On some Dallas County freeways, traffic moves so fast you can't drive the speed limit.

Armed with lasers, Dallas County sheriff's deputies recently went out to study the average speeds on various segments of the freeways all over the county.

NBC 5 was the only station to obtain the results of the study, which pinpoints the fastest spots on the freeway system.

One of the spots with the highest percentage of speeders is right in the center of Dallas.

Nearly all drivers speed in one section of Woodall Rogers Freeway. The test found that 92 out of every 100 drivers at Akard Street drove faster than the 50 mph speed limit.

The speeds get faster north of downtown Dallas -- and the number of speeders is about the same.

At Interstate 35E and Medical District Drive, the speed limit is 60 mph. But 95 out of every 100 drivers were speeding during the deputies' test. Forty percent of all drivers were going at least 11 mph faster than the limit.

When NBC 5 visited that location again with deputies, many drivers were traveling faster than 75 or 80 mph in relatively heavy mid-morning traffic.

Until recently, there's been nothing to slow them down.

No one was enforcing the speed limit on Interstate 35E north of downtown. Dallas police pulled their squads off the freeways to focus on city crime and deputies were only patrolling freeways in southern Dallas County.

"They know there's no one here right now," Sgt. Don Peritz said. "There's no one here right now. You can see it from the speeds."

But the sheriff's department is using the results of the study to target some of the fastest locations.

In January, it started writing tickets on Interstate 35E from downtown north to the State Highway 183 split. By March, deputies will cover that road all the way to Carrollton.

"That's one of the highest injury areas in the city," Assistant Chief Deputy Gary Lindsey said.

The department's target list also includes stretches of Interstate 35E south of downtown and sections of Interstate 30.

The study also found that average speeds only come down if police are visible on the same section of freeway day after day.

When the department frequently wrote tickets along Interstate 35 near Interstate 20 last summer, about half of drivers were following the speed limit. But when deputies pulled out of the area, only 10 out of every 100 drivers followed the limit.

"When police presence is no longer there, then everyone is speeding again, and we're going back to our routine," said Alex Del Carmen, a criminologist at the University of Texas at Arlington.

Dallas County deputies say they would also like to reduce speeds on Interstate 635, where the percentage of speeders could be just as high as some sections of Interstate 35 and Interstate 30.

But they said feel they can't effectively do so with the limited funding they have through an agreement with the city of Dallas to enforce speed limits.