City buses are surprisingly very bad for roads

November 10, 2013

In a comment exchange on some random liberal blog, a transit advocate was mysteriously opposed to bicycles-as-transit. He never completely explained why, but I think he was making assumptions about road damage and congestion that assumed a linear relationship to weight. However, that’s just not so. Damage-per-wheel is at least proportional to the cube of the weight on the wheel, if not the fourth power. It’s very non-linear, and non-intuitively a heavy vehicle with many wheels can do less road damage than a lighter vehicle with just a few.

Turns out, a city bus can do a LOT of damage, and per-passenger, it does over twice the damage of even a single-occupancy SUV. A plan-loaded city bus is almost as bad for roads as a fully loaded semi truck, and a crush-loaded city bus is worse. Whether you calculate it as marginal damage per passenger, or average damage per passenger, a bus passenger is hundreds of times more damaging to the road than even a fully-loaded cargo bike, and thousands of times more damaging than someone merely carrying themselves on a bicycle. Even the marginal road-damage cost of the first passenger to board a bus is a little bit worse than the road-damage cost of a single-occupancy SUV.

I’m not at all sure that these costs are rolled into the published “costs” of transit. Note that these are not the only costs; there are congestion costs, parking space costs, fuel costs, risk-to-others costs, risks-to-rides costs. But the road damage costs are really large.
(And if someone knows more about this than I do, do please check my math, I am working from published sources and conservative estimates. This was surprising to me.)

Pasted from Calca, lightly edited to remove irrelevant decimals and in one place line-wrapped.

First, data, sources, and assumptions:

Flyer_empty_weight = 28,880
http://en.wikipedia.org/wiki/File:SEATNewFlyer.jpg (picture showing paired rear wheels)
Flyer_wheels = 6
http://www.mbta.com/uploadedfiles/documents/Bluebook%202010.pdf (p. 64)
Flyer_plan_capacity = 54
Flyer_crush_capacity = 75

Passenger_weight = 150
Flyer_weight(passengers) = Flyer_empty_weight + Passenger_weight * passengers

Semi_weight = 80000
Semi_wheels = 18

Compact_weight = 2500
Car_wheels = 4

SUV_weight = 5500

Bike_weight = 250
Bike_wheels = 2

Cargo_bike_weight = 500
Light_bike_weight = 175

Road_damage(weight, wheels) = weight * (weight/wheels) * (weight/wheels) / 1,000,000,000
Bus_damage_pp(passengers) = Road_damage(Flyer_weight(passengers), Flyer_wheels) / passengers

Comparing damage per vehicle:

Road_damage(Semi_weight, Semi_wheels) => 1,580

Road_damage(SUV_weight, Car_wheels) => 10.4
Road_damage(Compact_weight, Car_wheels) => 0.98

Road_damage(Cargo_bike_weight, Bike_wheels) => 0.0313
Road_damage(Bike_weight, Bike_wheels) => 0.0039
Road_damage(Light_bike_weight, Bike_wheels) => 0.0013

Road_damage(Flyer_weight(0), Flyer_wheels) => 669
Road_damage(Flyer_weight(Flyer_plan_capacity), Flyer_wheels) => 1,405
Road_damage(Flyer_weight(Flyer_crush_capacity), Flyer_wheels) => 1,795

Bus damage, average per passenger at various loads, and ratios with bikes:

Bus_damage_pp(5) => 144.5
Bus_damage_pp(10) => 78
Bus_damage_pp(20) => 45
Bus_damage_pp(30) => 34.5
Bus_damage_pp(40) => 29.5
Bus_damage_pp(Flyer_plan_capacity) => 26
Bus_damage_pp(Flyer_crush_capacity) => 24

Bus_damage_pp(Flyer_crush_capacity)/Road_damage(Cargo_bike_weight, Bike_wheels) => 766
Bus_damage_pp(Flyer_crush_capacity)/Road_damage(Bike_weight, Bike_wheels) => 6,127
Bus_damage_pp(Flyer_crush_capacity)/Road_damage(Light_bike_weight, Bike_wheels) => 17,864

Marginal road damage; if the bus is running anyway, does it really matter that much if I take the bus? Yes, it does — even the first passenger to board a bus does more damage to the road than if they were driving an SUV. The heavier the bus, the greater the marginal cost per passenger, and it is always hundreds-to-thousands of times more damaging than riding a bicycle.

Bus_marginal_damage_p(passengers) = Road_damage(Flyer_weight(passengers + 1), Flyer_wheels) - 
                                    Road_damage(Flyer_weight(passengers), Flyer_wheels)

Bus_marginal_damage_p(0) => 10.48
Bus_marginal_damage_p(10) => 11.6
Bus_marginal_damage_p(Flyer_plan_capacity) => 17
Bus_marginal_damage_p(Flyer_crush_capacity) => 20

Bus_marginal_damage_p(0)/Road_damage(Cargo_bike_weight, Bike_wheels) => 335
Bus_marginal_damage_p(0)/Road_damage(Bike_weight, Bike_wheels) => 2,683
Bus_marginal_damage_p(0)/Road_damage(Light_bike_weight, Bike_wheels) => 7,822

Bus_marginal_damage_p(Flyer_plan_capacity)/Road_damage(Cargo_bike_weight, Bike_wheels) => 549
Bus_marginal_damage_p(Flyer_plan_capacity)/Road_damage(Bike_weight, Bike_wheels) => 4,394
Bus_marginal_damage_p(Flyer_plan_capacity)/Road_damage(Light_bike_weight, Bike_wheels) => 12,810

One Response to “City buses are surprisingly very bad for roads”

  1. psteckler Says:

    Also perhaps surprisingly, buses use more fuel and emit more CO2 per passenger-mile than cars: http://www.buses.org/files/ComparativeEnergy.pdf.

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