HAMPTON — Gordon Wood slipped his silver Chrysler Lancer into the Hampton toll plaza Wednesday morning, dropped his 75 cents into the basket, then sat with his wife Joan and watched nervously as a trio of stone-faced New Hampshire state troopers surrounded the car.

Nothing like this had ever happened at a toll booth before, the retired couple from Glen, N.H. thought. As they watched, they asked themselves one question: what did we do wrong?

"I thought the license plate fell off or the rear wheel was going," Joan said. "I didn't think we did anything wrong.

What they had done was drive the one billionth vehicle to use the New Hampshire turnpike system since it was established in 1950. They were pulled over to the toll booth office for a small reception and awards ceremony.

"I got to get my cap on," Gordon said, opening the trunk. A short man, with a slightly bemused look on his face, he pulled out a blue baseball cap with "Beaver Creek" stamped on it and slid it over the few gray hairs on his mostly bald head.

"If I had known, I would have shaved," he said, stroking the gray stubble on his face.

The couple had spent a week on a ski vacation in Beaver Creek, Colo., and were heading back home when they were interrupted by the ceremony.

"Bet you were glad you were on the road today," said Peter Carlson, New Hampshire's turnpike administrator.

Lincoln Rich, a retired toll plaza attendant who manned the booths the first day they opened on June 24, 1950, gave the Woods enough tokens for 52 free turnpike trips.

Rich modeled the uniform he wore on that first day on the job, but had undergrown it since, and the pea green suit hung on him like a well-tailored burlap sack.

The Woods also accepted a framed certificate .and a pass to New Hampshire's state parks before they obligingly faced the media horde and then headed north again.

Before leaving, they said they had been driving the turnpike since the early days'', but weren't on hand when it first opened.

"Good," Carlson said when he found out. "It would have looked too staged if you had."

The Woods weren't actually the one billionth customers to use the turnpike. Carlson said they were more of a symbolic one billionth, but were probably within a hundred thousand or so of the magic number.

Carlson said the exact billionth would have been impossible to find because the number of cars that pass through the toll booths each day aren't figured until the next day. Figures from Monday showed the one billionth car would pass through sometime Wednesday around 10 a.m.

The Woods, passing through just after 10 a.m., were close enough.

The Hampton toll plaza was picked to honor the one billionth vehicle because it was the first in the turnpike system, Carlson said.

The number of vehicles driving the turnpike has risen dramatically since it first opened. Thirty years had to pass before a half-billion cars drove it, but only nine years passed before the second half-billion cars went through.

The Deportment of Transportation estimates that only nine more years will pass before another billion cars use the turnpike.

Carison said the numbers are rising so fast because the population is growing, and more people are driving more cars more often.

Other Turnpike Fun Facts Carlson handed out during the ceremony: The New Hampshire turnpike system has raised $306.5 million in revenue for the state's highways; it made more than $29 million in fiscal year 1988, compared to $391,000 in its first fiscal year; if one billion cars were lined up bumper-to-bumper, they would circle the earth's equator 161 times; 94,062 vehicles passed through the Hampton toll plaza August 14, a single-day record for any of New Hampshire's seven toll plazas.