What are the benefits and costs of different road speeds?

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Pedestrians walking in a crosswalk as a car speeds by Pedestrians walking in a crosswalk as a car speeds by

Benefits of higher speed are generally well recognized and promoted as travel time savings, but the true economic costs of speed are often hidden. So, what are the real benefits and costs of speed in urban environments?

Speed has many critical economic impacts: the human and material costs of crashes, health effects of noise and air pollution, the effects of climate change from greenhouse gas (GHG) emissions, fuel costs and vehicle maintenance. However, these costs are often overlooked due to the outsized importance placed on reducing congestion for private motorized vehicle users. While those who speed may be benefiting (though perhaps not as much as is believed), everyone is forced to pay the costs.

Generally, traditional transport planning focuses mainly on the efficiency of trucking and logistics companies and of motorized road users – the typical advocates of the economic benefits of speed --, and does not adequately consider pedestrians or cyclists as road users. For example, driver waiting time is considered in economic modelling, but waiting time for pedestrians is not, even if they are the same person. Car-centric planning influences a myriad of policy decisions such as signal phasing at intersections that prioritize motorized road users, and facilitating inappropriately high speeds where pedestrians are present without regard to the economic value of trips made by foot. It is important to note that victims of pedestrian crashes also tend to come from lower income – and thus less politically influential – groups.

Fundamentally, road traffic speed greatly influences not only traffic safety and operations but also climate impacts and air and noise pollution. In addition to those mentioned above,  higher speeds on roads can generally include the following broad costs:

  • Loss of economic activity at the local level (a recent review of projects in North America showed that reallocating road space to pedestrians and cyclists, which is often the case with speed management projects in cities, results in net benefits for local businesses).
  • Loosing equity of access by increasing risks of pedestrians who have to cross highways to get to work or school and other vulnerable road users. This fosters inequality and poverty; and
  • Loosing opportunities for active transport which exacerbates many inactivity-related health problems such cardio-vascular disease.

Studies of the overall economic impact of speed are rare, and this reflects not only the neglect of the wide range of impacts of speed, but also the dominance of travel time in current analyses, which (mis)guides important transportation policy decisions. Where such studies have been conducted, the economically ideal speeds found are significantly lower than the prevailing speed limits. Figure 1 shows the economic costs of travel at various speeds on a divided carriageway (motorway - not a highway or rural road), in Iran. This graph indicates that the economically ideal speed for the motorway for the society is 73km/h.

Although studies on economically ideal speeds are mainly available for non-urban roads, studies shows that economically optimal travel speed for motorways are between 73 km/h and 85 km/h, well below the 100 km/h to 130 km/h speed limits often applied globally to motorways. In urban environments, the stop-start traffic, larger presence of vulnerable road users, and higher impacts on health of emissions make for much lower economically optimal speeds.

Figure 1: Impacts of speed on multiple components of travel cost

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Figure 2 shows the risk of fatalities for each speed for four different crash types (pedestrian crashes, crashes into rigid objects, side-impact crashes, and head-on crashes) and the importance of speed on the road safety. Speed is a risk factor in all road crashes, ranging from minor collisions to fatalities. These findings provide a strong recommendation for a system of safe speed limits for different road environments.

Figure 2 shows the risk of fatalities for each speed for four different crash types

Fig 2

Source: NSW Centre for Road Safety (n.d), ‘Safe System’ – the key to managing road safety. Fact Sheet 6. Roads and Traffic Authority of New South Wales

Adherence to speeds of 30 km/h or less where pedestrians are present can reduce their fatality risk to a very low level, and there are significant safety benefits for all other road users. Although these benefits are scientifically proved, no low-income countries, and only 3 percent of middle-income countries, have 30 km/h or less speed limits for urban roads, finds the Guide for Road Safety Opportunities and Challenges, recently published by the World Bank’s Global Road Safety Facility (GRSF).

Legislative measures that reduce city speed limits not only reduce injuries and deaths caused by automobiles, but also bring significant cost savings and health benefits. Moreover, with sound urban policies, lower urban speeds promote public transportation, reduce the need to allocate space for cars, increase the attractiveness of active transportation, and create more space for urban recreation and commerce, resulting in more livable and vibrant cities.

Thus, speed management is an inclusive solution for all road users globally and should be and are being facilitated by advocacy efforts by a wide range of NGOs and advocacy groups along with provision of information from researchers and with promotion from organizations such as the World Bank and GRSF.

Acknowledgement: Preparation of this blog was supported by two of the donors to the Global Road Safety Facility: UK Aid and Bloomberg Philanthropies. This blog was improved by suggestions from Blair Turner and Giannina Raffo of GRSF. The views and opinions expressed do not necessarily represent those of the World Bank.



Kazuyuki Neki

Junior Professional Officer

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