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The term speed bump is used to cover a range of speed reduction measures installed on our roads. Also called Speed Cushions or Sleeping Policemen, they have generated a lot of discussion in the last couple of decades, but seem to be here to stay as a relatively cheap method of slowing traffic down.
There are a number of designs in use across the country, but they can generally be categorised as one of the following:
Full-width Speed Bump
This sees a raised section of road spanning the full width of the carriageway, albeit sometimes with a narrow break against either kerb. This break serves the dual purpose of allowing cyclists to avoid the bump and also maintaining the run of the gutter for drainage purposes. When the road is unkerbed, however, the bump is normally the full width of the road.
The bump itself is often built out of Tarmac, laid in a roll on the surface of the road. There are a variety of design standards for different types of road, detailing maximum height, width and angles. However, as this simple type of speed bump is also popular on private roads leading to holiday parks, tourist attractions and so on, these design standards are not always met. The bumps can also be built out of other materials, notably brick, and these usually include a flat top, unlike the simple tarmac versions.
Part-width Speed Bumps
A development of the speed bump is the speed cushion, a small raised, squarish speed bump placed centrally in each lane. They are designed to be wide enough that larger vehicles such as buses can line up and pass unimpeded, whilst cars with their narrower track cannot straddle the bump. However, in practice, modern cars are not that much narrower than small vans, which are used as mini buses and ambulances (amongst other things) - the vehicles that the bumps do not want to impede! As such, many cars are also able to straddle the bumps. In addition, as many are installed on quiet suburban roads with on street parking, the bumps can get obscured by parked vehicles, leading to all vehicles being impeded by the need to straddle the central gap, rather than one of the bumps.
The bumps themselves were often constructed from brick when first introduced, with slopes to all four faces. More recently they have been constructed from small patches of contrasting-coloured tarmac or pre-formed rubber units. In all cases, painted white triangles on the slopes warn motorists of their presence. As noted above, the are placed centrally in each carriageway, so that there is a gap between the bump and the kerb, and a wider gap in the centre of the road. When the road is wide, they are normally joined by build-outs from the kerb, artificially narrowing the road at the location. They can also be used, in conjunction with a build out, to narrow a generally two-way road to a single carriageway to further slow traffic. In a few rare locations, they are placed 3-abreast, with the third one straddling the central line, forcing traffic to the kerbs if it wishes to straddle the bump.
Raised Junctions, also called Speed Tables, are used to slow traffic at junctions. Normally constructed of brick, or with brick ramps leading to a raised tarred area, they span the entire junction area, including all approaches. They are most commonly used in the UK at junctions in new housing estates, but have also been retro-fitted to other junctions. An early use of this type of speed bump seems to have been in the High Street in Burnham-on-Sea in Somerset, where each of the junctions had a speed table installed, along with a one way system as a half-way house to full pedestrianisation.
Also classified as Speed Tables, Raised crossings are nevertheless a different type of speed bump. Here the top of the table area is roughly the same length as the normal wheelbase of a long car or short van, or at most when on a bus route, the wheelbase of the bus. This minimises the risk of grounding. The top is generally painted with the Zebra Crossing lines, with the zig-zags starting at the base of the ramps. Again, the table itself can be constructed just from brick (creating a red and white 'Zebra'!) or from brick ramps with a tarred top.