Back in 1941, you could have hopped into your Delahaye 135 convertible and rolled along the Pennsylvania Turnpike at a steady speed of 70 miles per hour. And you would have been completely legal.

And you would have filled your gas tank for 18 cents per gallon.

When the turnpike opened in October 1940, there were no enforced speed limits except a posted 35 mph in the tunnels. In 1941, speed limits were set at 70 mph for cars and 50 to 65 mph for trucks.

Last month, the speed limit was raised in specific stretches from 65 to 70 mph. It hadn't been changed since 2005, when the limit was raised to 65 all along its length, except for tunnels, toll plazas and the curved portion near the Allegheny tunnel.

The new 70-mph speed limit on the turnpike was activated in July from the Blue Mountain (Interchange 201) to Morgantown (Interchange 298). PennDOT announced plans to raise the speed limit to 70 on two interstates: 88 miles of Interstate 80 from Exit 101 (DuBois) in Clearfield County to mile marker 189 in Clinton County; and 21 miles of Interstate 380 from Interstate 84 in Lackawanna County to Exit 3 (Pocono Pines/ Mount Pocono) in Monroe County.

"If all goes well, the rest of the 550-mile toll road system, including the Northeast Extension, could follow next spring," Turnpike CEO Mark Compton said during a press conference.

What makes the turnpike able to accommodate high-speed travel? Pennsylvania State Police Lt. Edward Murphy of Troop T, the unit in charge of turnpike patrols, said the turnpike was designed to meet federal standards. It was planned and constructed end-to-end, compared with other road systems where pieces of existing road were cobbled together.

"There are several factors in determining what the speed limit should be on a road," Murphy said. "The design of the highway is often the most important factor, but you must also consider traffic volumes and weather conditions, and use speed studies to determine the average speed of travelers.

"The Pennsylvania Turnpike was the first of its kind in the nation," Murphy said. "It was built to serve the people who would drive on it, and designed for that, instead of having its design controlled by terrain.

Erin Waters-Trasatt, Pennsylvania Department of Transportation deputy press secretary, said raising the speed limit from 65 to 70 on additional stretches of interstate won't be a decision to be made lightly.

"We're only piloting the program on routes 80 and 380, she said. "We'll analyze data in the spring and summer of 2015 and that's when we'll determine if any 65 areas will go to 70."

Act 89 of 2013 paved the road for enabling the Pennsylvania Turnpike Commission and PennDOT to be able to raise the maximum speed limit on limited access freeways within the state to 70 mph.

"Limited access" means there are ingress and egress ramps for the highway.

Lt. Murphy pointed out that the posted speed is the maximum speed allowed.

"Just because that's the maximum posted speed, doesn't mean that's the speed you should be traveling all the time," Murphy said. "That's the maximum speed under ideal weather conditions; you can be driving 60 miles per hour in a snowstorm and be ticketed for driving too fast for conditions."