The Fort McHenry Tunnel in Baltimore contains an induction loop system in the pavement. (Maryland Transportation Authority)
Columnist

My wife, Joan, and I regularly drive through the Fort McHenry Tunnel in Baltimore and have wondered about some surface markings in the lanes. At several places, there are tar markings that look like, for the lack of a better word, square-shaped “eyeglasses.” We’ve speculated on what the markings might mean, but haven’t been able to come up with a good answer.

Tom Malarkey, Silver Spring, Md.

As it happens, the markings don’t mean anything. That is, they are not signs, directing us to MAINTAIN SPEED or indicating that there are TOLL BOOTHS AHEAD. Rather, they conceal equipment that allows the tunnel’s overseers to monitor traffic conditions.

It’s called an induction loop system, and the process is pretty simple. Workers cut a shallow, moat-like trench in the pavement in the shape of a rectangle, circle or square. Laid into this trench are several loops of a coiled wire. The trench is covered with a rubbery material, and the wire is connected to a control box.

Those loops of wire — typically about six feet in diameter — create a magnetic field. When a big piece of metal enters the field — a car, a truck — it causes the field to fluctuate. That data is sent to the control box, where the pulse can be parsed in all sorts of ways.

Induction loop systems have been in use for decades, said Nadeem Chaudhary, senior research engineer at the Texas A&M Transportation Institute at Texas A&M University.

The technology has many applications. “At traffic lights, the traffic signal changes based on vehicle detections,” Chaudhary said. Where a minor street intersects with a main street, a traffic light might be kept green for the bigger road until a vehicle is detected on the side street. The main road’s light turns red, the side street’s light turns green, and the vehicle is cleared.

“In another case, they have detection on left turns, so they don’t serve an arrow unless there is a vehicle there,” Chaudhary said.

Induction loop systems can also raise the gate at a parking lot.

On a road such as Interstate 95 — the highway that flows through the Fort McHenry Tunnel — induction loops provide useful information.

“You can look at what percentage of the time there is detection of a vehicle,” Chaudhary said. “It’s called occupancy on the loop. If the occupancy is 10 percent of the time, you know that traffic is moving fine. If it starts to get to around 20 to 25 percent, you know there’s congestion.”

That information can be useful for short-term action — maybe reconfigure the direction of the lanes — and long-range planning: Build a bigger road!

An induction loop can identify the type of vehicle that’s passing above it, based on the vehicle’s size and number of axles. Position a pair of loops in sequence some distance from one another, and you can gauge a vehicle’s speed.

The benefit of an induction loop system is that it’s fairly easy to install and cheap to operate. The drawback is that installation requires closing a lane and cutting into the asphalt or concrete. The constant grind of traffic can cause the pavement to eventually crumble.

The Fort McHenry Tunnel opened in November 1985. It’s the world’s largest underwater highway tunnel, as well as the widest vehicular tunnel ever built by the immersed-tube method, where tunnel sections are constructed elsewhere, then floated to the location and sunk into place.

Kerry Brandt of the Maryland Transportation Authority told Answer Man that the tunnel’s induction loops have been supplanted by a newer system that uses cameras to track vehicle speed.

Other systems employ radar, Chaudhary said. (Both technologies have drawbacks. A camera’s view may be blocked by a large vehicle. Radar works best when vehicles are moving, not when they’re stopped.)

Gerardo Flintsch, director of the Center for Sustainable Transportation Infrastructure at the Virginia Tech Transportation Institute, said that induction loop technology shows promise for something else: charging cars.

Most electric vehicles have a range of 200 to 300 miles, requiring drivers to stop and plug in their batteries. “You can buy a Tesla for $120,000, but every two to three hours, you have to stop to charge it,” Flintsch said. “What they’re trying in different places, and we’ll probably be testing at VTTI, is whether we can charge the vehicle with these induction loops.”

Loops are placed in roadways so that the battery is magically topped up as an electric vehicle passes over them. The principle is already being tested in Europe.

The challenge is getting the technology cheap enough. “It’s still quite expensive for practical use,” Flintsch said. “My guess is that’s the solution we have for doing long-range electric vehicle trips.”

Twitter: @johnkelly

For previous columns, visit washingtonpost.com/johnkelly.