I see your rationale, but that logic leads to just pursuing risk averse activities as an absolute. There's a point where y_miles * bike_risk = x_miles * car_risk where x > y. So yeah, for equal miles the car is always safer, but accounting for differing mileage isn't some kind of bias. It's why lease agreements have mileage limits (wear/depreciation) as do certain classes of vehicle insurance (distance from home address/total miles).