Construction

This is certainly an area where vertical deflections have fallen into disrepute. It is the intention that vehicles should be subject to a steady vertical acceleration and that the acceleration will increase in proportion to the square of the speed. This should therefore ensure a relatively sharp comfort cut-off point, so for any given vertical deflection or, more precisely, change of vertical angle it will be relatively predictable what the maximum comfortable speed over the feature will be. Unfortunately the smooth profile needed onto the hump is usually disrupted by the installation of a kerb with the intention of holding the speed table together. There is usually a rise of a few mm just to get onto the kerb and if the horizontal profile of the road is poor in the first place, a kerb placed across the road will be bound to fail to meet the existing surface uniformly. So it is important to get the construction right. The graphs above indicate the actual profiles of various ramps on blacktop speed tables that I measured shortly after construction. All were supposed to be 75mm high with a slope that matches the purple one to the left. In most cases the slope was accurate at the upper part but too steep towards the base resulting in some tyre impacts. Most of the "offending" ramps were corrected.

The poor ramps at Borehamwood where vehicles "drop" some 40-50mm onto the main carriageway then level over a short distance illustrate the problem well. Large vehicles bounce and can be noisy. By contrast the join between carriageway and on-ramp in Duke St, Tavistock where the carriageway has been saw-cut to allow a near perfect join. [images to follow]

They are particularly well constructed and bedded in to the carriageway surface so that there is no significant impact as at so many speed tables; but faster vehicles would still have some slight impact as there is virtually zero vertical radius at the beginning of the up-slope.

This is an example of a speed cushion constructed in a residential road. To ensure that drivers cannot go around the cushion the kerblines have been built out. A drainage system has to be provided - in this case bypass channels have been included; these require regular maintenance. Speed cushions have maintenance problems over the longer term. The constant running of vehicles over their edges often results in plastic deformation of those built in flexible materials and can cause some breaking up of those built in brick or concrete.

Single speed cushion in pinch-point on narrow residential road. This is a particularly quiet residential road but the pinch point requires gullies or drainage bypasses

Summary of vertical deflections

There is no doubt that vertical deflections work. They are almost too effective and so are mostly used in residential areas only. Shopping centres where buses operate and flows are high rarely use vertical deflections, or where they do there is usually a compromise somewhere. In both Borehamwood and Tavistock the vertical deflections are significantly too gentle to reduce speeds of light vehicles sufficiently because of the problems with buses and HGVs. The use of differential slope humps such as H- or S- humps would be helpful.

But if you are determined that vertical deflections are not for your road then let's look briefly at horizontal or lateral deflections.