Speed limits on British roads are by no way a new phenomenon and their history can be traced back as far as 1861.

However, it was not until the 1960′s that the Government created the statutory regime of the Road Traffic Regulation Acts which controlled the setting of speed limits by highway authorities – generally local authorities. Where restrictions such as 30 mph speed limits without the presence of street lighting existed, the Department of Transport had to authorise the limits and create traffic orders.

World-leading traffic engineering

B1 30-limit-sign-ckThis system, based on decades of research and years of experience resulted in a national working practice which ensured that roads were engineered to encourage safe speeds with the need for only minimal signing.

National standards for both engineering and signing were created. These produced sustainable and compliant restrictions meaning that drivers in general did not speed or, drove within a design threshold that was safe for the highway environment.

This in turn resulted in only a small percentage exceeding the nationally accepted design / enforcement threshold.

The benchmark publication ‘Circular – Roads 01/93′ issued by the Government, stated that “local speed limits are normally unnecessary where the character of the road itself limits the speed of most vehicles (95%) to a level at or below the limit under consideration”.

Think 20 mph. If a road network in an area is so meandering and narrow that no one can reasonably drive above 20 mph, then why apply a speed limit by way of an order (expensive) or providing signing (further expense). Effectively, all you are doing is telling drivers at what speed they are driving.

The Transport Research Laboratory (TRL) reinforced this position by publishing independent research that found that when a speed limit was imposed on a road that was lower than the design speed of the road, and only doing so by the placing of traffic signs, the resultant speed reduction averaged out at a mere 5%. So in the case of dual carriageways, with a safe design speed of 70mph, the modern trend for imposing say a 30 mph limit, would mean than most drivers would continue to drive safely at 66.5 mph (assuming that they are confident that no enforcement cameras are in place).

Effectively, highway authorities considered the speed that most drivers tended to drive a stretch of road, (the 85th percentile – see below) and assuming that the road was not producing a disproportionate number of accidents, that defined the chosen speed limit.

Before the arrival of Safety Camera Partnerships and the politicisation of speed limits, the Association of Chief Police Officers (ACPO) following government research, policy and consultation, set the national prosecution threshold at 10 mph + 1 mph as the design and engineering of roads. In addition, and in line with ACPO policy, each limit was created 2/10th of a mile longer than necessary to provide a buffer zone for drivers to naturally slow to compliant speed and to cater for the natural acceleration into higher limits. The resultant generally compliant vehicle flow speeds meant little or no enforcement was necessary and any engineering solutions required were small and easily installed.

Where limits were found not to match typical speeds, a highway authority had two choices:

  • Engineer the road to produce a sustainable speed reduction (no enforcement needed) or;
  • Increase the applied limit to reflect the natural limit of the geometry and traffic flow on the road.
  • The first impacted upon limited budgets and the second was seen as a political nightmare in the age of the ‘speed kills' mantra.

    Speed cameras arrive

    B2 speed-camera-by-cinema-ckCampaigning by those not associated with the enforcement industry, and often with an anti-car agenda, leading up to and post millennium, led to the introduction of technology driven enforcement (speed cameras).

    In order to qualify for cameras, authorities had to demonstrate both a history of collisions and defective limits.

    The government had dealt with defective limits within its working practices and rules set out in ‘Circular – Roads 01/93′. In order to produce defective limits and therefore qualification for camera enforcement, some authorities actively sought to impose limits they knew would not be complied with just to meet the statistical requirements set by the new Safety Partnership practices.

    Speed cameras were seen by some as good politics, allowing specialist (but highly important) road policing officers to be re-deployed to other duties whilst providing a mechanism for local authorities to ‘pass the buck' on road safety to others in the safety camera partnerships – police and the camera partnerships themselves. Historically, the police were seen as trusted experts on matters of speed limit setting, now this role was marginalised politically, unless the Chief Constable agreed with the ‘policing by numbers' approach to traffic speed management.

    Despite the fact that many speed limits were being set clearly too low, it was now fair game to blame (and fine) motorists for driving at a speed that used to be considered acceptable. It is far easier and cheaper, for an authority to impose a limit it knows drivers will exceed than to undertake research, engineer permanent solutions and provide a compliant restriction.

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