Speed-limited buses effective ‘safety car’ on 20mph streets

bus transport in london

Nobody knows for certain why pretty much every car available for sale in Britain today is capable of breaking so comprehensively the national speed limit, but an even greater mystery is why most public service vehicles are not fitted with speed limiters. Research has demonstrated that buses fitted with speed limiters act as effective ‘safety cars’ – vehicles that slow the speed of other traffic.

Two years ago Transport for London (TfL) began testing an after-market intelligent speed assistance (ISA) system on two bus routes in London – an initiative that formed part of its commitment to halve the number of people killed and seriously injured on London’s roads by 2020.

According to the European Transport Safety Council:

  • Data collected during the trial was compared with pre-trial data to evaluate the system’s effectiveness. ISA was particularly effective when travelling through 20mph zones – helping to ensure other vehicles in the area adhered to the limit.
  • All buses fitted with ISA remained within the speed limit 97-99% of the time. The few incidences of speeding
    occurred on downhill sections – the trialled system did not automatically apply the brakes; it simply prevented acceleration over and above the local speed limit.
  • The percentage of time spent travelling above the speed limit reduced from a range of 15-19% to 1-3% in 20mph
    zones and 0.5-3% to 0-1% in 30mph zones (+/-50km/h).
  • No adverse effect on behaviour was recorded, despite an expected increase in riskier overtaking by surrounding traffic. Indeed, a ‘pace car’ effect saw a reduction in average speeds in 20mph zones.
  • Although there was no significant difference in fuel consumption, there was some evidence of improved emissions in some of the 20mph zones.

Following the success of the trial, it is expected that by the end of this year, over 500 buses will have the technology fitted and all new buses will start to have the technology fitted at point of manufacture. However, it is expected to take a decade for the whole London fleet of 9000 buses to be renewed.

Why do we need speed limited buses?

Quite simply because most drivers refuse to respect speed limits. Lower speeds are particularly important on streets where people live and work or children play and this is the reason many towns and cities have introduced 20mph limits. However, research by the Department for Transport found that 81 per cent of cars recorded at nine sites across the country in 2016 broke the 20mph limit, with 15 per cent travelling at over 30mph – this despite evidence showing that a person struck by a car at 20mph stands a 5 per cent chance of being killed, but that increases to 45 per cent at 30mph.

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  1. edmund white


    The answer is simple, not that it will happen. Ban the sale of cars/vans an such, that can do higher speeds than the speed limits. Stop pushing sales on there 0-60 performance . If you give anyone a car that will do 120-30mph but then say but your not allowed to go that fast, your mad, the urge to try it out will be there.. Not any good for the engine either when the best you can do is 25-35 most of the time in towns

  2. Mike


    Speed limiters on all vehicles is the answer. Once cars can’t go faster than the speed limit, the need for ridiculous over-performing cars will hopefully abate. When will the powers that be wise up to the fact it is now the 21st century and we’ve got a major problem with the planet, a significant contributor of which is the personal car? Let me guess? When the planet is dead?

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