Buses are implicitly subsidized by road maintenance spending. Road wear and tear occurs according to the fourth power of axle weight, which effectively means almost all of the wear and tear is incurred by the heaviest vehicles, which include buses.
Axle weight and vehicle weight aren't the same (or even very closely correlated). A bus will weight ~3-4x more than a car, but has wider tires, and carrying far more people. As such the weight of a bus is likely similar to or lower than an equivalent number of cars.
Here in Seattle, the busy roads with older lanes used for buses are obvious, because they have two deep canyons while the lane next to them is fine. In fact King County Metro has to pay millions in fines to the state because the buses are excessively heavy.
No roads without bus traffic have the same type of damage.
Roads still need maintenance even if nobody uses them, so a significant portion is split evenly across all traffic.
Busses are light compared to 18 wheelers and other heavy equipment, they also replace many cars and SUV’s which keep getting heavier.
Finally that rule of thumb isn’t really that accurate, “A 1988 report by the Australian Road Research Board stated that the rule is a good approximation for rutting damage, but an exponent of 2 (rather than 4) is more appropriate to estimate fatigue cracking.” Rutting really isn’t that significant in most cases, but can instantly destroy road surfaces when fully loaded construction vehicles etc drive over something once.
> Roads still need maintenance even if nobody uses them, so a significant portion is split evenly across all traffic.
Your former doesn't imply the latter. Here in Seattle we even still have cobblestone roads without heavy traffic and they spend very little money on them.
We have extensive rutting damage on the lanes use by busses and requires more expensive, deeper road base when they get replaced. This cost is due to the heavy traffic.
Even if squared, the buses are still 22 tons instead of 2-3 tons. 49 times more damage isn't good.
22 tons are huge busses and overkill unless you actually need that much space, and tend to have 4 axles. ((22 / 4)/(3/2)) ~= 13.5x a heavy SUV but could be replacing 30+ vehicles.
Also that visible ware is noticeable because it hasn’t been replaced. Looking worse when you resurface on the same schedule isn’t an actual cost.
But those are what we have and they have 3 axles, not 4.
We also have many concrete roads and closely-spaced axles, if they had them, would not help.
> Looking worse when you resurface on the same schedule isn’t an actual cost.
I addressed this: they have to dig much deeper and replace with much thicker road. Much more expensive. It's not "looking worse", it's actively dangerous to cyclists and other road users, so the surface must be replaced more often too.
Closely space axels work fine for road surfaces they don’t help on bridges but that’s a separate concern. You can see a plethora of heavy military vehicles etc which use extra axles to avoid getting stuck in the mud due to plastic deformation IE rutting. EX: The 22 ton KTO drives has to deal with rutting on vastly worse road surfaces like mud. https://en.wikipedia.org/wiki/KTO_Rosomak
But this is where you need to do a deeper analysis than just a simple rule of thumb. Even adding extra wheels to the same axle makes a big difference to road surfaces.
> so the surface must be replaced more often too.
Level of ruts you see are considered acceptable or they would be replaced.
However, ultimately the same entity is paying for the busses and road maintenance. If lighter busses saved taxpayers money that’s what they would use which is a major sign your analysis is inherently flawed.
>They don’t replace nearly enough cars and SUV’s to make up for the difference in fourth power of axle weight
A modest bus holds 40-50 people. Most commuter traffic is single driver, single vehicle. I don't know to which power the difference in axle weight would have to be to surpass the efficiency gains of replacing 40 to 50 American sized SUVs with a city bus, but I suspect it's more than four.
At the heavy end, SUVs weigh about 3 tonnes, while at the light end buses weigh about 12, a 4x difference. 4^4 = 256. So if the claim about the fourth power is true, you'd need to replace 256 SUVs to break even on wear, which is obviously impossible.
(I don't really understand how the fourth power of axel weight thing can possibly be true, though. Why would joining two vehicles together into a mega vehicle with double the weight and double the wheel count suddenly cause the combined vehicle to inflict 16x more wear than before you joined the two together? It makes no sense.)
Plus the SUV is usually point-to-point, leave home, go to work, come back. Whereas the bus is going back and forth ten times per day.
In Europe, the numbers differ even more. Lighter weight cars typically 1.5-2 tons, a new London bus can be upto 18 tons when loaded - that's ~5-16 units of wear for the car to 104,976 units for the bus...
But this is all supposing we're optimising for road wear, which isn't really the point of a bus system.
A Ford F-150 weighs about 2 tons and has two axles, for an axle weight of 1 ton. 1^4=1.
A garbage truck weighs maybe 30 tons and has three axles, for an axle weight of 10 tons. 10^4=10,000.
So if you drive an F-150, you’re doing as much road damage driving down the street 10,000 times as the garbage truck does once. Rural areas that don’t have garbage trucks and just expect everyone to haul their garbage to the dump in the back of their pickups are onto something.
