It is not a joke and the English administrations are interested in it more than seriously.
Each time a vehicle passes over the two metal plates mounted on the springs of this retarder, they move up and down by storing the energy thus developed. Depending on the weight of the vehicle, it is between 5 and 50kW that are generated (on average 10kW for a classic car) that can be used to power lights and illuminated traffic signs. "
The quoted power values do not mean anything because they are given in kW (power unit and not in kWh, energy unit). The energy created depends on the duration of generation of the current which according to us is undoubtedly rather weak (a few tens of seconds at most).
Finally, the problem of storing this energy remains to be solved and at € 37 per unit (with or without road works?) It will take several million passages to make them profitable.