I-10 in Houston, TX (source: Wikimedia Commons)

Highways, revisited

How we can make highways cleaner, more efficient, and a lot less frustrating

Although we take them for granted today, highways are a relatively new city fixture. Not long ago these roads were built through the hearts of every major US city, drastically changing transportation of people and goods and impacting where we live and work. They were originally promised as a way to move people through and around cities more quickly, and in that capacity, they do their jobs well…except during the times we need them the most.

Highways have become synonymous with the American dream: cruising down the open road through the countryside on our way to our single-family home with a picket fence. Unfortunately, they have also become synonymous with rush hour traffic jams and severe pollution. Given the high cost of maintaining highway infrastructure, it is time for us to reevaluate our future transportation options. If we save ourselves the cost of building and maintaining new infrastructure while also increasing efficiency to speed travel and reducing carbon emissions, we will truly achieve something special. Hopefully, this article will point us in the right direction.

Traffic and Traffic Jams

Traffic is, unfortunately, a reality of a successful city. That is to say, congestion itself may not actually be a solvable problem in a desirable city. Think of a trip to an amusement park: the newest and most fun rides tend to have the longest lines, while the older, more boring ones tend to have little to no wait. Cities are the same way – the most exciting attractions tend to draw the biggest crowds.

Highways and roads, in general, are the city equivalent of waiting in line. Urban planners call this concept derived demand: when people are forced to do something like wait in line to do their desired activity: in the case of the amusement park, riding a roller coaster. Just as amusement park planners build waiting areas for rides (think about all those switchbacks), city planners build roads to accommodate all the people coming into the city for work, baseball games, concerts, restaurants, etc.  

This looks fun! (Photo Credit: Wikipedia)

Just like theme parks, highways are designed to handle their peak load; designers build lines to accommodate wait times based on their predictions of how many people will be on the road when its busiest. On rainy or cold days, park visitors may find themselves wandering through fields of empty switchbacks, whereas on a sunny summer day, visitors may find the line overflowing. Highways have the exact same problem: at peak times, they may not have enough room to accommodate everybody who wants to use them – the result is a traffic jam.

So if the problem of traffic is unsolvable, why do we even bother trying? We can probably never fully eliminate congestion, but we can change the type of traffic we have to reduce its environmental impact. Cars take up a lot of space on the road and emit harmful fumes into the air. Consolidating people into fewer cars would cramp the people, but help us cut pollution. As city populations continue to grow, we have to start planning for ways to fit more people on the road. This means fitting more people into the space we have and ensuring they’re on the road for as little time as possible.

Life in the fast lane

Since traffic is fundamentally a problem of trying to fit too many people into too little space, the logical next step is to determine how much space we need to accommodate the traffic. Imagine that you were sitting next to a highway counting the number of cars that pass by you in a single lane of traffic. As the highway approaches its maximum capacity before cars start having to slow down, you’d observe about 32 cars go by per minute or about 1 car every 2 seconds. As the highway fills up, the number of cars passing you would slow as the speeds drop to well below the 55 mph speed limit. If you were to multiply it out, you’d find that a highway can handle about 1,800 cars per lane per hour.

The Math (notice how units that are common between the numerator and denominator cancel):

Adding lanes is often proposed as a solution to the traffic jam problem, but that has already proven itself to be ineffective. As we build new highways, we invariably find that they are overcrowded almost as soon as they open. The I-405 in LA is a case study in this phenomenon — right after a new lane opened, travel times actually increased! At peak highway demand, or “rush hour,” everybody is trying to use the highway at once which overcrowds the highway and pushes travel speed down. On top of that, expanding highway capacity does nothing for the smaller roads that bring cars to and from the highway. Think again of our amusement park: adding lanes would be like adding more line space – it would simply allow more people to wait for a long time without actually helping deliver them the experience they want any quicker.

Exit strategies?

Since adding lanes to the highway has been proven ineffective, let’s explore another solution on the table: using the space we have more effectively. Perhaps we can revisit the formula from above and add in another term: capacity.

These equations show that at complete free flow, a lane that is exclusively used by high-frequency and high-capacity buses can have almost as much throughput as a highway lane at full capacity. Instead of having cars bunched together at dangerously close intervals, the bus has room to maneuver and still can move almost the same number of people. As more people crowd onto the highway at peak demand and traffic slows, the bus lane will still continue to move people efficiently. The following is an interactive example to prove the point:

Below is a standard Level of Service chart that highway engineers would use. If you were sitting on the side of a highway counting cars all day and measuring their speed, you would likely come up with a plot similar to this. The fewer cars on the highway, the faster they can all go, but as the highway fills up, they start slowing down, until the highway reaches a point that the amount of congestion has completely almost completely stopped all movement. Many Americans (and people around the world!) waste hours per day during their commutes sitting at the bottom of this curve.

But if a highway’s capacity is generally fixed, how do we move more people? The answer is efficient transportation modes. The typical car carries 1.2 people and covers about 128 square feet of road space (8 ft wide & 16 ft long not counting the space between vehicles). If we could find ways to fit more people into space we already have, we could help eliminate traffic congestion. Public transportation is one way to go about this: buses and trains have a far higher throughput than private cars. Walking, cycling and other non-motorized (or active) modes also have higher spatial efficiency than private cars.  

I have put together a small demonstration that uses the cheapest motorized alternative to relieve traffic congestion: the bus. In this example, you will be able to see the difference between putting a bus lane on a typical 4-lane highway, thus reducing the highway to three lanes and a bus lane. Depending on bus ridership and frequency, and how congested the highway is, you will be able to see what implementing this type of policy can do for a highway throughput. Your output chart will look something like this (which is what you will see with the default inputs to the app):

Check out our highway traffic model interactive here

As you adjust the sliders on the app, consider the following questions:

  • How does the capacity of the highway change with traffic speed?
  • Can you get the highway with a bus lane to outperform the highway in the off-peak condition?
  • Can you get the highway with a bus lane to underperform versus the highway in peak conditions?
  • If you were to design a system to optimize your local highway’s performance based on this model, what would you do?
  • How do you think the numbers would look if you put a train system in instead of a bus?
  • What are some advantages and disadvantages of scaling up to a train?

Play around with the simulation and see how changing how we use the lanes on our highways could allow for more efficient travel. My generation – and yours – will be tasked with not only managing transportation projects but financing them as well. So next time you hear that a multi-billion dollar highway expansion project is in the works, be skeptical – the science hasn’t proven that it will solve the problem.

It’s time for us to rethink our highway policies.


Leave a Reply

Your email address will not be published.


Follow by Email