The message
from "dennis@home" contains these words:

http://petitions.pm.gov.uk/85-mph/?

What's the significance of 85 mph?

1.5 x the energy if you crash?

close - 1.47

1.5 x the stopping distance at 70 mph?

Not quite so close - 1.42 using the Highway Code figures as a guide.

The petitioners age?

No idea but I doubt it. By that age drivers should be considering very
carefully whether they should continue driving at all.

The calculations above of course contain v^2 and kinetic energy in
particular is a vital consideration in the seriousness of an accident.
Surprising therefore that the maximum speed of heavy commercial vehicles
is only marginally more restricted than that of cars. A 40 ton lorry at
56 mph has the same kinetic energy as a 2 ton car at 250 mph. If kinetic
energy was the major determining factor 40 ton lorries would have to be
restricted to a maximum speed of 16 mph to match the 2 ton car at 70.

--
Roger Chapman