KYLE GREEN The Roanoke Times

For now, the speed limit on Interstate 81 in Virginia will remain posted at 65 mph, except for 14.7 miles near Roanoke and segments in Harrisonburg and Winchester where the limit is 60 mph.

ERIC BRADY The Roanoke Times

VDOT will decide whether to raise the speed limit to 70 mph from 65 mph on interstates and major roads in Virginia.

Just because the speed limit on Virginia highways can go to 70 miles per hour on July 1 doesn't mean it will.

Gov. Bob McDonnell signed a bill Monday to raise the maximum potential speed limit on interstates and other major roads from 65 mph, effective in the new fiscal year.

But it's the Virginia Department of Transportation that decides whether to implement an actual increase and, if so, where.

That process could take four months or longer, transportation officials said Thursday.

Before it decides, VDOT intends to scrutinize each eligible section of road from a physical standpoint and crunch traffic, accident and law enforcement figures. It is not clear how long that will take, nor what priority it will receive. Until that work is done, nothing changes.

The upshot: Don't accelerate yet.

"These studies will take time, and recent staff reductions may slow down progress," said Jason Bond, a VDOT spokesman in the Salem District.

For now, I-81 will remain posted at 65 mph, except for 14.7 miles near Roanoke and segments in Harrisonburg and Winchester where the limit is 60 mph.

Eligible roads include up to 1,130 miles of interstate and 150 miles of multilane, divided, limited access highways, according to General Assembly staffers.

That would include sections of U.S. 460 Bypass in Montgomery County, where speed limits were raised in August 2008 to 65 mph from 55 mph.

A cost estimate for the project was not available.

VDOT's own tracking shows that many motorists have already driven at the planned new limit and presumably still do. In 2008, the last time the data came out, the average speed for cars on I-81 was 70 or 71 mph in several places in Southwest Virginia.

State police officials were disinclined to describe how troopers would respond to a speed limit increase.

Corinne Geller, state police spokeswoman, said the agency's official policy is "we enforce the speed limit."

Because resources are limited, however, troopers tend to pull over the drivers who pose the greatest risk -- those exceeding the limit most egregiously, she said.

Geller disputed the notion that there is a trigger point above the speed limit at which troopers issue tickets. Therefore, questions about the supposed trigger point going up along with any speed limit change were pointless.

Geller also said she could not discuss how higher speed limits might affect an existing Virginia law that deems going 80 mph to be reckless driving, a misdemeanor.

Should that be raised? She said she could not comment because the change in the law has not taken effect.

Del. Bill Carrico, R-Grayson County, who sponsored the House version of the speed limit bill and is a former state trooper, said the lawmakers might change the 80-mph threshold for reckless driving but not right away.

"We're talking about doing something to address that next year," Carrico said.

The options include imposing an enhanced fine but no misdemeanor charge for speeders clocked between 80 and 90 mph, Carrico said.

Outside a Christiansburg restaurant near I-81, in the court of public opinion, the speed limit increase authorization has critics.

Jim Worley, 75, of Pembroke predicted a 70 mph limit would lead to drivers routinely going 75.

"It's too darn fast," he said.

Charles Miller, 66, of New Castle agreed.

"Sixty-five is plenty fast," he said. "It should be 55."

During deliberations in Richmond, some who tried to block the rise to 70 mph argued to lawmakers that faster cars, trucks and motorcycles will have worse accidents. It's pure physics, critics said.

A 5 mph increase in a vehicle's speed in a collision results in an exponential increase in the kinetic energy released, warned Chris LaGow, an insurance lobbyist.

McDonnell said not to worry.

"This slight increase in our speed limit will be safe for motorists and help get Virginians to their destinations a little quicker each day. This is an important early step towards our common goal of improving transportation in the Commonwealth," according to a governor's news release.

State officials said 32 states already have 70 mph speed limits, and 13 allow motorists to drive 75 mph, the release said.

So much for reduced dependence on foreign oil.

The faster a vehicle goes, the more fuel it burns to go one mile, scientists say.

According to data the federal government developed from a 1999 study by Oak Ridge National Laboratory in Tennessee, gas mileage decreases "rapidly at speeds above 60 mph," says the U.S. Department of Energy Web site called fueleconomy.gov.

"You can assume that each 5 mph you drive over 60 mph is like paying an additional $0.24 per gallon for gas."