Nref = Nx (Wx / Wref)^4
Nref = number of trips of reference vehicle
Nx = number of trips of other vehicle
Wref = weight of reference vehicle
Wx = weight of other vehicle
The quartic formula really works in our favor for bicycle commuting: if a bicycle (with rider) is 1/20th the weight of a car, the bicycle will produce 0.000625% the wear of the car. So if commuting were the only purpose of the road, you would essentially never need to replace a road again.
This does make adding (larger) delivery vehicles quite bad. If the average delivery vehicle is twice as heavy as the average passenger car which was formerly used, each trip will produce 16 times as much wear. Looking up the curb weights for a Chevy cargo van (5000 lb) and a Toyota Corolla (2800 lb), the actual wear factor is 10x.
This does make adding (larger) delivery vehicles quite bad. If the average delivery vehicle is twice as heavy as the average passenger car which was formerly used, each trip will produce 16 times as much wear. Looking up the curb weights for a Chevy cargo van (5000 lb) and a Toyota Corolla (2800 lb), the actual wear factor is 10x.
[1] http://www.nvfnorden.org/lisalib/getfile.aspx?itemid=601