20 Mph Speed Limits – Cost Benefit Analysis

20mph speed Limit
20mph speed Limit

After returning from New York last Monday, I’ve been met by a barrage of 20mph signs around Oxford. Most roads in the city centre have been limited to 20mph. As both a cyclist and driver, what are the costs and benefits of this decision?

Benefits of 20mph Zones

  1. Slower speed of cars will make cycling more enjoyable and less dangerous. At lower speed, accidents are less likely and also less likely to be fatal. At 30mph, 50% of collisions prove fatal, at 20mph, the fatality rate is only 10%
  2. Slower speed and greater safety may encourage more people to cycle and walk. This creates a critical mass of less cars on the road.
  3. Lower number of cars on the road will help reduce pollution and create a better environment.

Costs of 20mph

  1. Slower journey times for drivers.
  2. More petrol consumption, 20mph is less efficient than 30mph. More petrol is consumed at this lower speed.
  3. Chance of increased congestion. Lower speeds mean slower moving traffic and possibly  more traffic jams. Though, if the lower speeds deter cars, congestion could improve.

Evaluation.

  • The main issue is will the 20mph speed limit be enforced? It is one thing to put a sign saying 20mph, but do drivers may any attention unless there are speed cameras? I have still seen cars speeding past at 40mph.
  • Note: the costs to the drivers are mainly private costs – increased time, increased petrol consumption. Many benefits are external benefits e.g. less chance of fatal accidents to other people.
  • The real beneficiaries are those who cycle, walk and have children who like to play in the road. Car drivers are the least to benefit with facing longer journey times. However, the extra journey time is fairly marginal as many of the roads limited to 20mph are not fit for fast speeds anyway.
  • If 20mph speed limits do lead to lower fatalities that is a pretty big benefit. If we assign a value of life saved at £2million, then the potential benefits are significant.
  • As a cyclist I’m very happy, to have 20mph speed limits, as a driver I have to admit I’m not used to driving at 20mph. But, I’d be quite happy if they introduced speed cameras to enforce it.

See also: Benefits of 20 mph zone

4 thoughts on “20 Mph Speed Limits – Cost Benefit Analysis

  1. Re your valuing a human life at £2m, the last time I looked at the figures (roughly 5 years ago), motorway safety features were evaluated on the basis that a life is worth £100,000.

    We ought to have consistency right across the board on this issue. That is, the Health Service, road designers, aircraft designers and so on should all abide by the same “human life value” figure. The reasons for this are similar to reasons for free trade.

    To illustrate, if the price of apples in Scotland is much higher than in England, then we benefit by shifting apples from Scotland and selling them in England. Likewise, if two different organisations use different “human life” values, then lives can be saved at no cost to the nation by shifting safety expenditure from one organisation to the other.

  2. Sorry: I got the apples illustration wrong. The apples need shifting from England to Scotland. Duh!

  3. Your stated benefits are counter to existing evidence.

    “1. Slower speed of cars will make cycling more enjoyable and less dangerous. At lower speed, accidents are less likely and also less likely to be fatal. At 30mph, 50% of collisions prove fatal, at 20mph, the fatality rate is only 10%”

    Analysis taken from Cowley Road and Portsmouth showed a rise of 5% in major accidents and a fall of 11% in minor accidents. If you look strictly at deaths and significant injuries to cyclist you will see in Oxford at least that none of the people involved in the accidents were speeding. The majority of deaths took place with vehicles travelling under 20mph, and only 2 were at over 50mph. The most common cause of death / major injury in cyclists is a heavy goods vehicle, turning left at a junction and going over a cyclist. Statistics back up the view that slowing cars down to the speed of the cyclist will cause this major incident to happen more regularly.

    Oxford has on average 100 major incidents yearly and 380 minor incidents. That means that the speed limit change has created 5 more major incidents and prevented 42 minor incidents. Minor incidents do not require hospital treatment, major incidents are hospital treatment or death.

    I would suggest that the 42 minor incidents can be calculated to cost less than £2000 each, where as the major incidents cost from £10000-2000000 (if I use your cost of human life assessment). Even conservative estimates show that there is a detrimental effect on road safety for cyclists according to government data.

    “2. Slower speed and greater safety may encourage more people to cycle and walk. This creates a critical mass of less cars on the road.”

    Portsmouth has not shown a reduction in car numbers in its reports, suggesting this effect does not exist.

    “3. Lower number of cars on the road will help reduce pollution and create a better environment.”

    Portsmouth has not shown a reduction in car numbers in its reports, so the same number of cars travelling at less efficient speeds leads to an increase in pollution.

    Essentially the system is less safe for cyclists and more polluting. The benefit which you failed to cite is noise pollution. In some vehicles engine noise will be higher at 20 than 30 depending on gearing ratios, but I am reasonably certain that even if this was the case for the majority tire road noise and air displacement would override this issue.

    Now I have not seen any analysis of road accidents involving vehicles and if these have decreased, there may be benefits for the motorist for this. Sadly though a standard CBA for cyclists at least shows they have lost out from this scheme.

    I would have preferred the money to be spent on road safety that was proven to work, such as collapsible lamp posts, cycle lanes or improved road surfaces. Luckily at least car manufacturers are doing their be to help with pedestrian sensors, collision detection and automatic breaking. Over time these will have massive impacts in road safety, this traffic scheme however will not.

  4. As a Cyclist and a pedestrian for most journeys i have experienced no benefit in 20mph zones they in my experience make the situation 100 times worse. At least in a 30 cars can legally overtake me. I have had more near misses in 20mph zones than anywhere else of the road. I would rather ride along the batheaston bypass than through a 20mph. It really is safer.

    As a car driver and when i’m at work as a delivery driver i have on many occasions experienced violent behaviour and aggressive driving that i never suffered in a 30mph zone.

    So thank you 20mph zone campaigners you have made my life an utter misery. The sooner we get police out pulling over bad drivers regularly and not relying on stupid useless signs and cameras the better.

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