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Driving Taxes for the 21st Century

Both Max and Lucas have recently written on this blog about the need to price gasoline appropriately. I agree with them…mostly.  I mean, how could I disagree with them? I’m the one driving the gray Prius with the license plate “TAX GAS”.  But, as I and the others who have advocated for higher gas (and diesel) taxes have recognized all along, it is an imperfect way to price the externalities of driving.  And it is likely to get worse.


A good idea, but not the whole solution

For more than a decade, the students in my MBA course on Energy and Environmental Markets have listed the externalities from driving and then discussed how well taxing gasoline prices those externalities. The list usually looks something like this:

  1. Greenhouse gas emissions
  2. NOx, particulates, and other local pollution emissions
  3. Energy security
  4. Congestion
  5. Accidents

The students generally get the emissions externalities right away, and the energy security externality pretty quickly. Congestion externalities — my decision to get on the freeway slows down all the other cars on the road — sometimes takes a bit longer.  Accident externalities — my decision to drive increases the chances that another car will hit or be hit by me — are almost always the last to be pointed out.  Most students are surprised to learn that congestion and accidents are the largest externalities from driving.


Slowing down other cars is often the largest negative externality from driving (

Then we get to the livelier part of the discussion: is taxing gas an effective way to have drivers internalize the externalities that they create?

Before I discuss the answers, let’s recognize that no public policy perfectly targets the problem it is meant to address. Every tax break is utilized by someone it was not intended to benefit and goes unnoticed by someone whose behavior it would have changed in exactly the hoped-for way.  Subsidizing the purchase of an energy efficient refrigerator sometimes causes a household to go from having one refrigerator to two, keeping the old one running in the garage or basement.  Fuel economy standards get people to buy more efficient cars, but don’t encourage them to drive any less.

Still, even if perfection is unreachable, we need to understand the policy imperfections and work to improve them.

The discussion of gas taxes and greenhouse gas emissions is always very satisfying, because it turns out that the correlation between burning gasoline and emitting GHGs is nearly perfect. Every gallon results in about 20 pounds of CO2 emissions.  So, if you want to put a price on GHGs, taxing gasoline is pretty much the same thing when it comes to emissions from gasoline-powered cars.  One smiley face for gas taxes. 


Much of the problem comes from a small number of old smokies (Oak Ridge National Laboratory)

But the students also start to see red flags as they apply that logic to the other categories. The high correlation with GHG emissions evaporates when it comes to NOx and other local pollutants. These emissions, which contribute to ozone and other health-damaging pollutants, have a very low correlation with the amount of gasoline the car uses. Old cars are massive polluters compared to new cars, due to great technology improvement in pollution control systems. And even within the same year and model, there is huge variation in the quantity of these emissions, as our MIT colleague Chris Knittel has shown in work with Ryan Sandler. Taxing gasoline is not an effective way to go after the small share of cars that put out most of the local pollution.  A frowny face for gas taxes.

Energy security is always a bit hard to explain, but it generally means some combination of greater risk to our economy when we import a lot of oil, and greater security risk when oil sales enrich the autocratic leaders of oil exporting countries.  As US oil production rises and world prices fall, it’s less clear that this is a big externality, but it is clearly still highly correlated with the amount of gasoline one uses.  Another smiley face, though probably a less important one.

By now the students see where this is going, despite the fact that I have told them of my license plate at the beginning. Congestion is also likely to be poorly correlated with the amount of gasoline a car burns. Some people drive on crowded freeways at rush hour, while others drive on uncongested roads or at off-peak times. I’m not aware of any good studies on the variation in congestion externalities across drivers, though someone at Waze/Google should be able to tell us a lot on the subject. That one almost certainly gets a big frowny face.

Accidents are more complicated because some of this externality is internalized through your insurance rates. But work that Max has done with Michael Anderson points out that insurance does a poor job of internalizing the accident-risk externality, because of low insurance requirements and limited cases of liability.  Max and Michael find that a gas tax does a pretty good job of representing the fact that heavier cars are more likely to hurt other people, but it still doesn’t capture the variation in where and when people drive, or much of the variation in how carefully they drive.  Hard to know for sure, but gas taxes probably aren’t great.

Perhaps the most interesting part of this debate is not how well taxing gas captures externalities today, but how that will change in the next decade or two. Gas taxes will almost certainly remain an excellent way to price greenhouse gas emissions and, to the extent they are relevant, energy security externalities.

Technology, however, is increasingly giving us much better ways to address the other externalities, though not without their own issues. Onboard computers will be able to inexpensively monitor tailpipe emissions so we can know exactly how much pollution a car has put out in the last year (though tampering with the equipment may still be a problem – see the VW debacle).  GPS will be able to report to that computer how many of the miles were driven on roads that Google was coloring yellow or red at the time and, with some sophisticated algorithms, even calculate how many delay minutes you imposed on the drivers around you.


GPS could easily tell your onboard computer when you’ve been driving on congested roads (LA freeways at 7:13am today)

At the cost of a modicum (OK, a whole lot) of privacy, we could price pollution and congestion externalities to an extent that perhaps only an economist could love.  The difficult conversations we have been hearing lately about the trade-off between privacy and social responsibility will come to vehicle transportation.

And the possibilities for pricing accident-risk externalities are even more exciting/disturbing.  That onboard computer will know how close you came to hitting the other car or tree or pedestrian, as well as every time you accelerated too quickly or braked too hard.

By now, you may be thinking, “wait, when onboard computers are monitoring that much information, they will also be driving the car.”  Maybe so.  But given the blowback I’ve heard from drivers who seem to think that the right to drive old polluters is also protected by the Second Amendment, I don’t think see everyone giving up their vehicular autonomy any time soon.

And sometime in the next decade we will have to face up to the fact that electric or hydrogen or biofuel powered vehicles have the same congestion and accident effects as the ones powered by hydrocarbons.  As they become a larger share of the fleet, gas taxes will become even less effective for these major externalities, though still a fine way to capture GHG emissions.

In 20 years, if vanity license plates aren’t obsolete, I will have to get a new one: MEASURE AND TAX ALL DRIVING EXTERNALITIES.  Hmmmm. That may have to go on my LED bumper sticker.

ADDENDUM: As commentors have pointed out, wear and tear on roads is also an externality, because we don’t pay for the damage our vehicles do to the roads.  That is clearly correct.  There seems to be some disagreement about how much road damage increases with weight (though it is recognized to be more than proportional), and about how much damage occurs due to weather apart from vehicle use.

Severin Borenstein View All

Severin Borenstein is Professor of the Graduate School in the Economic Analysis and Policy Group at the Haas School of Business and Faculty Director of the Energy Institute at Haas. He received his A.B. from U.C. Berkeley and Ph.D. in Economics from M.I.T. His research focuses on the economics of renewable energy, economic policies for reducing greenhouse gases, and alternative models of retail electricity pricing. Borenstein is also a research associate of the National Bureau of Economic Research in Cambridge, MA. He served on the Board of Governors of the California Power Exchange from 1997 to 2003. During 1999-2000, he was a member of the California Attorney General's Gasoline Price Task Force. In 2012-13, he served on the Emissions Market Assessment Committee, which advised the California Air Resources Board on the operation of California’s Cap and Trade market for greenhouse gases. In 2014, he was appointed to the California Energy Commission’s Petroleum Market Advisory Committee, which he chaired from 2015 until the Committee was dissolved in 2017. From 2015-2020, he served on the Advisory Council of the Bay Area Air Quality Management District. Since 2019, he has been a member of the Governing Board of the California Independent System Operator.

22 thoughts on “Driving Taxes for the 21st Century Leave a comment

  1. There is significant uncertainty in estimates of energy security externalities. US strategic petroleum reserve can dampen risks to the economy of events like the 1973 & 1979 oil crises or Hurricanes Katrina & Ivan. The external costs of protecting imported crude oil supplies is the subject of much conspiracy theory. It certainly is clear some portion of US military presence in the Middle East and for example, a supply choke point like the Strait of Hormuz, is a cost not paid at the pump. Many people suggest there are much greater costs associated with US activities to ensure access to foreign crude oil supplies. Increases in domestic production of oil in the US will not substantially decrease the need to protect the flow of imported crude oil.

  2. “As commentors have pointed out, wear and tear on roads is also an externality, because we don’t pay for the damage our vehicles do to the roads.”

    My understanding is that expected road damage is approximately proportional to the fourth power of axel loading, so to a first approximation all road damage is done by large trucks and buses. Certainly this squares with my experience on I-80 in the Sierras, where the right lane (in which trucks drive) is often in terrible condition relative to the left lane.

  3. While I appreciate the focus on ‘driving’ related to personal automobiles, your scope seems to miss the broader question that should really examine basic transportation. Why just look at personal automobiles? Why should buses, commercial trucks, trains, and all other entities that that use the same roadways and right-of-ways not also be included? Examining the externalities you’ve raised also seems to place little or no value on how taxes impact the overall benefits that roadways and right-of-ways create. A road creates the opportunity to use a vehicle and to get/move from one place to another, which in turn creates a broad package of costs and benefits. Taxes seem to make sense as a way to balance societal investment if the costs outweigh the benefits. Why else collect a tax?

  4. Not so fast on energy security, even noting that your endorsement of the concept is tepid Is it better for us to have energy plutocrats like the Kochs inside the tent corrupting our politics, or offshore? As we’re not going to burn all the oil, I guess “drain America first” has lost some of its ironic force, but still…

    All in all, I worry more about our security of access to real quality of life essentials, like coffee and chocolate.

  5. “…insurance does a poor job of internalizing the accident-risk externality, because of low insurance requirements and limited cases of liability”

    Also, car insurance doesn’t reimburse people whom your accidents inconvenience by, for example, shutting down a lane of traffic on a busy freeway. And there are the costs of sending police out to secure the scene of a messy accident, which (as far as I know) are borne by taxpayers, unless they send a bill for their time and services to the insurer in the same way that an ambulance service would.

  6. We had roads before we had cars, and we had forms of mobility before we had cars. The original lobbying group for “paved roads” — circa 1880’s — was the Good Roads Committee of the League of American Wheelmen (now League of American Bicyclists). Most of the costs of load roads and streets are paid from local taxes (property taxes, sales tax, various other taxes) levied by cities and counties. I think there’s a fair question whether these should be shifted to vehicular taxes, but the current framework gives bicyclists a sound argument for shared use of the roads, and defense against bicycle licensure fees.

    The leading analyst of transportation externalities is Todd Litman of the Victoria Transport Polity Institute, For more than 25 years, Todd has been updating a model on transportation costs. The current iteration of Transportation Costs and Benefits Analysis is available on the VTPI website (This began when he was a graduate student at The Evergreen State College; I was on his thesis committee). The cost of driving exceeds $2/mile when all of the elements he examines are considered (and, yes, Sev, accidents are huge part of this). The full list is inserted below (the blog reply format does not make formatting intuitive).

    Vehicle Ownership Fixed costs of owning a vehicle.
    Vehicle Operation Variable vehicle costs, including fuel, oil, tires, tolls and short-term parking fees.
    Operating Subsidies Financial subsidies for public transit services.
    Travel Time The value of time used for travel.
    Internal Crash Crash costs borne directly by travelers.
    External Crash Crash costs a traveler imposes on others.
    Internal Activity Benefits Health benefits of active transportation to travelers (a cost where foregone).
    External Activity Benefits Health benefits of active transportation to society (a cost where foregone).
    Internal Parking Off-street residential parking and long-term leased parking paid by users.
    External Parking Off-street parking costs not borne directly by users.
    Congestion Congestion costs imposed on other road users.
    Road Facilities Roadway facility construction and operating expenses not paid by user fees.
    Land Value The value of land used in public road rights-of-way.
    Traffic Services Costs of providing traffic services such as traffic policing, and emergency services.
    Transport Diversity The value to society of a diverse transport system, particularly for non-drivers.
    Air Pollution Costs of vehicle air pollution emissions.
    Greenhouse Gas Pollution Lifecycle costs of greenhouse gases that contribute to climate change.
    Noise Costs of vehicle noise pollution emissions.
    Resource Externalities External costs of resource consumption, particularly petroleum.
    Barrier Effect Delays that roads and traffic cause to nonmotorized travel.
    Land Use Impacts Increased costs of sprawled, automobile-oriented land use.
    Water Pollution Water pollution and hydrologic impacts caused by transport facilities and vehicles.
    Waste External costs associated with disposal of vehicle wastes.

  7. Aren’t road repair and construction of new roads on the list? I’ve seen civil engineering estimates on the order of $1-2/gallon tax for decades, just to get caught up, assuming all road repair is paid for by gasoline taxes, which doesn’t make sense because weight and miles are more important.

  8. Would wear and tear on public roads also be an externality? Yes, gas taxes are supposed to cover these costs, and possibly do, but the damage is not totally a function of the amount of gasoline used, as seen by the concern by various jurisdictions over electric vehicles escaping those taxes.

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