Speed cushions are several small speed humps installed across the width of the road with spaces between them. The idea is that wider-tracked vehicles like buses, ambulances and fire appliances are not slowed by them.
In 30 MPH areas, they have some effect in slowing down traffic, but in an increasing number of areas, particularly on cycle routes, the limit has been lowered to 20 MPH. Speed cushions don't work in 20 MPH zones. They don't slow down other wider-tracked vehicles like goods vehicles, vans and 4x4s, they don't even slow down a lot of cars. This Islington Council document says "speed cushions are less effective in reducing speeds than full width road humps". According to this report by Rob Gifford of the Parliametary Advisory Council for Transport Safety, to achieve a mean 'beweeen hump' speed of 20MPH, cushions may not be appropriate without other measures.
Speed is not the only issue with cushions. As I've pointed out before, and as Freewheeler points out, they encourage drivers (and riders) to aim for the gaps between the cushions and adopt an incorrect, dangerous road position. In other words, cushions can actually make a road more dangerous for cyclists.
According to Brake, they cost about £2000 per set of cushions. So the cost per mile, with one set of cushions per 100 metres is about £36,000. A lot of money for a measure which is at best less effective than other alternatives, and at worst results in more danger.
Now, one justification I have seen for speed cushions as opposed to road humps is that emergency services response time may be impaired by road humps. I don't buy that argument. There's no evidence for it that I've seen. General traffic congestion on major roads is likely the main determinant of response time. But rather than address congestion, council and Highways Agency planners and traffic engineers are part of a political system that ignores the consequences of over-dependence on cars and the attendant congestion and pollution. They wilfully turn a blind eye to the fact that people are dying because ambulances can't get to and from incidents quickly. They also turn a blind eye to the fact that very large numbers of people are dying because of road crashes and because of traffic-generated pollution, not to mention heart disease and cancer, the consequences of a sedentary lifestyle. They blame it on road humps instead.