Charge cyclists a toll on the Golden Gate?

So there's again talk of possibly charging a toll for cyclists and pedestrians to cross the Golden Gate Bridge. Logistical nightmares aside, there's some interesting math running around the intertubes today. The original credit on this goes to John Holtzclaw of the Sierra Club.

The damage to roadways from motor vehicle traffic in the standard formula used by engineers is the speed x (axle loading) to the 4th power.

Speed on GG Bridge:
bicycle -- 10 mph
SUV -- 50 mph
ratio SUV/bicycle = 5

Axle loading:
170 lb person on 30 lb bike -- 200 lb
SUV -- 4000 lb
ratio SUV/bicycle = 20
20 to 4th power = 160000

So SUV damage = 5 x 160000 = 800,000

To be fair, bicycles should be charged the same rate as cars for the damage they cause. Charging bicycles 1 cent = $0.01 is equal to charging SUVs $8,000. If bicycles were charged 1 cent for every 10 bridge crossings, SUVs would be charged $800 per crossing. While there isn't an equivalent formula estimating pedestrian damage, if it is conservatively assumed to be 10 times that of bicyclists, a 1 cent charge to pedestrians is equal to charging SUVs $800.

Ref: Johnsson, R, THE COST OF RELYING ON THE WRONG POWER: ROAD WEAR AND THE IMPORTANCE OF THE FOURTH POWER RULE, Transport Policy, Volume: 11 Issue: 4 ; 10/00/2004