Speed Limits

Speed Limits

How Effective?

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

  • Physical measures that restrict the speed at which a road can be travelled (or comfortably travelled) can be highly effective in reducing vehicle speeds and accidents:

    • The introduction of road humps can result in large accident reductions:

      • 71% reduction in accidents at 34 sites with the introduction of road humps. (Webster, 1993, as cited in DfT, 2007)

      • 60% reduction in accidents with the introduction of road humps in 20mph zones. (Webster and Mackie, 1996, as cited in DfT, 2007)

      • 89% reduction in accidents for an average speed reduction of 13 mph upon the introduction of road humps. (Hampshire County Council, 1996, as cited in DfT, 2007)

      • 86% reduction in accidents upon introduction of speed cushions. (CSS et al., 1994; Northamptonshire County Council, 1998, as cited in DfT, 2007)

      • 97% reduction in accidents at sites where ‘thumps’ (thermoplastic humps) were introduced. (Webster, 1994, as cited in DfT, 2007)

    • Narrowings and chicanes can reduce vehicle speeds and reduce accidents:

      • Overall by around 7mph (though individual schemes achieved between 1 and 19mph). (Hass-Klau and Nold, 1994, as cited in DfT, 2007)

      • Road space reallocation achieved a reduction in mean speed of 7-8mph. (Kennedy et al., 2005, as cited in DfT, 2007)

      • Traffic islands and pedestrian refuges provide modest speed reductions of 1-5mph though extra care should be taken to ensure risk for pedal cyclists is not increased through their implementation. (Thompson et al., 1990; Cloke et al., 1999; Boulter, 2000, all as cited in DfT, 2007)

      • Chicanes reduce vehicle speeds to 20mph where the deflection angle is greater than 15 degrees and to 25mph where the deflection angle is greater than 10 degrees. A 54% reduction in injury accident frequency was observed. (Sayer et al., 1998, as cited in DfT, 2007)

    • Various remedial treatments have been found to be effective in lowering speed choice through psychological mechanisms:

    • 2-7mph reduction in speeds from vehicle activated signs. (Winnett and Wheeler, 2003, as cited in DfT, 2007)

    • 6-7% reduction in speed where advance curve warning markings (SLOW) accompanied by a bend warning curve and transverse markings were used. (McGee and Hanscom, 2006)

    • 3mph reduction in speed (measured at the apex of a bend) where transverse rumble strips were used on the approach. (Barker, 1997)

    • Optical bars/ transverse markings

      • 0-5mph reduction in speed.

      • (McGee and Hanscom, 2006)

      • 57% reduction in speed related crashes as a result of transverse yellow bar markings being introduced on the approach to roundabouts.(Helliar-Symons, 1981)

    • 12mph reduction in vehicle speeds where converging chevron bars on an exit ramp of a motorway were introduced. (Drakopoulos and Vergou, 2003)

    • Gateways

      • 1-2mph further reduction in speed for basic gateway treatment when compared with just the introduction of a speed limit sign.

      • 5-7mph reduction with high visual impact measures (e.g. dragons teeth/coloured road surfacing).

      • Up to a 10mph reduction where physical measures are used (e.g. narrowings etc.). (Wheeler and Taylor, 1999, as cited in DfT, 2007)

  • Safety cameras have a clear and positive impact on vehicle speeds and safety (see also the ‘Safety Cameras Synthesis):

    • The RAC has undertaken a comprehensive review of the effectiveness of safety cameras finding that deployment of speed cameras leads to appreciable 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. It is estimated 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. (Allsop, 2010)

    • 20% reduction in injury crashes in Norway upon introduction of safety cameras on rural roads. (Elvik, 1997)

    • In Australia where fixed speed cameras were introduced at high risk sites, crashes fell by almost 20%; casualty crashes fell by 23%, injury crashes by 20%, and fatal crashes by nearly 90%. (ARRB, 2005)

    • 33% reduction in injury crashes for rural safety camera installations. (Gains, Heydecker, Shrewsbury and Robertson, 2004, as cited in OECD, 2006)

    • Hidden safety cameras on a motorway in New Zealand yielded a further 11% reduction in ‘open road’ crashes and 19% reduction in casualty rate when compared with visible cameras. (Keall, Povey and Frith, 2001)

    • Average speed cameras were installed on the A14 between Huntington and Cambridge in 2007. Analysis of the accident rates after installation has shown that a reduction of accidents of 20% can be attributed to the implementation of these cameras. (DfT, 2011)

    • 10km/h speed reduction on an Australian motorway with the introduction of average speed cameras. (Stefan and Winkelbauer, 2006)

    • Reduction of violations to below 1% on a Dutch stretch of freeway with the introduction of average speed cameras. (RWS, 2003)

  • Variable (mandatory) speed limits reduce vehicle speeds and can improve safety:

    • The introduction of variable mandatory speed limits on M25 in 1996 achieved a 15% drop in injury crashes. (DfT, 2010)

    • In St Louis, US: 4.5-8% crash reduction. (Bham et al., 2010)

    • In France on the A7 road:

      • 48% reduction in crashes. (Serti, 2006, as cited in Traffix Group, 2009)

      • 77% reduction in serious crashes. (ASF, 2007, as cited in as cited in Traffix Group, 2009)

    • In Germany (autobahns): 20-30% reduction in crashes. (Robinson, 2000, as cited in Traffix Group, 2009)

    • In Finland: 13% reduction in risk of injury. (Rama and Schirokoff, 2004, as cited in Traffix Group, 2009)

    • In Netherlands: 35% reduction in serious crashes. (FHA, 2003, as cited in Traffix Group, 2009)

  • Signs that are activated by vehicles according to their speed (Vehicle Activated Signs (VAS)) and signs that display to drivers their speeds (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).

    • In the UK the effect of VAS that display a curve/bend warning sign when a vehicle is travelling above a certain threshold speed (set at the 50th percentile speed) was investigated at three rural curves. A reduction in mean speed of between 2 and 7mph (3 to 11 km/h) after one month was observed. (Winnett and Wheeler, 2003, as cited in DfT, 2007)

    • A 1.4mph speed reduction was observed at sites where SIDs were operational in the Royal Borough of Kingston-Upon-Thames. The speed reduction observed varied from 0.6 mph to 2.6 mph. The proportions of drivers exceeding 30 and 36 mph were significantly reduced at 10 out of the 11 sites. There was evidence of a novelty effect, with SIDs being most effective in the first week of operation. (Walter and Knowles, 2008)

    • 6-8mph reduction in vehicle speeds was found when DSMD were used at transitions to an urban area; this effect was still present after one year. (Sandberg, Schoenecker, Sebastian and Soler, 2006)

    • DSDS effectiveness decreases with time and are only effective over a short distance. (Ardeshiri and Jeihani, 2013)

    • The impact of different types of DSDS have been compared (numeric, numeric coloured and text based signs that say ‘slow’, ‘slow down’ or ‘thank you’). All DSDS led to a reduction in vehicle speeds: average speeds reduced by between 0.7 and 3.1km/h, typically from 31mph, and 85th percentile speeds by 1-3km/h, typically from 37mph. Verbal coloured signs were the most effective. The impact reduced as time elapsed. (Gehlert, Schultze and Schalg, 2012)

  • Road geometry can have a significant impact on vehicle speeds, particularly:

    • Parking was found to reduce speeds on links and at junctions by 2-5mph.

    • The largest effect on speeds was found to be associated with reducing lines of sight. A reduction from 120 to 20m reduced approach speeds by approximately 20mph on links and 11mph at junctions. (York et al., 2007)

    • An interruption to a journey, caused by stopping at a red traffic light, can result in failure to resume the speed of travel prior to the interruption. The addition of a reminder cue could offset this interruption.
      (Gregory et al., 2014)]

    • The more restricted drivers become the more likely they are to comply with speed limits; potential restrictions include street parking, bike lanes, pedestrian crossing, or the absence of shoulder lanes.
      (Gargoum and El-Bayouny, 2015)
  • Date Added: 22 Aug 2013, 04:15 PM
  • Last Update: 05 May 2017, 11:59 AM