The need to get from A to B is something we’re all familiar with.

In the US, most people spend about an hour traveling to and from work – not to mention all the other sorts of traveling we do. But despite all the work engineers put into making those journeys go smoothly, it seems like things go wrong more often than they should.

Maybe you’ve been delayed at the airport, encountered signaling issues on the subway, or got caught in a traffic jam. While the weather often plays a role, one of the biggest reasons that transportation systems run into issues is that they have to deal with another persistent, difficult, unavoidable element: us. If you want to design a good transportation system, you have to understand the people who are going to use it. [Theme Music] Transportation engineering is one of the largest disciplines of civil engineering.

Every highway, railroad, shipping lane, and airline route relies on transportation engineers to guide the way. Suppose you were tasked with setting up a subway system for a city. You’ll need to plan, design and construct every last element before it can function.

What’s more, transportation systems are, almost by definition, heavily connected – every piece needs to support all the others. It’s no use having state-of-the-art trains on brand new tracks if the stations aren’t built where people need them. And once a system is up and running, you’ll need to constantly monitor how it copes with demand.

That will help you make decisions like how to improve access to stations and how often to run trains. The same kind of thinking applies to plane routes and shipping systems, although roads and highways have some extra considerations. The physical aspects of engineering we’ve learned about so far play a big part of the design work.

You need to build components out of the right materials and put in the right safety features. For example, you’ll want to build the subway’s tracks with enough mechanical strength to avoid buckling because of an overloaded train. And the key element in transportation engineering is people.

We’re a core part of the system. Unlike other disciplines we’ve looked at, this kind of engineering isn’t about processing inanimate materials into something that we use or consume. It’s about moving people in a way that’s safe, convenient, and comfortable.

That means considering social requirements in addition to physical ones. For instance, how do you decide where to build tracks, and which locations they should connect? Ideally, you’d build enough infrastructure to support every journey that anyone would want to take.

But, like everything else in engineering, cost is one of the biggest constraints you have to work with. Since you probably have a fixed budget for setting up the system, you need to prioritize what what gets built and where. And to do that, you have to understand what kinds of journeys are going to be made.

So with a subway, some people will want to visit their friends or go shopping, while others are just trying to get to work. The layout needs to support those different needs, while making enough from the passenger fares to keep the system up and running. So the social and economic goals of your city have a big role in the design of the network!

In general, transportation engineering is often about taking a bunch of different priorities and combining them in the best – but rarely perfect – way. And that applies on every scale! Your new subway system, for example, might have a point where two lines merge into one, taking passengers from different starting locations to the same stop.

But if two trains approach at the same time, how do they both pass through? Obviously, no one wants a collision. So your job as an engineer is to design a system where the odds of that happening are as small as possible.

Since we’re dealing with heavy objects travelling at high velocities, the routes need to be designed to allow changes to those velocities well in advance. While a car traveling at highway speeds can safely stop over a distance of a hundred meters by applying the brakes, a passenger train travelling at the same rate needs over a kilometer before it comes to a halt. That’s also why air traffic controllers have to constantly monitor and communicate with planes that are flying at similar altitudes.

Because a passenger plane can’t be maneuvered quickly, the flight paths need to be designed to ensure that flights from different locations overlap as little as possible. Schedules need to keep them out of each other’s way, while also avoiding turbulent weather. But to avoid collisions at your train junction, you’ll need signaling.

To give each train enough notice when it has to stop, you’ll need regular signals at intervals along the track. Those intervals are also known as block sections. Signals give useful information about what’s happening on the tracks several sections ahead, like whether there’s another train or if it’s safe to travel at full speed.

To allow enough time for the train to respond and stop, block sections tend to be no shorter than the necessary stopping distance. Older signaling systems rely on a set of lights above or beside the tracks that a driver can interpret and respond to. For your brand-new subway, you’d probably build in wireless signaling capabilities to relay information straight to the train cabin, to the driver or an automated system.

If two trains are approaching the same junction, signaling would tell one of them to slow down several block sections ahead while allowing the other to pass through. And that choice can be made in a few different ways. If one train is slightly closer to the junction, it might make sense to give it priority while the other is told to wait.

But if the other train was delayed earlier on its route, you might want the signals to give it priority, even if it’s farther away, to help get it back on schedule. Or, if you want to get as many people to their destinations as quickly as possible, you might let the train likeliest to be carrying more passengers through first. The details will depend on the particulars of the routes and the needs of your system.

Railway signaling involves managing a whole network! Modern approaches are also sophisticated enough that safety mechanisms, like electronic sensors, will automatically stop a train if the signals indicate danger ahead. When they’re working, the combination of signals and tracks give trains a big advantage: they go where they’re supposed to, when they’re supposed to.

But many people mainly get around with vehicles like cars, trucks and motorbikes. In Britain, for example, about two thirds of all journeys are taken in an automobile. With every driver free to move independently, roads lose much of the predictability that railroads enjoy.

Potential collisions aren’t confined to known junctions – they could happen every time someone changes lanes! And even if accidents don’t happen, all that switching back and forth causes something we’re probably all familiar with: traffic jams. To understand why, it actually helps to think back to fluid dynamics.

In general, a highway system is well-designed if as many cars as possible are flowing through the system as fast as safely possible. So, on a given stretch of highway, you can think of the “efficiency” of the system as the flow rate of cars. The flow will be the product of the density of cars passing through and their average speed – much like the flow of water through a pipe.

Except the driving force here isn’t water pressure. Instead, it’s every driver’s individual desire to get to where they’re going. If there’s only a few vehicles on the road, it’s not a problem.

The cars don’t have to fight for lane space or slow down because of other drivers. Everyone cruises along at roughly the speed limit. That’s what engineers call “normal flow”.

In normal flow, you can increase the total flow rate just by adding additional cars. In other words, more people move through the system and the highway is doing its job. Add too many cars, though, and the problems start.

In normal flow, the extra cars don’t affect the average speed of each vehicle. But once the density of cars reaches what’s called a critical point, the lack of space between cars starts causing restrictions. Everyone drives a little slower to accommodate the other cars around them.

Add even more cars, and you start getting stoppages. When one car slows down or stops, the others behind it have to slow down too, and so on. This brings down the average speed of all the cars on the road.

Even though there are more cars, the flow rate actually decreases past the critical point because their average speed has dropped so low. That’s what engineers call forced flow. Everyone’s desire to get ahead actually makes the problem worse.

Each car trying to desperately squeeze through every opening makes it harder for the jam to ease up and for the cars to flow steadily again. There might be other routes drivers can use to get to their location, but in general, every driver wants to take the shortest route, which seems pretty sensible on the face of it. But when everyone uses the same route, forced flow occurs.

You might think that building another road or adding more lanes would improve things. In practice, though, most people still take the route they think will be fastest, which only makes things worse! So highway engineers tackle problems like this in sometimes unusual-sounding ways.

One solution comes from German mathematician Dietrich Braess. Instead of adding more roads, you can actually improve traffic flow by taking them away. That might seem counterintuitive, which is why it’s called Braess’ Paradox.

After all, how can the flow be improved by removing a highway? Braess’ Paradox works because when the seemingly-fastest route is taken away, drivers end up using several alternative routes instead. Even if those alternate routes are longer, they only take a fraction of the cars each.

That keeps speeds higher and the flow rate closer to normal. On average, people will even get to where they’re going faster on the longer route. Of course, that’s just one solution that only works for some highways.

In general, you need to accurately model the flow of traffic and develop sophisticated ways to keep it flowing. Many of the the same principles used for signaling on railroads carry over to programming traffic lights and even the design of roads themselves. The way roads merge and connect has a big impact on how drivers act and the eventual flow patterns.

In the future, if fleets of driverless cars take to the road, it could require whole new principles of highway design – much like automatic train networks did for railways. But even if humans won’t be behind the wheel, transportation engineers will still be there, ensuring that those journeys are designed to be safe, fast, and convenient. In this episode we looked at transportation engineering.

We saw how transport systems are designed from the outset, what decisions need to be made, and how to operate them with the help of signaling. We learned how systems like highways are built around the behaviors of their users, how considering traffic flow lets us assess a highway’s efficiency, and ideas transportation engineers can use to improve it. Next time we’ll explore how engineers create communication systems like the one you’re using right now.

Crash Course Engineering is produced in association with PBS Digital Studios. Wanna keep learning? Check out Origin of Everything, which explores the history behind stuff in our everyday life, from the words we use, the pop culture we love, the technology that get us through the day, or the identities we give ourselves.

Subscribe at the link in the description. Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people.

And our amazing graphics team is Thought Cafe.