Some 20 years ago a book came out “Stuck in Traffic” by the brilliant Anthony Downs. One of his key points was the “Iron Law of Congestion”, sometimes called “Triple Convergence”, and now called “Induced Demand” which basically said if you expand a road, the extra capacity gets used up by people switching routes, modes, and time of travel. We might also add other effects of road expansion include changing destinations for non-work trips (making longer trips), making trips that would otherwise be foregone, and even changing jobs and houses, as well new development. While Downs did not discover this idea, (e.g. Lewis Mumford had said something similar) he popularized it.
Since Downs wrote the book in 1992, remarkably little has changed in how we travel. Not nothing of course, (travel rose for a few more years and has leveled off overall, and dropped in more recent years on a per-capita basis) but a lot less than you would expect given the changes in information technologies over the same period. No-one is satisfied with this status quo. Everyone is crying out for something different. We believe we can do better than daily congestion, excess pollution, devastating crashes, and all the other ailments associated with our existing transportation systems. In recent posts I identified peak travel, and made a speculative scenario of how traffic might disappear “on its own”. But of course, that won’t happen everywhere, and there are lots of things we can do to manage better.
Roads are perhaps the slowest changing technology. Once laid, they are difficult to move. Parts of the Appian Way, from two millennia ago, are still in use. Famously London could not change its street grid after the 1666 London Fire destroyed most of its buildings, despite an able plan from Sir Christopher Wren.
We can think of transportation as a layered system. There is the earth, on top of which are rights-of-way, within the rights of way are pavements (themselves layered). On the pavements are markings denoting lanes and directions. Above these are traffic control devices like signs and signals. Only then do we get to services, people driving their vehicles, trucks carrying freight, passenger buses, taxis, bicycles, pedestrians, etc. all riding on the layer of roads.
To the dismay of many transportation planners and engineers (many of whom got into the field to build things), the physical layer of the surface transportation network in the United States is largely complete. The projects that are left are projects that were too expensive to build the first (or second, or third) time, (much like the Second Avenue Subway in New York). These Zombie projects do not die, while construction is essentially irreversible, non-construction is easily reversed. In the end though, these are tinkering at the edges. Given the small amount of new construction, most travel 20, 30, or 50 years from now will be on roads that already exist.
Until we go airborne for short trips, we are highly constrained. As the world gets more developed, building new roads is progressively more expensive. The world adapts to the infrastructure that is provided, and builds as close to the right-of-way as possible, making expansion that much more difficult.
While we have scarce right-of-way (scarce in that it is limited, and finite, and at times fully utilized given the applied technologies for its use). We lack time. We have limited energy. This waste has both supply and demand aspects. Yet collectively we don’t do relatively easy things that would reduce the waste of these scarce resources.
- Most roads are under-utilized most of the time. We have plenty of capacity outside the peak.
- Most of the pavement is unused even at peak times, there are large gaps between vehicles both in terms of the headway between vehicles  and the lateral spacing between vehicles (we drive 6′ wide cars in 12′ lanes, often on highways with wide shoulders).
- Most seats in most cars are unoccupied most of the time.
- Most cars are carrying around far more weight than required to safely move the passenger. While bigger cars tend to be safer for the occupants, they are less safe for non-occupants. This is an inefficient arms race.
- Most roads are so wide we use them for storage of vehicles most of the day.
- There is a tremendous amount of excess delay at traffic lights, especially at off-peak periods, wasting time (and space).
- Most trips during peak periods are not work trips and have temporal flexibility, yet these trips travel in the peak because they are underpriced.
- Most trips produce negative externalities (pollution, congestion, noise, risk of crash) in excess of the price paid by their driver. They produce so many of these externalities because they don’t pay for their full cost.
So instead of expansion, we should instead think about ways to use that scarce right-of-way (and our scarce time and energy) more efficiently.
- Where there is congestion, we should price roads to encourage use in the off-peak and discourage use in the peak. This revenue should be used for the operations and maintenance of roads and should largely replace existing funding sources (fuel taxes, vehicle taxes, property taxes). Prices need to be systematic, not just on specific routes, to maximize system efficiency. We should avoid having a random set of underutilized toll roads, while free roads remain congested.
- Pollution and noise and crash risk should have their own externality charges.
- We should encourage narrower vehicles and, for instance, provide two six-foot lanes for narrow vehicles in place of one twelve-foot lane where we can, and promote use of driverless cars so that cars can use less space. Lane widths are standard, and changing them would require changing standards. Ultimately we should move to a model where we don’t need pre-defined lanes, but rather have vehicles move as near each other as possible without colliding, such as we do when walking in crowds.
- Cars should be lighter. If all cars were lighter, everyone would be safer. The greatest risk is when big car/truck meets small car. In particular we should encourage use of neighborhood cars that are specialized for local, lower speed travel. This may or may not require private vehicle ownerships as opposed to vehicle rental/sharing.
- We should promote technology to enable real-time, ad-hoc ride-sharing (with compensation for the ride provider) to better utilize excess capacity within vehicles. (In many places the compensation is illegal, as it looks and smells like a taxi, which are highly regulated).
- We should narrow up roads where we can, and use strategies so that people can share more cars, so we need fewer of them, so we don’t need to spend as much road space for vehicle storage. Road widths are again set by standards, often determined by the fire department (which does not want to back up their trucks).
- We should be able to eliminate many traffic signals with appropriate use of roundabouts (and later with driverless vehicles). Given the vacant space available on roads, if vehicles and inter-vehicle communications were better, we should be able to arrange real-time coordination of vehicle movements and have as a goal eliminating almost all stopped delay at undersaturated intersections that are today signalized. Pedestrian/vehicle conflicts might still remain, and require controls.
This is hardly a complete catalog of what we can and should do, but I hope the key point, there is plenty of pavement already, we just need to use it more wisely, comes through.
 If we follow the “2 second rule” (2 seconds between two successive vehicle’s front bumpers) (or 1800 vehicles per hour), at 60 mph we have a vehicle density of 30 vehicles per mile, or 176 ft per vehicle. Obviously with congestion, we are wasting time because we don’t increase throughput and we decrease speed, though we increase density. Vehicles are typically 26 ft, so we are using about 7 vehicle lengths for every vehicle we are moving at free flow speed near maximum stable throughput on a pipeline section without a bottleneck.