Background

In 1986 the first UK regulations allowing experimental road humps were published. These allowed the use of humps on a road subject to 30mph or less and where a speed reducing feature was always present for drivers entering the road or roads from any direction. The requirements were stringent and required full width kerb to kerb humps thus affecting drainage.

Traffic Advisory Unit Leaflet 1/87 from the DoT advised that the various levels of the carriageway could be varied in accordance with the (then) legislation (UK). This allowed speed "tables" to be introduced allowing pedestrians to cross a carriageway at footway level saving the use of drop kerbs. A number of authorities had already started to use such facilities indicating that the Highways Act allowed the variation of the relative levels of the highway and so speed tables and road humps were born. The early research identified likely maximum speeds for various vehicles of different sizes and different hump dimensions.

The research is now history and does not need to be expanded here, but I am concerned that there still does not seem to be much scientific basis behind the current thinking on vertical deflections. It is still widely quoted that hump height is the main feature that controls speed when it is clear that gradient or, more precisely, change in gradient (vertical angle) is more likely the critical factor. Those authorities who limit hump height to say 75mm may be unnecessarily restricting the use of long gradient speed tables which would successfully and gently raise the carriageway (nearer) to footway level to the considerable benefit of pedestrians.

Early schemes understandably sought to stretch a given expenditure on traffic calming as far as possible. The humps were designed at about the maximum height for maximum speed reduction and at maximum spacing. Such schemes often resulted in drivers accelerating and braking severely between (overspaced) humps. This was not the intention behind traffic calming and a better use of vertical deflections was needed to ensure more consistent speed control.

Schemes that I developed for Gillingham Borough Council (now Medway Council) were based on closer hump or table spacing but with less severe features so that a much steadier speed profile could be achieved. To some extent this is a compromise.

Careful attention needs to be paid to the change of vertical angle at the point where drivers run their vehicles onto the up-ramp of a hump or speed table. It is this angle, and the type of vehicle riding it, that seem to dictate the speed at which traverse becomes uncomfortable. Similarly the end of the down ramp needs careful attention if the vehicle is not to 'drop' off the ramp to carriageway level. The use of conventional 900mm bull-nosse kerbs across the carriageway is usually inappropriate. It is rarely possible to follow precisely the existing carriageway profile. Any step must be eliminated. This problem occurred at Borehamwood; whereas in Devon at Tavistock and Princetown the blacktop joints at the base of the up (and down) ramps eliminate the uneven joints associated with kerbs and hence virtually eliminated any impacts which might lead to jolting and consequent noise and vibration usually associated with such laterally placed kerbs. In time they usually break out anyway.

Another factor is the vertical radius between the approach surface and the ramp. In conventional humps and speed tables there is theoretically no such radius and vehicle tyres could impact on the surface of the ramp. In some cases positive changes of vertical angle (i.e. where the obtuse angle is less than 180°) could result in impacting as the vehicle tyre straddles the two planes but this is only a problem on steeper ramps or dips and troughs. So in very low speed environments there may be a case for providing a vertical radius between the two slopes not less than that of the largest tyres likely to use it. This way the vertical deflection might operate in a more gentle manor. But the maximum change of vertical angle remains be the main factor controlling speeds, and if correctly designed and constructed there should be no impacts to create jolting in the vehicle, excessive noise or ground vibration.

Site Surveys

It became apparent from the outset that it was necessary for a good site survey to be done identifying relevant features along the road such as pedestrian and vehicle crossovers, street lighting, drain gullies and falls, service covers, pedestrian focal points as well as the usual traffic flows and speeds. Despite the relaxation allowing tapered humps I still make a thorough site survey before starting to design a scheme. I have often been able to make use of the existing drainage gullies and kerb weirs when introducing speed tables. This illustrates a speed table in The Vineries, Gillingham, Kent where a complete scheme was introduced with full kerb to kerb speed tables without any additional drainage works. Although pedestrian numbers were not particularly high I was able to locate the tables close to their desire lines. Note the superelevation on the bend - this new estate road should not have been designed this way contributing to the need for traffic calming.

Let's move on to look at humps and tables in more detail.