How wide-area 20mph speed limits are better value and more effective than engineered schemes
Updated: Feb 27
Many local authorities assume that 20mph speed limits will be ignored by drivers and insist that physical calming measures - including speed bumps are the best solution for implementing 20mph speed limits. In fact, it is a common myth that drivers ignore 20mph speed limits in areas without traffic calming. Most drivers do comply without the need for additional measures.
It is also the case that often local authorities (including Herts County Council) will insist on engineered measures for roads with an existing average speed greater than 24mph. There is much evidence, though, that setting 20mph speed limits on roads that previously saw an average speed greater than 24 mph will be the ones that see the most benefit. This has been seen in Highway Authorities such as Brighton , Bristol  and Portsmouth  where 20mph limits brought in on faster roads (notably without physical calming), where most casualties occur, have seen the greatest speed reductions. Other inexpensive traffic calming measures can also be used to deal with compliance issues, such as signage, lines, staggered parking bays and planters.
The Population Paradox
Another justification that's given for using physical calming measures is that there are often only small reductions in speeds on roads that have introduced 20mph speeds limits. However, even with seemingly small decreases in average speeds, research has found that every 1mph fall in average speeds reduces casualties by 5-6%. There is also the case for the numerous other benefits of 20mph speed limits. The population paradox demonstrates how small speed reductions over wide areas bring more benefits than big interventions in specific places (eg accident hot spots). Road danger reduction which limits drivers to 20mph on most built up streets prevents around 20% of casualties and means a wide range of other benefits for communites such as better air quality, reduced noise pollution and increased social interaction.
Value for Money
Of course, the other big problem with engineered solutions (including speed bumps) is that they are very expensive and time-consuming to install. In fact, engineered schemes cost, on average, fifty times more than wide-area sign only schemes (with community engagement and education). For the same cost of providing say 250 people on a single road with speed bumps, you could provide a community of 12,500 people a wide-area, sign-only 20mph scheme with education and community feedback.
20mph in Watford - the Mott Report
In 2017, 20's Plenty for Watford and Watford Borough Council commissioned a report (you can read about it here) from Mott Macdonald - engineering consultants - to look into the cost implications of engineered versus wide-area 20mph schemes in the town. The study found that Herts County Council had already spent more than £3 million to install 20mph limits covering just one third of the town. This would mean that their Speed Management Strategy (SMS) approach would cost around £10 million to cover the whole of Watford - the smallest, by area, local authority in the country.
Following the latest update to their Speed Management Strategy in December 2020, Herts County Council have since announced that they propose to spend £7 million on 20mph schemes over the next four years. There are no details, as yet, of where the money will be spent. However, there will still be a heavy reliance on physical calming measures (including speed bumps), particularly on roads that have an existing average of 24mph or more. Using the Watford case study as an example, it's clear that this money will not stretch very far.
If, in contrast, the County Council were to adopt wide-area 20mph limits, with a sign-only schemes and community engagement, the cost of introducing 20mph speed limits for the majority of Herts is estimated at around £3.3 million (based on an average cost of £3 per head of population). Normalising 20mph speed limits is, in fact, seven times more cost effective than using speed bumps and other physical calming measures and can be rolled-out over a much wider area.
Of course, the real game changer will be the introduction of Intelligent Speed Assistance (Speed Limiters) which will be installed in all new models of car in 2022 and all new cars by 2023. ISA uses a speed sign-recognition camera and/or GPS-linked speed limit data to advise drivers of the current speed limit and automatically limit the speed of the vehicle as required. This will mean that speed bumps will become more and more obselete, as much greater compliance with speed limits will be inevitable. In fact, drivers will have to make a conscious decision to speed. Vehicles fitted with ISA will act as pacers for any cars following them, meaning that almost no speed enforcement will be needed at any speed limit.
Vulnerable Road Users
Adopting a policy of installing small-area bumps rather than default, wide-area 20mph schemes can also fail to provide a duty of care for children and other vulnerable road users. Children up to the age of twelve-years-old cannot cross the road reliably because they lack the accuity to judge speeds of over 20mph effectively (looming). This places them at greater risk from road danger and means that they are more likely to cross the road into oncoming traffic. The polluting-effects of speed bumps (both air and noise) have a further impact on vulnerabe road users and they also pose a problem for many drivers with a disability (especially those with back injuries) as the action of going over the bumps is hard on spines. The NICE (National Institute for Health and Care Excellence) Guidance on Outdoor Air Quality and Health even goes as far as recommending the removal of speed bumps in 20mph areas, to reduce air and noise pollution levels and promote smooth journeys.
 WS Atkins: effectiveness of 20mph speed limits https://www.gov.uk/government/publications/20-mph-speed-limits-on-roads
 WS Atkins interim evaluation http://www.wirralpedestrians.org.uk/files/20mphzoneresearch.pdf