Kill a cyclist, get community punishment

Yes because E is proportional to the square of the velocity.

I understand the equation, I'm just not sure it's relevant. If you're hit by a vehicle at 40mph then you will very rapidly be accelerated up to 40mph regardless of whether it's a car, van, or HGV because they are all much heavier than you and so none of them will be slowed down appreciably due to conservation of momentum.

In any case this isn't relevant, since we have the direct measurements of the relationship of impact speed on probability of fatality that @dancing james posted.