An­swer Man in­ves­ti­gates: What are those weird pave­ment mark­ings in­side Fort McHenry Tun­nel?

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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 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.” For pre­vi­ous col­umns, visit wash­ing­ton­


The Fort McHenry Tun­nel in Bal­ti­more has an in­duc­tion loop sys­tem built into the pave­ment.

John Kelly's Wash­ing­ton

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