85th percentile speeds but there is very little evidence regarding the impacts of 20mph limits on the number of vehicles driving at high speeds (i.e. over 30mph). We think that residents’ concerns about vehicle speeds are often about a minority of vehicles that go above the speed limit, which is why we are interested to monitor this if we proceed with the scheme.

Evidence from 20mph schemes elsewhere has informed DfT guidance that states that on roads where average speeds are above 24mph it is unlikely that a 20mph limit scheme using only signs and road markings will achieve general compliance with the limit. The streets that we have selected all have average speeds no higher than 24mph. DfT has commissioned a research project to assess their effectiveness across the UK over a range of outcomes and impacts including speed, collisions, road users’ perceptions, and air quality. The project was due to report in 2017 but it has yet to be published.

Why have these particular streets been selected?The Council receives a large number of requests from residents and residents’ associations to address traffic speeds in particular roads. As part of our investigations, we sometimes commission surveys to measure traffic speeds. In the majority of cases, we find that 85th percentile speeds are below the speed limit of 30mph. This sometimes comes as a surprise to residents, who tell us either that most vehicles are travelling faster than they would like, or that they are concerned about the minority of vehicles that travel above the speed limit.

These surveys provided a starting point for our selection of the 20mph limit streets. Firstly, because the existence of a survey for a given street is evidence that local residents have raised concerns about traffic speeds in that street in the past four years. Secondly because we need traffic survey data in order to assess two key selection criteria: whether the average speed is already at or below 24mph, and the extent to which the 85th percentile speed is above 25mph.

In our assessment, we rejected roads where the 85th percentile speed was below 25mph and awarded a point for each additional mile per hour above that threshold. Where streets had two speeds recorded, one for each direction, we took the higher of the two. We also considered the number of vehicles per hour that exceeded the existing 30mph speed limit.

Streets were also more likely to be considered if they were:

  • on a designated cycle route or if they were close to a cycle hire docking station, or
  • near a school or another local attractor such as a park or a market

We also considered cost, which is driven by the number of signs that would be required. For this reason, we avoided very long roads with lots of junctions.

Why was the St Helen’s and Dalgarno area picked?We want to pilot a 20mph limit across a reasonably large area, in addition to a selection of roads, in order to see if there is any difference in impacts. A St Helen’s councillor had asked the Council to implement a 20mph speed limit in the ward.We were also looking for an area with obvious boundaries, so that we could create a discrete recognisable area, and keep the additional signs required to a minimum. The area we have proposed has railway lines and the Westway on three sides, and all of the roads within it are managed by the Council.

How will we monitor the impacts of the pilot scheme?We will compare traffic speeds before and after new limits are introduced, and ask residents’ opinions. We shall look at average and 85th percentile speeds, and we will also assess whether the scheme results in fewer vehicles exceeding 30mph. We will also seek residents’ views on the impacts of the lower limits.

We cannot assess the effect on collisions as there needs to be at least three years of ‘before’ and ‘after’ data – the proposed scheme would not exceed an 18-month timeframe.

Will this increase congestion, journey times or air pollution?Most studies agree that the effect of a 20mph speed limit or zone is dependent on vehicle type and the nature of the road, and has a mixed overall effect on emissions resulting in no significant net effect.

A steady driving speed with minimal acceleration and braking results in lower emissions. Because most vehicles will have a shorter range of speeds they are likely to accelerate and brake for less time. As vehicles tend to travel more closely together when travelling at lower speeds, 20mph limits could even reduce traffic queue lengths.

Clearly a vehicle that travels at a consistent speed of 30mph will reach its destination in less time than a vehicle that travels at a consistent speed of 20mph. However, in urban road conditions, most vehicles do not travel at a consistent speed. So any increases in some journey times are likely to be very minor.

Most studies do not account for how speeding restrictions may affect people’s choices around how they travel. We think lower speed limits will create environments in which it is more pleasant to walk and cycle. We want to encourage these forms of transport in order to reduce congestion and emissions.

How will the new limit be enforced? Will there be additional speed cameras and/or speed humps? What about traffic signs?As with existing speed limits, the new 20mph limit could be enforced only by the police. The Council has no powers to use speed cameras. There are no plans to introduce physical ‘traffic calming’ measures (such as speed humps) as part of this scheme. The aim is for this new limit to be self-enforcing.

It is unlikely that everyone will drive within the 20mph limit, but we do expect to see speeds fall. We have selected streets where the average speed is at or below 24mph so they have a good chance of achieving general compliance.

There will be some additional signs to implement the scheme, but we will keep them to a minimum and use existing posts and lamp columns wherever possible.

Will the Council benefit financially from this scheme? How much will this scheme cost?The Council will gain no additional revenue as a result of this pilot scheme. As noted above, we have no powers to issue fines for speeding. Final costs will depend on the detailed design of signs and road markings but we estimate that the cost will be between £60,000 and £70,000. We have made provision within this year’s Local Implementation Plan funding from Transport for London and Council capital budgets to cover this cost, should the scheme go ahead.

Why is this scheme not borough wide?The Council’s motion was to initiate a pilot scheme on a number of selected roads. Being a pilot scheme, it makes sense to monitor outcomes on specific streets or areas. We want to see evidence of the impacts of 20mph limits before we consider further options. A borough-wide scheme would cost significantly more, and take much more time, to implement than our current proposal.