I don’t know why I didn’t see this earlier, World Streets proposes a Street Code

The idea is works is that legal responsibility for any accident on street, sidewalk or public space, is automatically assigned to the heavier faster vehicle. This means that the driver who hits a cyclist has to prove his innocence, as opposed to today where the cyclist must prove the driver’s guilt (not always very easy to do).

I suppose there is a conflict if the heavier vehicle is not necessarily faster, or if you get the irrational (drunk) pedestrian or cyclist, but it seems a good heuristic that will give those who impose the greatest unsafety externality the incentive to yield.
Note that all vehicles are expected to yield to trains, because trains can’t brake quickly (and they were there before cars in general), despite the fact that trains were heavier.