Speed Limits

Speed Limits

Summary:

  • Effective speed management comprises a series of interventions that can have great benefits for road safety; one intervention is the setting and signing of speed limits.

  • According to OECD (2006) most countries have general speed limits within the following ranges:

    • Urban roads: 18-31 mph (30-50 km/h).

    • Main highways or rural roads: 43-62 mph (70-100 km/h).

    • Motorways: 56-81 mph (90-130 km/h).

  • World Health Organization (2017) provides guidance for safe speeds:

    • Roads with possible conflicts between cars and unprotected users – 30 km/h.
    • Intersections with possible side-on conflicts between cars – 50 km/h.
    • Roads with possible frontal conflicts between cars – 70 km/h.
    • Roads with no likelihood of frontal or side-on conflicts between road users - ≥100 km/h.

  • Historically, speed limits have been set in accordance with the prevailing speed of traffic. OECD (2006) suggests that this approach is no longer viewed as appropriate; in response new principles are emerging:

    • Setting speed limits based on an assessment of the combined risk relating to the infrastructure, travel speeds, volume and mix of traffic by type (including vulnerable road users).

    • Setting speed limits based on the safe system approach.

  • In addition, there is a recent emphasis of the notion that speed limits should be credible or self-explaining to encourage self-compliance (DfT, 2013; Van Schagen, Wegman and Roszbach, 2004, as cited in SWOV, 2012; Goldenbeld, Van Schagen and Drupsteen, 2006, as cited in SWOV, 2012; Houtenbos et al., 2011, Lee et al, 2016) and applied consistently across the road network reflecting the function of the road to which they are applied.

  • Even small changes to the speed travelled by the driving population lead to large and measurable changes in risk. A 5% increase in mean speeds leads to an increase in injury crashes of 10% and an increase in fatal crashes of 20% (Nilsson, 2004).

  • Meta-analyses show that lowering the speed limit alone by 10km/h leads to a decrease in mean speeds of 3-4km/h (OECD, 2006).

  • A 10 km/h reduction, from 110 km/h to 100 km/h on roads sections in Australia was found to result in a 27.4% reduction in the number of casualties when matched with the control roads (Mackenzie, Kloeden and Hutchinson, 2015).

  • Reducing speed from 90 km/h to 70 km/h on a considerable number of highways was found to result in a 5% decrease in crash rates, after taking trend into account (De Pauw, Daniels, Thierie and Brijs, 2014)

  • Reducing speed limit from 50 km/h to 40 km/h was found to produce significant reductions in mean free-flow speed and speed variance (Islam, El-Basyouny and Ibrahim, 2014)

  • On the other hand, a study from Israel suggest that increasing speed limits on highways withby 10 or 20 km/h could result in increases in safety of 18 to 21% (Harari, Musicant, Bar-Gera and Schechtman, 2017)

  • To maximize their impact, any changes in speed limits should be accompanied by appropriate enforcement, engineering and educational measures (OECD, 2006; DfT, 2013).
  • In most countries, national speed limits are communicated to drivers through driver training and licensing, through a highway code, and reinforced through signs on the road.

  • Signs are the primary way of communicating the speed limit of a road to drivers, especially where this changes from one location to the next. Principles of effective speed limits signing are: uniformity, consistency, simplicity, relevance and legibility.

  • Clear signing where a speed limit changes, coupled with repeater signs, helps the effective communication of speed limits to drivers (OECD, 2006).

  • Without clear speed limit signs that are in accordance with national legislation, speed limits cannot normally be enforced.

  • Gateway treatments are useful where a speed limit reduces by a large amount (Kennedy et al., 2005).

  • If speed limit compliance is poor, there are several options available that might be used to improve compliance:

    • Clarify speed limit signing and introduce additional measures such as gateway treatments where there is a large variation in speed limits between two sections.

    • Introduce engineering measures that have a psychological impact on driver speed choice.

    • Physical engineering treatments that reduce vehicle speeds.

    • Enhance enforcement.

    • Ensure that the road features are safe at travelled vehicle speeds.

    • Educate the public on the importance of reducing their speed.

  • Traffic calming measures that physically restrict the speed at which it is possible (or comfortable) to drive the road can be highly effective, resulting in speed reductions of around 7mph, from 36.5mph to 29.5mph, and accident reductions of up to 97%, from 13.0 to 0.4 accidents per year (DfT, 2007).

  • Various remedial treatments have been found to be effective in lowering speed choice through psychological mechanisms; these measures can reduce vehicle speeds by up to 12mph.

  • Effective enforcement at a single location introduces a localised deterrent to drivers. Safety cameras can be highly effective in reducing vehicle speeds and the number and severity of injury collisions. The RAC Foundation has undertaken a comprehensive review of the effectiveness of safety cameras finding that deployment of speed cameras leads to reductions in speed in the vicinity of cameras and substantial reductions in crashes and casualties at those locations in addition to that which is attributable to regression-to-the-mean (Allsop, 2010). Allsop estimates that, in the year ending 2004, safety camera operations at more than 4,000 sites across Great Britain prevented 3,600 personal injury collisions and saved around 1,000 people from being killed or seriously injured. In a later comprehensive review of the effectiveness of average speed cameras, undertaken by RAC and RSA, the finding suggests a 36% reduction in the mean rate of fatal and serious crashes, after accounting for trend, RTM and site selection periods (Owen, Ursachi and Allsop, 2016). On the A14 in the UK, average speed safety cameras achieved a 20% reduction in crashes. When safety cameras were introduced on rural roads in Norway there was a 20% reduction in injury crashes (Elvik, 1997, as cited in Elvik, Hoye, Vaa and Sorensen, 2009); at high risk locations where safety cameras have been introduced, fatal crashes have been reduced by nearly 90% in Australia (ARRB, 2005). Average speed cameras can reduce vehicle speeds by 10km/h, from around 85km/h to around 75km/h (Stefan and Winkelbauer, 2006) and violations to below 1% (RWS, 2003, as cited in OECD, 2006). On road sections between 100 m upstream and 1 km downstream of the speed cameras a statistically significant reduction of the number of injury crashes by 22% was found (Hoye, 2015). Automated speed camera programme introduced in France between 2002 and 2006 saw a drop in death among young people by 40% (Atchinson, 2016).

  • Variable mandatory speed limits applied on motorways reduce speed differential and smooth traffic to reduce congestion. Upon introduction to a section of the M25 in 1995 a 15% drop in injury crashes was observed (DfT, 2010). Similar results have been observed across the world.

  • Vehicle Activated Signs (VAS), Dynamic Speed Monitoring Displays (DSMD), Dynamic Speed Display Signs (DSDS) and Speed Indicator Devices (SIDs) all have a positive impact on vehicle speeds (reduction in mean speeds of between 2 and 7 mph. Whether the impact of such devices is long-term remains an issue of contention.

  • Intelligent Speed Assistance (ISA) can help improve drivers’ compliance with speed limits by alerting them when they are travelling above the posted speed limits (WHO, 2017). ETSC recommends that all European countries should adopt legislation for fitting all new vehicles with an overridable Intelligent Speed Assistance system (ETSC, 2016). 
  • Date Added: 22 Aug 2013, 04:15 PM
  • Last Update: 05 May 2017, 11:55 AM