Traffic management: speed limitations
Traffic calming aims to reduce the dominance and speed of motor vehicles. It employs a variety of techniques to cut vehicle speeds to improve safety and environmental quality in urban areas. Usually, traffic calming applies as an area-wide measure. If applied only to a particular street, it runs the risk of pushing accidents, pollution and cut-through driving into neighbouring areas. Traffic calming originated in residential areas but is increasingly used in whole city areas.
Traffic calming techniques include low-speed zones – 20 or 30 km/h – and/or the use of calming infrastructure including speed humps, lumps or bumps, stop signs and traffic circles. Often, both speed limits and infrastructure elements combine for effective traffic calming.
For connected vehicles, speed limitations can be distributed online from Traffic Management Centres. Mandatory onboard automatic speed-limiter features and localisation equipment ensure compliance with speed limits on any road section at any time.
Reducing speed limits from 50 km/h to 30 km/h can reduce fuel consumption – and related emissions – by 7%, if rapid acceleration and deceleration are avoided. A 10 km/h speed reduction from 60 km/h to 50 km/h can achieve a 5% reduction in CO2 emissions. There is much debate as to whether and to what degree speed limits achieve CO2 and other pollutant-emissions reductions, however. The most fuel-efficient driving occurs at around 50–60 km/h.
The costs of implementing a low-speed zone consist of road infrastructure costs (changing signage, speed bumps etc.), information costs (communication of the new speed limits) and enforcement costs (installation of radars, policing, prosecution costs etc.). Fines imposed on drivers caught not respecting the limit can help recover costs and potentially generate revenues for local authorities.
As an indication, the British city of Portsmouth spent GBP 630 000 to implement a 30 km/h zone across 1 200 streets.
Low-speed zones and other traffic-calming measures reduce road accidents and related costs. Case studies demonstrate a reduction of traffic-related injuries of between 25% and 64%.
Where low-emission zones increase vehicles' fuel efficiency, pollutant emissions are reduced.
Low-speed zones can encourage more-aggressive driving outside the low-speed zone, and less-smooth driving styles within the zone. The latter especially applies where speed bumps or other infrastructure elements are used, which are likely to cause additional decelerations and accelerations. For example, CO2 emission increases due to a speed bump can be in the range of 4% for one bump over 1.5 km, to 60% for two bumps within 300 metres.
ITF (2021) Transport Climate Action Directory – Traffic management: speed limitations
https://www.itf-oecd.org/policy/speed-limitations
Ahn and Rakha (2009) A field evaluation case study of the environmental and energy impacts of traffic calming. Transportation Research Part D: Transport and Environment, Volume 14, Issue 6, Pages 411-424 : https://doi.org/10.1016/j.trd.2009.01.007
Ahn and Rakha (2016) A field evaluation case study of the environmental and energy impacts of traffic calming.
Boulter et al. (1999) Driving cycles for measuring passenger car emissions on roads with traffic calming measures.
Boulter et al. (2001) The impacts of traffic calming measures on vehicle exhaust emissions https://core.ac.uk/download/pdf/17301234.pdf
Copenhagenize Consulting (?) Analysis of 30 km/h Zones.
Crabbe and Elsom (1998) Air quality effectiveness of traffic management schemes: UK and European case studies. https://doi.org/10.1023/A:1005924529092
Federal Highway Administration (FWA) (1994) . https://safety.fhwa.dot.gov/PED_BIKE/docs/case19.pdf
Federal Highway Administration (FWA) (1994) FHWA CASE Study No. 19 Traffic Calming, Auto-Restricted Zones and Other Traffic Management Techniques. https://safety.fhwa.dot.gov/PED_BIKE/docs/case19.pdf
Litman (1999) Traffic Calming Benefits, Costs and Equity Impacts http://www.vtpi.org/calming.pdf
Meers and Roth (2001) Road safety and ecological sustainability working together. http://atrf.info/papers/2001/2001_Meers_Roth.pdf
Panis and Broekx (2006) Impact Of 30 Km/H Zone Introduction On Vehicle Exhaust Emissions In Urban Areas.
Panis, L. I., Broekx, S. and Liu, R. (2006) Modelling instantaneous traffic emission and the influence of traffic speed limits. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.589.9937&rep=rep1&type=pdf
Woolley et al. (2002) Impacts of lower speed limits in Southern Australia.
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