Vertical deflections - H & S-humps

A few years ago I needed to traffic calm a road on which buses were travelling regularly and where there was a need for pedestrians to be assisted across the road. Speed cushions were inappropriate as they failed to provide for pedestrians and are not that good for buses either. I realised that I needed a speed table which incorporated some of the principles of a speed cushion but without the lurching problem.

The principle of the speed cushion works just as well for the speed "hollow" - an inverted "cushion" where the road has a cushion shaped excavation in it. Clearly the speed hollow on its own could be a dangerous feature and would require drainage and some means for ensuring that it does not collect debris. But mould a speed hollow into the up-ramp of a speed table and you have a device that works. This is the H-hump and has been used on the continent for some years and has been reported in Fife, Scotland. My design is illustrated in principle in the figure below and the complete arrangement is shown for a typical road after that.
The drawing illustrates the basic two way shuttle-working H-hump and clearly shows the derivation of the "H". The principle is that the wider-track vehicles have much longer climbs onto the speed table, while light vehicles must in part at least use the steeper slopes to traverse the table. In this case, as in all pinch-points, cycles should be able to bypass the feature. On reasonably level roads no additional drain gullies would be necessary.

In effect there are two different slopes to the on-ramps, one for cars and light vans and a much gentler slope for wider tracked vehicles such as buses, fire engines and HGVs. In my design I recommend a ratio of slope lengths of 3:1 so that the limiting crossing speeds for all vehicles should be about the same, probably around 25-30mph. I suggest that there is normally no need for the feature on the down-ramp on two-track carriageways. The Scottish design incorporates the "hollows" on both on and off directions so there are four per table and they always add drain gullies on the uphill side. On most normally profiled carriageways rain will drain to the side and additional gullies are rarely necessary above those required for normal kerb-to-kerb speed tables.

In this design the hollows give much steeper slopes for light vehicles on the approach ramps only.
Cyclists will tend to use the nearside long ramps.
The build-outs are illustrative of how to manage the table on a road otherwise too wide.
(Tactile surfaces and other details not shown.)

The otherwise excellent traffic calming scheme in Borehamwood would benefit considerably from H-humps. Speed control is intended to be at about 25-30mph.

This is the H-hump at Feltham. Because of the side-fall to the nearside, no water would gather in the hollow so these need not be so expensive as has been suggested. Although not perfectly built it is nonetheless effective and buses were seen to traverse relatively comfortably.
Another view showing more clearly
the slightly convex profile of the approach ramps.

S-humps

These were developed as an alternative to H-humps which were considered difficult to construct. They are in effect a pair of rounded off cushions moulded onto the end slopes of a speed table. This means that they can still cause some lurching to large vehicles, a particularly serious problem for buses unless the driver is able to make an almost perfectly aligned approach and passage over the table, whereas the H-hump does not require such precision. It is little more than a glorified speed cushion, but having a flat-top could be used for pedestrians.

Drawing illustrating S-hump. Wider track vehicles have a gentler slope,
but the inner rear wheels are affected and will rise and fall relatively sharply;
at least there is a guarantee that no additional drainage is required.

It can be seen that these features would require careful construction. But that is not restricted to H & S humps alone. I am aware of cushions that have caused problems by being placed on the crown at the correct height relative to the centre line of the road, but because of the crown the apparent height of the cushion above the road for light vehicles has been excessive causing some grounding. Similarly poor detailing of otherwise straightforward tables and humps has led to poor operation. So it is worth looking at some aspects of construction.