Speed is a risk factor for absolutely all crashes, ranging from the smallest fender-bender to fatal crashes (Elvik,2004). Reductions in speed limits are intended to improve road safety by decreasing travelling speed and thus reducing the risk of crashes occurring and the severity of crashes that do occur. Specifically, speed limit reductions in urban residential areas have been recommended by researchers and practitioners alike (Islam et al., 2014).
As urbanization and motorization continue to grow, a speed limit of 30 km/h should be standard in all places where cars, cyclists, and pedestrians interact. Setting a speed limit of 30 km/h (20 mph) where people and traffic mix, make streets safer, healthier, greener and more liveable. Streets that promote safe walking and cycling can reduce car dependency and harmful vehicle emissions that contribute to climate change (WHO, 2021).
Most importantly, people have a 90% chance of surviving after being hit by a car or a truck going at 30 km/h, but less than 50% at 50 km/h or higher (Benefits of lower speed limits, 2021). The critical impact speed for serious injuries is 20 km/h for pedestrian collisions and 30 km/h for most other types of passenger car collisions. The risk of death is almost five times higher in collisions between a car and a pedestrian at 50 km/h compared to the same type of collisions at 30 km/h(Why 30 km/h?)
Based on crashes in France the mortality risk of pedestrians, when hit by a car was low (about 1%) at an impact speed of 30 km/h, but increased by a factor of 2 at 40 km/h, a factor of 6 at 50 km/h and a factor of 18 at 60 km/h (European Road Safety Observatory, 2021).
In July 2016, the city of Edinburgh lowered the speed limit on almost all of its roads from 30 mph to 20 mph. The city center, main streets, and residential roads all became 20 mph zones. The zones with a reduced speed limit saw 38% decrease in the number of road crashes (371 fewer crashes per year), including fewer crashes involving cyclists and pedestrians (VICE, 2022).
Graz in Austria was the first major European city to adopt a zone of 30 km/h limit, applying to all city areas, in 1992. Nowadays, in this city covering some 127.58 km², 80% of roads are limited to 30 km/h. One of the aims of this measure was to improve air quality and to reduce reliance on cars (City30-brussels). There has been a 12% reduction in the number of road crashed and a 24% decrease in serious injuries (ROSPA,2017).
Brussels is a 30 km/h zone established on 1 January, 2021. Maximum speed is 30 km/h on all roads in the Brussels Capital region, except of the major axes where the speed limit remains 50 or 70 km/h. Five months after installing the general speed limit of 30 km there was an overall 10% decrease in the number of road crashes, while one year later, a 50% reduction in road fatalities was identified (City30-brussels).
Bilbao in June 2018, began reducing the speed limit to 30 km/h with the triple aim of reducing noise, pollution and increasing road safety. It was named European City of the Year at the 2018 Urbanism Awards and is the first city in the world with more than 300,000 inhabitants to have introduced a 30 limit on all of its streets. Between 2019 and 2020, the city had reduced by 23% the road crashes (Eurocities, 2021). Indeed, the groups that were initially most resistant to the measures (taxi drivers, traders and delivery drivers) are now happy to acknowledge that the lower speed limit does not cause them any problems and in fact improves traffic flow (City30.brussels, 2022).
A study by the French national research institute Cerema found that the number of pedestrians injured or killed in Grenoble had been 50% reduced since the city has set 30 km/h as speed limit (City30-brussels). As for Lille, it is the first city in France (on August 2020) with over 500,000 inhabitants that has lowered its speed limit to 30 km/h almost everywhere gradually. The main benefit after three years is that the number of cyclists had significantly increased by 55% more comparing to 2016 (City30.brussels,2022).
Referring to the capital of France, Paris, since the 30th of August 2021 vehicle traffic has been limited to 30 km/h, except for selected roads such as the Champs Elysées (50km/h) and the main ring road, the Boulevard Périférique (70 km/h) (La vitesse limitée à 30 km/h, 2021). Despite some criticism, officials in the French capital said their decision was in line with a survey that showed 59% of Parisians were in favor of the new speed limit (Paris introduces citywide 30 km/h speed limit, 2021). The implementation of this measure led to a 25% decrease in the number of road crashes and a 40% decrease in those considered serious and fatal (Polis, 2021).
Areas with 20mph limits, such as the city of Portsmouth and Camden in London had a reduction of 22% and 54% in road crashes accordingly (British and Swiss public back 30km/h limits, 2014). Furthermore, cutting from 50 to 30 kph in Munster, Germany has as result the drop of number of people severely injured in road crashes by 72% (Reducing speed limits, 2021).
The first widespread evaluation of the equivalent 20mph zones in the UK found that injury crashes were reduced by 60%, and child injury crashes were reduced by 67%. In the 20mph zones in Hull, UK, there was a decrease in total crashes of 56% and in fatal and serious injuries of 90%. The largest reductions were pedestrian casualties, which were reduced by 54%, child casualties were reduced by 54% and child pedestrian casualties were reduced by 74% (van den Dool, 2017).
In 2004, Helsinki introduced large-scale 30 km/h restrictions and then the system was extended in 2019. First it was used in the city centre and some residential areas, then speed restrictions were modified in effectively all the streets. After the 2004 change, in the streets with lower speed limits traffic crashes resulting in personal injury decreased by 9 percent. In places where the speed limit was changed from 40 km/h to 30 km/h pedestrian injuries decreased by 19 percent and vehicle damages by 34 percent. The biggest improvement was observed in the city centre, where the number of traffic-related injuries decreased by 42 percent. There is also a recent, 2022 poll concluding that 77 percent of Helsinki citizens feel safe in traffic, and only 5 percent answered that they find transportation in the Finnish capital dangerous (None of the European Cities that lowered the speed limit to 30 km/h regrets it, Arato, 2023).
Several cities worldwide have started setting speed limits of 30 km/h in large parts of the City.
Brussels and Paris have introduced 30km/h limits in order to improve air quality and reduce noise pollution and traffic collisions. Indeed, a 30 kph speed limit reduces noise by 3 decibels (Reducing speed limits, 2021). The reason for reducing Zurich’s speed limit was first and foremost to reduce noise pollution.
Switzerland's government is currently discussing 50 and 30 limits at national level, but Zurich has already gone ahead and installed 30 km/h limits both day and night on 400 km of roads (out of a total of 750 km). After the implementation of this measure, a 25% reduction in road fatalities was identified. The incidence of car-pedestrian crashes was cut by 16% and the number of injured pedestrians by 20%. By the end of 2021, the Dublin City Council's Transportation Department intends to do the same. Luxembourg has extended its 30km/h zones to all built-up areas, as have Oslo and Helsinki, where not a single pedestrian or cyclist was killed in a road crash last year (Benefits of lower speed limits, 2021).
Based on a Welsh Government report, the direct estimated costs of introducing the 20 mph default will be £32.3M. This report estimates the casualty savings of 20mph, in the first year alone, to be just over £92M; nearly three times higher than the implementation cost. The benefits, over a three year period, are between eight and nine times higher than the costs of implementation (£275.8M). Road crash casualty savings in the first year alone are almost three times the implementation costs (Andrian Davis, 2022).
The effects of the implementation of 30 km/h zones are multiple. First, it is far easier for motorists to merge with traffic travelling at 20mph than at 30mph. Furthermore, motor traffic volumes decrease, since slower speeds encourage active, sustainable, and shared travel. Moreover, buses operate more efficiently. The reduced length of traffic queues reduces bus journey, increases reliability and makes buses become a more attractive alternative to the car. Additionally, children are more likely to walk or cycle to school on their own and older people are more confident in venturing outside their homes, trying to cross the street, or of driving their own cars at a more reasonable (i.e., slower) speed, rather than always at 30mph.
The strong relationship between speed and the risk of injury and of death applies to all road users involved in crashes. Legislative, enforcement, and road engineering actions to reduce urban speed limits will not only reduce crash injuries and deaths, but will also provide significant cost savings and health benefits delivered by transport noise and air pollution reduction, and increased pedestrian and cyclist active mobility (Neki,2021). All of these benefits accumulate in addition to those monetised, but still grossly underestimate the vast range of potential benefits from the implementation of 20mph and the interactions between lowered speeds, better health and well-being, health care spending, commercial benefits, social benefits and the climate emergency (ETSC, etsc, 2022; Andrian Davis, 2022). When implementing them requires nothing more than some new signage, it is one of the technologically easiest ways of improving safety and reducing emissions (Reducing speed limits, 2021).
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