As a UK manufacture of traffic calming devices, I agree with the comments of your reader from Liverpool about the ramp gradient being more important than the height on a road hump or speed cushion. In fact, all the dimensions on a cushion are critical in order to perform in a specific way and so we supply 16 different sizes of cushions to suit every application. But what the engineer specifies is not always what he gets from the contractor. This is where a preformed construction consistently provides the correct profile and in the case of our products also accounts for 20 tonnes of recycled rubber a week being used to reduce accidents and save lives – without tearing tyre inner shoulders to shreds