Tag Archives: 20mph

Yes, Bristol’s sign-only 20mph intervention reduces speed (yet another study finds)

Photo at courtesy of @Bristol20mph

We’re big fans of the Essential Evidence series of one page summaries of peer-reviewed literature on current transport policies and practice. This is produced by Bristol based Dr Adrian Davis in order to ensure that academic research and evidence informs implementation of planning and policy. Although sadly one recent summary shows that politicians are far more resistant to this than professionals and communities (No 187: Examining the politics of transport planning).

Summary No 185 is another confirmation of the robustness and effectiveness of the city-wide Bristol scheme, The effectiveness of a 20mph speed limit intervention on vehicle speeds in Bristol. Key points include:

  • Prior research has confirmed that there is a 6% reduction in collisions with each 1mph reduction in average speed
  • Reduction of individual vehicle speeds on those roads which received the intervention was 2.66 mph
  • Compliance to the posted speed limit improved following the intervention in both 20 mph and 30 mph roads

Our response to the 20mph consultation

20mph speed limits on most streets and roads in Bristol has been one of Bristol Cycling’s campaigns (Twenty’s Plenty) from the beginning. We encouraged as many BCyC members as possible to respond to the consultation in 2018 (7 Reasons Einstein would support 20 mph) and we were pleased at the overwhelmingly positive response the council received. We put in our own formal submission as well, as below or click here Response to 20mph consultation 2018.


The ability to cycle safely on low speed, low traffic streets is essential in providing a comprehensive cycle network in Bristol. In 2017 Avon and Somerset Police recorded 232 injuries to cyclists in Bristol. Based on the Department of Transport’s estimates this is likely to be a significant under-estimate of actual numbers.  Despite this Bristol Cycling Campaign believe that 20mph speed limits have made a meaningful difference improving conditions for cycling in Bristol and we strongly urge the 20mph limits to be retained.

The UWE BRITE study supports the effectiveness of 20mph speed limits in reducing deaths and serious injuries, saving an estimated 4 lives and preventing 170 injuries every year, and as a result saving an estimated £15m of public money by doing so. Nor is there any clear rationale for relaxing limits. Commercial research undertaken last year by ‘In Car Cleverness’ suggests that average traffic speeds within a 5 mile radius of Bristol City Centre are under 13mph. Increasing speed limits would only have the effect of encouraging stop-start driving and extending waiting times at pinch points and junctions, compromising road safety in the process.

Bristol Cycling Campaign estimate there are 28 schools on or immediately adjacent to roads being considered for higher speed limits as part of this review. Increases to these speed limits would negatively affect the safety of children in these areas and others. A vehicle travelling at 30mph has more than double the kinetic energy of a vehicle travelling at 20mph dramatically increases the stopping distance and the risk of a pedestrian being killed in the event of an impact.

Some additional benefits of 20mph limits that have not been mentioned in the council’s evidence documents are the potential of 20mph limits to:

  • Reduce air pollution and fuel consumption. 20mph reduces acceleration of vehicles, the most energy intensive stage of driving. Braking from 20mph instead of 30mph puts less energy through brakes, tyres and the road surface. Thanks to this lower energy a fully enforced 20mph limit would reduce air pollution from exhaust and non exhaust emissions.
  • Reduce noise. Numerous studies have shown that cars travelling at 20mph instead of 30mph a perceived as generating half the noise.
  • Reduce congestion, through improved vehicle flow. This is not shown in the council modelling because city-wide models are macrosimulation (average speed models) they assume a vehicle travels at steady speeds across the city and stop start behaviour is not included.
    Given the benefits outlined above Bristol Cycling Campaign believe there is no negative effect for imposing speed limits which are ‘too low’ for a given road and urge that where non-compliance with the existing speed limits is high that traffic calming and enforcement measures are introduced to bring average vehicle speeds down to compliant levels rather than compromising road safety by relaxing limits.

Bristol Cycling Campaign believe these measures should be combined with traffic reductions measures to reduce through traffic and vehicle movements generally in residential areas via filtered permeability of traffic modes favouring walking and cycling. This would both improve road safety for pedestrians and cyclists, and improve residential streets as a place to live.

Bristol City Council is currently considering the options for a Clean Air Zone (CAZ). The same technology proposed for this could be used to enforce 20mph, providing a list of benefits, that if conveyed to the general public, could be more popular than a CAZ.

20 mph and the Clean Air Zone

Bristol City Councils 20 mph review finishes today. In this consultation, the potential of 20 mph to help meet the National Air Quality Objectives (NAQOs) for NO2 and PM10 emissions, for which Bristol exceeds, has almost completely been ignored.

Instead, a Clean Air Zone (CAZ) has been proposed to tackle this as part of central governments Clean Air Framework. A CAZ would attempt to reduce air pollution by encouraging residents and businesses to purchase a cleaner car. A scrappage scheme has even been mooted as a way to kick-start this.

A CAZ might encourage some people to leave their car at home but it would have nowhere near the benefits of 20 mph limits.

One of the key infrastructure requirements for a CAZ is Auto Number Plate Recognition (ANPR) cameras. These are the same yellow cameras used along the M32 to enforce speed limits. They are one of the most effective measures for speed enforcement, ensuring drivers stick to the speed limits within the zone. Surely it would make sense to simply install these cameras to enforce the 20 mph speed limits that are already in place?

From Bristol Cycling’s perspective, 20 mph zones are only positive for Bristol. We recently highlighted the key benefits of 20 mph. In summary, 20 mph limits:

  • Reduce Collisions and lessen their severity when they do happen. A driver seeing a danger travelling at 30mph would have travelled 9 metres before they even pressed the brake pedal and another 14m to come to a stop. At 20 mph the overall stopping distance is half that. This makes a collision less likely in the first place. The kinetic energy of a 1419kg Nissan Qashqai car travelling at 20 mph is 15.7 Wh. If the vehicle didn’t stop in time, this would be the energy felt by the object it hits. Increase this speed by 50% to 30 mph and the kinetic energy goes up by 125% i.e. it more than doubles to 35.4 Wh. To put this in comparison, the energy of a 90kg cyclist at 20 mph is 0.9 Wh and a 70kg jogger at 5mph, 0.05 Wh. This video of an appalling collision in Wales demonstrates the huge amount of energy in a moving car.
  • A car braking from 30 mph to a stop puts more than double the energy through the brakes, tyres and roads than the same car stopping from 20 mph, increasing particulates (PM) from the brakes, tyres and road, contributing to poorer air quality. It also puts more strain on the road surface making damage and potholes more likely.
  • 20 mph has also been shown to reduce noise from vehicles. A car travelling at 20 mph instead of 30 mph can emit almost 10db less noise. 10 db is perceived by the human ear to be a halving of noise. Studies in Sweden, Germany and the UK, have consistently found 2—5 db noise reductions at the roadside from 30 kph (20mph) limits.
  • 20 mph Improves traffic flow and reduces journey time. Countless studies have found 20 mph reduces the concertina effect of traffic and reduces congestion.
  • Acceleration is the most energy-intensive stage of driving. Lower speed limits mean less time on the accelerator pedal and reduced fuel consumption. Considering a hypothetical a 16km journey with 25 stops and starts a car only accelerating to 20 mph will spend nearly a minute less on the accelerator than if it reached 30 mph. Although this does not directly correlate with emissions, a detailed study in London found (unenforced) 20 mph limits reduced NO2 air pollution concentrations at the roadside.

Over the past few weeks, Bristol Cycling has been campaigning hard to raise awareness of the 20 mph consultation. The criticism we hear time after time is that 20 mph isn’t enforced* and that “no one really travels at 20mph”. This isn’t a reason to scrap it, it is a reason to do it properly and see what the real benefits could be.

We’ve not heard anyone suggest the CAZ could be left to chance police patrols to enforce, so why has 20mph?

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7 Reasons Einstein would support 20 mph

ACTION:  Please take a moment to respond to the Bristol 20mph Review

Our lives fundamentally rely on energy. It puts food on our table, gets us to and from work, powers our offices and factories. The more we use, the more money it costs us. Our demand for it causes wars, our generation of it emits harmful gases.

Almost every issue 20mph raises is energy related and the answer to each, along with many of our urban challenges, can be found by looking for the lowest energy solution.

Energy dictates the severity of a collision, how much fuel is burnt to move somewhere and the volume of emissions that activity will release. The more energy we put through car components and the road surface the quicker they will break and wear out. The faster a vehicle moves the more noise it generates.

Yet energy is rarely mentioned. Despite apparent concerns over climate change and air pollution, sales of energy-hungry SUVs are soaring, up 24% across Europe this year and on course to become the most common cars on our roads.

So in this time of “fake news” and manipulation of facts, what better way to argue a case than with some basic physics. Just as keen cyclist Albert Einstein might have done. Here are the seven reasons (also as an infographic here):

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Bristol consulting on 20 mph speed limit

Bristol are consulting on 20 mph speed limits in the city (20mph Limits Review). We believe the city-wide ‘Total Twenty’ approach has been good for walking and cycling, and the evidence supports this. We’re concerned that the review seems to be wholly focussed on reversing this and allowing 30mph on many roads. There is almost no mention of eduction or enforcement measures to embed the change.

We’re asking everyone to respond to the review. There are suggestions below to make it quick and easy. 

The exercise has benefitted from a UWE Monitoring Report which has found that Bristol’s 20 mph scheme has saved lives and reduced the number of people injured on our roads. Although we all know that speed limits are inadequately enforced, the UWE report found that road traffic speeds have decreased. We’ve made our contribution with a fact filled post on 7 Reasons Einstein would support 20 mph.

When elected, the Mayor, Marvin Rees, promised to “get Bristol moving” and said he would review the operation of the 20 mph scheme. This consultation exercise is part of that and asks whether the public would like speed limits increased on 78 roads and reduced to 20 mph on 5. The balance of roads to be consulted on, increased speeds v speed reductions, shows where Marvin’s heart is.

While the consultation on reducing the speed limit on the token 5 roads identified, especially Kellaway Avenue and Hotwell Road, is welcome, it is a great shame that the opportunity was not taken to consult on bringing in a 20 mph limit to roads such as those round the Downs, the ones that cut through Old Market or to Coronation Road in Bedminster.

Even more surprising is the apparent lack of concern with the well-documented additional risk posed by higher speeds, especially to vulnerable road users. In fact 28 of the roads being considered for faster speed limits either have schools on them or very close by. Some, such as Cranbrook Road, have been the focus of parent-led campaigns for safer routes to school. The Council’s response: to let vehicles travel even faster.

The Council collects and analyses death and injury incident reports by the police. “Accident cluster sites” in Stokes Croft, Park Street and Church Road as well as notorious roads like Gloucester Road are all included for consideration for faster speed limits. The evidence shows slower speeds lead to fewer deaths and fewer and less severe injuries. Faster road traffic speeds will make these roads even more dangerous.

It is very important that cyclists make their voices heard and oppose faster speed limits for vehicles. The Council consultation can be found at: https://bristol.citizenspace.com/city-development/20mph-limits-review/

The survey is LONG but you only need to answer the first part, the rest is optional. Don’t feel obliged to give an answer for every single road, maybe just stick to the ones in your ward or neighbouring ones.

For each ward you will be asked for your view on changing speed limits on selected roads, with a list of prompts for why. This graphic suggests the important ones for cycling. If you think the increased dangers to schoolchildren and other vulnerable road users is important, you’ll need to write this under “other” reasons as it’s not something the consultation has taken on board.

In general we invite you to click the ‘Other’ box to make something like this point:


It’s important that almost every road, and especially residential and retail, has default 20mph. Too many exceptions creates uncertainty about what the citywide default speed is and undermines the overall effect that we’re moving towards


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