I'd say the licensing regimes should be set at a level that's proportionate to the number of deaths of others caused by that kind of vehicle, i.e. consider the number of other road users killed per year by every 10000 cars/bikes/bus-passengers/pedestrians/.... (perhaps as a QALY calculation to take into account serious injuries as well). Modes where that number is higher should have their licensing requirements made stricter, modes where that number is lower should have their licensing requirements relaxed.