What distinguishes Andrea Pearson from her fellow commuters at the Baltimore Harbor Tunnel is the little red box attached to her windshield.
It resembles a pocket pager: a half-inch thick with the dimensions of a credit card. But this modest gadget could soon make the toll collector as obsolete as the elevator operator.
No more stopping. No more searching for exact change. The device is a key component in a system that uses radio waves to identify and bill motorists who may never again have to stop -- or even slow down -- at a toll booth.
"It's an excellent idea," said Ms. Pearson, a Northeast Baltimore resident who drives to work each day in Linthicum. "It'll make the lines go a lot quicker. It's about time, actually."
As part of a 90-day experiment that began last week, one lane in each direction of the Harbor Tunnel has been equipped to handle the "TollTag" electronic toll collection system. The Maryland Transportation Authority paid the system's manufacturer, Amtech Corp. of Dallas, $25,000 to temporarily install it.
Ms. Pearson is one of 259 regular tunnel patrons who have been recruited to try out the device. If the test proves successful, electronic toll collection systems could be set up at all the harbor crossings, including the Fort McHenry Tunnel and the Francis Scott Key Bridge, in about two years.
"We don't have the capacity to keep expanding and expanding toll plazas," said John A. Agro Jr., the authority's executive secretary. "This could help move traffic a lot more effectively."
Similar systems have been up and running along the Oklahoma Turnpike, the Dallas North Tollway, and the Crescent City Connection in New Orleans, for two to three years. A coalition of seven toll authorities in New York, New Jersey and Pennsylvania is experimenting with radio wave systems to collect tolls along Interstate 95.
Here's how the system works:
An antenna mounted at the toll lane broadcasts a low-level radio signal toward the approaching vehicles. In the temporary setup used at the Harbor Tunnel, the unit broadcasts continuously. In a permanent installation, the antenna is activated by a sensor buried in the pavement.
A tag mounted on the vehicle's windshield reacts to the transmission. The tag, which is battery-operated, is able to relay an identification code as it bounces a signal back to the antenna. The results are analyzed by an electronic "reader," and the customer is identified on a computer system.
Each participant has an account, which is automatically billed. With the toll paid, the driver is cleared to go ahead.
In the Harbor Tunnel experiment, which started last Monday, commuters were asked to deposit $37.50 in an account in advance. They pay 25 cents for each crossing, a 15-cent discount from their usual one-way toll.
The whole process takes a fraction of a second. But to double-check their results, officials have given motorists a book of tickets and toll collectors have been tearing one off before each crossing -- as they would with a regular commuter pass.
In the first three full days of operation, only one mistake was detected -- and that was thought to be the result of human error.
"The experience so far has been pretty good," said Thomas J. Fallon Jr., tunnels administrator.
The chief advantage to be gained from all this is speed. A tunnel toll collector can process up to 400 vehicles an hour; an electronic system could handle 1,200 transactions in the same period of time.
That should permit the authority to save money. About 35 percent of the more than 160,000 vehicles that cross the harbor each day are driven by regular commuters. It would cost an estimated $1 million to have the necessary hardware installed in all the lanes at the Key Bridge and the Harbor and Fort McHenry tunnels. But the system would pay for itself in the savings in salaries and other operating costs, Mr. Agro said.
And then there is the matter of convenience. Theoretically a car doesn't even have to slow down at the toll booth. The Oklahoma "PikePass" system lets traffic breeze by at 65 mph, and Amtech claims a 99.9 percent accuracy rate at high speed.
In addition, state officials envision a day when all of Maryland's toll facilities use the system, including the Chesapeake Bay Bridge. They, in turn, could be interconnected with the other northeast states. Motorists could zip up I-95 without having to bring along a roll of quarters.
"You may never have to stop at a toll booth again," Mr. Agro said.
There are reasons to be skeptical, however. Five years ago, the authority tried using an Amtech radio system with employee cars at the Key Bridge. It failed badly: Tags were sometimes picked up by antennas in adjoining lanes.
According to Amtech, those problems have been corrected. "We didn't know about cross-lane reads," said David P. Kerr, an Amtech computer specialist. "It was like chicken wire and electrical tape compared with what we have now."
Metal can interfere with the TollTag system by blocking out the radio transmission, and so can human flesh. Amtech warns that the tag won't work if the driver puts it in his hands and wraps his fingers around it.
There are also concerns about privacy. Theoretically, the radio wave system would give the authority an electronic record of a motorist's comings and goings for a period of months and perhaps years.
To counter that concern, Mr. Agro said the transportation department will ask the legislature to require that toll collection information be kept confidential. He said he also wants the General Assembly to give him the ability to use video cameras to catch people who don't pay their tolls.
The automated system is designed eventually to be unstaffed. Without cameras, there will be no way to catch violators, Mr. Agro warned.
Since the mid-1970s, the authority has maintained an optical system to automatically collect tolls at the Thomas J. Hatem Memorial Bridge, which spans the Susquehanna River along U.S. 40.
It works like a supermarket scanner -- motorists paste a decal on their windshield that can be read with a laser beam. Officials say it has worked fairly well, but has limitations: It can't distinguish among millions of vehicles as the radio wave system can do or process cars as swiftly.