(Vertical deflections uniform across the carriageway)

Round-top humps

Standard DfT round top humps have been in use in the UK since 1986 and on an experimental basis for up to 12 months before then. In the early days they had to run from kerb to kerb so required a drain gully on the uphill side at each end of a normally cambered road. Some of the schemes that I designed in 1988 were on roads where fortunately the drain gullies were originally installed in pairs and the scheme could be devised and introduced at minimum cost. This is still the best way to do them if you can. In Wellington Road, Sandhurst where it was desired not to close a rat-run but to stop the excessive speeds that were occurring a scheme of humps was devised to deal with this problem. This was one of those roads that was always waiting for legislation to catch up with the need and the scheme was soon funded once humps became legal. A series of humps on Marlborough Park Avenue, Sidcup, Bexley has reduced through traffic and reduced accidents from around 5 in three years before to zero since installation about 5 years ago.

Sinusoid humps In essence these are round top humps with a significant radius at the bottom of the ramp. They differ by only a very few mm. from the construction of a plane (speed table ramp), as indicated in the plot. Here a table at 75mm high has optional sinucoid or plane; the greatest difference between their respective levels at a particular point is just 8mm.
		Construction is quite difficult and sometimes happens "by accident" when resurfacing or surface dressing a road with round-top humps.

Speed tables

These are probably the commonest form of hump in use in the UK at the moment. They may now rise to 100mm but generally 75mm is the preferred maximum height. But it is not the height that is critical to the speed at which traffic drives over them. As already discussed it would appear that the change of vertical angle is the factor which defines the maximum comfortable speed for any given class of vehicles, by which I generally mean cars and light vans. The problem with speed tables, especially those constructed with bricks, is that the join from and to the original carriageway level is often constructed so badly that there is more speed deterrent effect in the various tyre impacts on kerbs jutting out from the surface than there is from the true vertical acceleration were the transition to be smooth. At the lower speed end of the scale I would prefer there to be a vertical radius of not less than that of the largest vehicle tyre that will roll onto and off the table, i.e. some 750mm, but I suspect that this is not critical. In this way there would be no possible impact and therefore reduced noise and vibration, but the vertical acceleration would be the same.

Speed tables of course provide excellent facilities for pedestrians except for the visually impaired who prefer the benefit of a kerb face, but the use of tactile surfaces now commonplace overcomes this problem. Wheelchairs and pushchairs pass onto speed tables easily.

Some speed tables have tapered ends to allow for drainage; this is only recommended where there is minimal need for their use as a pedestrian facility, but it has to be admitted that the savings in providing for drainage can be substantial. As an alternative, special kerbs incorporating drainage channels can be used.

Hollows/drainage channels

I mention this as I have seen surface drainage channels across minor roads and observed that they are very good at constraining speeds. They may be considered to be part-time fords. It is important that the vertical radius of any concave surface is greater than that of the largest tyres including tractor tyres otherwise there will be an impact as the tyre hits the far side of the hollow. Probably the best shape is an inverted sinusoid to try to avoid this. For fords there will usually be plane slopes to a level or nearly level ford bed.

Some minor urban streets have drainage channels along the middle of the road and this has an advantage that the road slopes away form the buildings alongside, an important matter if the road is unkerbed and delivery vehicles need to park close to the buildings which they serve.

But there is a need to look at devices which can differentiate between larger and smaller vehicles, so let's now move on to Speed Cushions.