Vertical deflections - Speed Cushions  

Vertical deflections affect hard sprung vehicles more than cars and light vans; so there is a need for devices that are able to differentiate between these vehicles so that speeds are constrained more evenly and emergency vehicles are held up less. The commonest device in use in the UK is the speed cushion and is illustrated above. They must be placed centrally on the intended vehicle paths and there must not be gaps that enable drivers to avoid them.

Cushions three abreast on a wider road

Speed cushions of the dimensions illustrated are effective at capping speeds to around 25mph and are thus ideally suited to main road locations where traffic speeds have become excessive but where traffic volumes are not expected to be reduced as a result. They work well because drivers have to align their vehicles well to get minimum deflection and this in itself slows down traffic. Buses have an advantage using speed cushions as they can cause very little vertical acceleration if the driver is able to align well. Do not use speed cushions on bus routes unless you can be sure that parked vehicles will not interfere with correct passage. A bus must traverse precisely or it will lurch up to 75 mm over a cushion causing over 150mm sideways lurch on the upper deck. (This problem happens to a small extent at mini-roundabouts where buses cannot usually avoid driving over the centre spot.)

Speed cushions need to be placed on the vehicle running track; but this can mean complications. On a typical 7.3m road there is too much space between adjacent cushions and light vehicles can avoid the cushions by using this gap. It is not quite possible to create a layout where two large vehicles can pass each other centrally straddling their respective cushion without leaving too large a central gap. Sometimes this can be overcome by placing a traffic island between adjacent cushions.

For a typical road of 7.3m, parking bays can be provided on one side of the road with a prohibition of waiting on the other. The island may be provided to prevent drivers from bypassing the cushion (right to left) in the absence of parked vehicles, but it would require the bays to be clear of 6-8 vehicles for this to be a significant risk at speed.

Centrally placed speed cushion with cycle bypasses as used at Gillingham, Medway
where traffic calming was installed on a route regularly used by emergency vehicles.
Speed cushions, or humps, have a particular role at pinch-points. I designed a scheme for Gillingham BC on a road which was also an emergency vehicle route. Parking on this road (6.7m) was occasional and it was observed that in an emergency a fire engine would run along the centre of the road. So the device designed to control speeds to about 25 mph was a pinch-point with a central speed cushion. The pinch-point includes cycle bypasses for the benefit of cyclists and to avoid additional drainage connections and is illustrated below.

It has normally been my practice NOT to install priority working on pinch-points unless there are specific reasons such as to ensure that a queue does not form on one approach which might tail back to a junction. A series of pinch-points may have alternating priorities but there is often confusion and little evidence of their safety. It saves a lot on sign costs too!

More about priority systems can be found in the page on pinch-points.

  Single speed cushion in pinch-point - one of a series along Childscroft Road, Rainham, Gillingham. Note the cycle bypasses which are not only easy to provide, they reduce the cost by providing drainage bypasses too.
While speed cushions are effective at controlling speeds on routes used by buses and other large vehicles, they do not particularly help pedestrians; there is a need for devices that both help pedestrians and reduce speeds differentially according to track width and that is where H & S-humps have an important role.