UPDATE 2023: I’ve learned one more thing about the road damage formula. It’s not just axle weight to the 4th power, you must then multiply that by number of axles. Since most bikes and cars have 2 axles most of the numbers in this post stay correct, but a 5 axle big rig will actually do 2.5x more damage than calculated below. Adding more axlese still reduces damage by better distributing weight and lowering the number being raised to the 4th power, but it isn’t magic.
Been on a road lately and noticed how absolutely busted it was? Have you also noticed how vehicles today are far larger than in the past? These two things go together because vehicle weight is the main factor that determines road damage. State legislatures across the country are ignoring this simple fact and instead trying to blame EVs or even bicycles for our crumbling roads. The smarter/fairer way to fix our roads is by switching to a fee based on the actual road damage that vehicles cause. Replacing gas taxes and EV road fees with this would lead to a fairer and better system. Drivers who are currently freeloading off our system will pay more, but the rest of us will profit and the world will become greener. Read on to find out how it could all work.
How Roads Get Damaged
Note: The above chart from streets.mn incorrectly assumed that a 9 ton big rig would have 2 axles holding 4.5 tons each. Big rigs actually have 5 axles so it needs to be 22.5 tons to have 4.5 tons per axle and do 410 times as much damage. I've corrected this on their otherwise wonderful chart.
The first thing you need to know to understand my proposal is how roads actually get damaged. The exact formula for road damage depends on a lot of factors, but the most important two are axle weight and miles traveled. When scientists did a thorough study of how quickly roads deteriorated in the 50's they found that road damage increases by axle weight to the 4th power. There's some debate as to whether road damage is primarily due to axle weight or to total vehicle weight but the conclusion either way is that heavy vehicles do far more damage to roads than light ones.
A 4.3 ton Hummer does more damage per mile than 21 2 ton passenger cars. Miles traveled increases road damage linearly, so one car driving 200 miles does as much damage as two cars driving 100 miles each. The 4th power rule still applies so a single 22.5 ton shipping truck with 4.5 tons per axle does as much road damage in 1 mile as a 2 ton passenger car does in 410 miles. Most states in the US limit total truck weight to 40 tons and a 5 axle truck carrying this much would do as much damage per mile as 4096 2 ton passenger cars! There are a fair number of exceptions to this, like Michigan allowing 82 tons per truck, but they still generally limit axle weight to around 10 tons or less.
Age and Weather Deterioration
Even if no vehicles drive along a road it will slowly degrade. Roads in Hawaii that have been traffic free for decades due to volcanos show just how long a road can last if nothing is driving over it. The 30-50 year old road pictured here is looking pretty good, outside of the giant mass of lava rock on top of it. I'm unsure how long it will last in this state of disuse, maybe 100 years? Certainly far longer than the 18-25 years engineers expect roads with a usual amount of traffic and maintenance to last. It's fair to pass the initial construction costs of a road to all users, but the costs created by actually driving on it dwarf these and our fees should account for them properly.
The Gas Tax Can't Cut It
Now that you understand how road damage is actually caused you can start to see how the gas tax can't fairly pay for our road maintenance. Big shipping trucks average around 6 MPG, with 7.25 being consider quite good. US passenger vehicles averaged around 22 MPG in 2017. That means that shipping trucks use a bit under 4 times as much fuel for every mile they travel, but that doesn't come close to accounting for the 400-4,000 times as much damage they often do per mile. They do face other fees, like a 32% higher tax on diesel than on regular gas, but these do not come close to matching the extra damage they cause. Big trucks drove a bit over 1 mile for every 10 miles light duty passenger vehicles traveled in the US in 2018 but do far more than 10 times the damage of passenger vehicles. They're freeloading on the current highway funding system.
Of course Passenger vehicles have also ballooned in size and weight. Americans have made a massive shift to larger SUVs over the past few decades. These accounted for 50% of new autos sold in the US in 2019! The engines of these vehicles have also become more efficient, burning less gas to pull the same amount of weight, but the gas tax has remained largely unchanged in most places. A heavier truck that gets the same MPG as an older lighter model will pay the same amount of gas tax while causing significantly more road damage.
All of this means that in America the gas tax hasn't fully funded roads in decades. When you look at it comprehensively road funding costs every household in America over $1,000 beyond what they pay in gas taxes. This doesn't even include the local property and sales taxes that generally fund local streets. People are on the hook for this even if they don't drive cars at all. Think about that next time someone tries to tell you that pedestrians and cyclists aren't paying their fair share for roads.
EVs Complicate Things
The upcoming shift to electric vehicles will provide an even greater challenge to road funding as they don't use gas at all and are often heavier than their gas counterparts. Rather than fairly charging EVs for the road damage they cause legislatures across the country are tacking flat fees onto EV ownership. These fees range from $50 to $225 per year currently and some states have proposed fees as high as $1,000/year! It should come as no surprise to readers of this blog that these fees are often backed by money from fossil fuel interests that are trying to slow EV progress.
State EV fees will almost never correctly charge drivers fairly. For example, my dad payed Virginia an $88.20 fee for his EV last year. If he'd just kept his old gas car and used it for the same driving he'd only have paid around $25 in gas tax. This is exactly what fossil fuel lobbyists are hoping for when they get these fees passed. Of course being flat fees means that they may also undercharge some road users. The shuttle company that drives Teslas over 100,000 miles per year will never be charged appropriately by a flat fee.
These flat fees also fail to address the elephant in the room, vehicle weight. A 6,600 lb Cybertruck will be charged the same as an 1,800 lb Aptera even though it does over 180 times more road damage per mile. That'll just keep pushing Americans to buy bigger and heavier vehicles that do more damage to our roads. This will eventually result in another shortfall of road funding in the same way our switch from small cars to huge SUVs has. EVs can be made smaller and lighter, but this won't happen so much if heavy vehicles aren't charged their fair share of road maintenance fees.
Hidden Costs of Big Trucks
The trend towards bigger vehicles is doing a lot more than just destroying our roads. To start with the obvious, big SUVs take more materials to build. Kees van der Leun did a quick estimate in 2020 and found that around 9 million extra tons of steel was used to supersize cars into SUVs that year. This isn't all the steel used in SUVs, just how much extra they required compared to traditional cars. To contrast, every wind turbine built in the entire world in 2019 combined probably only used about 6 million tons of steel. That's a lot of material to be wasted on big vehicles that have just one occupant and no load much of the time.
Safety is another big reason to get drivers to switch to smaller vehicles. Many SUV owners bought their vehicles for safety, but the reality is that they're just as risky in a crash as smaller cars (ask Tiger Woods). The sad flip side is that SUVs are far more likely to seriously injure or kill the people they collide into, particularly if they have bull bars on them. Part of this risk is accounted for in higher SUV insurance costs, but much is simply born by society through hit and runs and unpaid medical bills.
Note: Buying a cheap used Leaf to use for trips that don't require hauling saves so much on gas and maintenance vs a big truck that it is essentially free and will quickly start saving the owner money.
Bigger vehicles often have big hoods in front that make it harder to see and avoid hitting things that aren't so tall, like kids. The new post office truck is a welcome exception to this trend, but consumer trucks are in a race to make the largest, most vision obstructing hoods possible. This is part of why vehicle crashes are the leading cause of death for kids in the US. If a road use fee that included axle weight to the 4th power caused truck drivers to only put miles on their large vehicles when they really needed their size it would have benefits far beyond paying for road maintenance appropriately.
What About Work Trucks?
A common complaint about a weight and mileage based road damage fee is that it would unfairly penalize those who use heavy trucks for work. Those who are simply using their 2 ton pickups as commuter cars can follow Mr Money Mustache's work truck advice and switch to smaller vehicles, but what about the people who actually need to haul literal tons of stuff? The simple answer is that they should pass the cost of the damage their vehicles cause on to their customers directly. This would increase the cost of things like construction and fracking that truly require large trucks by taking away the subsidy they are currently receiving from all of us when we pay to fix the roads that they destroy. The people who are actually requesting those services should be the ones who pay for the road damage the trucks they require cause and the rest of us should keep our money.
Beyond the simple fairness of appropriately charging large trucks, a weight based fee could also encourage better ways of doing things. Light weight materials would cost even less to ship and thus become more likely to see use in buildings and products. Rails can also handle huge weights with less maintenance than roads. If roads aren't so subsidized efficient trains may transfer more of our freight. Because of the 4th power rule two smaller trucks combine to do less damage than 1 large one, so shipping companies could start downsizing trucks and employing more drivers to lower their road use fees. In densely populated areas this could go even smaller like the 10,000 electric rickshaws Amazon is deploying in India, or the delivery cargo bike trailers that have started popping up all over Manhattan. These smaller vehicles do less damage to roads and also cause less traffic. Think of how frequently delivery trucks block full lanes of traffic while dropping off a package. Bikes can avoid disrupting traffic by pulling directly up to someone's door. This also saves delivery workers time, with many actually delivering more packages per day after switching to cargo eBikes. Of course I'd be remiss if I didn't refer back to my own post which explains how these eBikes likely get close to 1,000 MPGe, an efficiency level no car or truck will ever achieve.
How to Implement It?
The next major complaint about a mileage based road damage fee is that they'd be hard to implement. The straw man argument made here is that they'd require privacy invasive GPS trackers on all cars. This of course ignores the fact that most people have already accepted having their locations tracked by Google and other tech companies for years. Also, a mileage based fee does not require that level of tracking. In many states the odometers of cars are checked during their annual safety inspection. In the rest odometers get logged whenever a car is sold. These existing practices can be used to charge people for the miles they've driven without invading their privacy.
We could implement GPS tracking for super heavy cargo trucks since they do so much more damage per mile. Reporting where a truck has driven is already somewhat required by IFTA fuel tax reporting and many already have GPS trackers because of this. The privacy implications of this feel less fraught as these trucks are driven by employees on the job, and not by people just living their daily lives. The cost of GPS trackers is so low now that this could be done very cheaply, and it would allow precise delivery of funds based on where trucks are traveling. When Amazon opens up a shipping yard in your town there won't have to be a huge political fight to get them to pay for the extra road damage their trucks will cause, it'll just be recorded on the GPS and paid accordingly.
The other big complaint is whether we'd have to start weighing our cars. This is disingenuous and simply taking their gross vehicle weight should be enough. For light trucks we could add some percentage of their payload capacity, say 50%, to this figure. That'd give even more disincentive for people to drive huge trucks around with nothing in the bed, and might actually spur manufacturers to make some decent smaller trucks again. These aren't the only possible answers, but they seem easy enough and show to implementation of a road use tax is do-able.
Once we open our eyes to how the true costs of our roads are incurred a weight and mileage based road fee is pretty obvious. Yes, there can be quibbles about how the cost of initial road construction should be factored in, but the basic idea that heavier vehicles should pay far more per mile driven is sound. The monetary and environmental benefits of encouraging lighter vehicles are huge, and we need to stop missing out on them. If it helps properly fund our roads and increase our safety at the same time, all the better.
Incredibly janky port of comments from my old WordPress site:
2022-01-16 at 9:37 pm
The question of bus-equivalence is an interesting one – the road damage on Don Mills in Toronto is notable – raised ‘bus knuckles’.
HOWEVER this does not remove the value of identifying and charging externalities to recoup subsidies.
As I am fond of saying to people who want a ‘road tax’ levied on cyclists, “I can’t wait for my rebate cheque”
Incredibly janky port of comments from my old WordPress site:
2021-03-25 at 11:07 pm
Another great article and captures the bulk of the issue. A few caveats worth mentioning from this traffic engineer:
– This all accelerates an order of magnitude where studded tires are allowed and the tire fees don’t come close to covering it.
-Road damage isn’t the only direct cost. The needed right of way, pavement, signals, etc needed to supply capacity in the form of new lanes (including bike lanes and sidewalks) is another factor. For vehicles, this points to a GPS tax structure that charges more at peak times. Less peak demand = smaller roads needed.
– Fundamental to this all is a transportation funding structure that makes new highways cheap, but does little for those outside a car, safety, etc. and creates unsustainable maintenance obligations at the state and local level.
– To venture into politics, this is one area where the “pay your own way/personal responsibility ” party is willfully blind to the massive suburban SUV subsidy.