DALLAS (CBSDFW.COM)– They say time is money. Kristen Youmans of Dallas certainly feels that way. “How much is 30 minutes of my time worth,” she asked herself. “If it’s a $5 charge, I would say, yeah, I’ll do it”

Road crews are in the process of expanding a 13-mile stretch of Interstate 635 from Greenville Ave to Luna Road. When it’s finished four years from now, drivers will be asked, how much is their time worth?

“I would pay the extra couple of bucks to get where I’m going really fast, instead of having to wait an extra 20 to 30 minutes in traffic,” said Jennett Gentry of Dallas.

The LBJ project will add three more lanes in both directions. However, these extra lanes won’t be free. They’re called “managed” lanes and drivers must pay to use them.

Here’s the catch. The slower and more congested the managed lanes are; the more it will cost motorists. It may sound backwards, but the designers say it makes perfect sense. “50 mph is the threshold that we are required to maintain in our contract,” explained Andy Rittler of the LBJ Infrastructure Group. “If we go below that, then the toll will increase,” Rittler added.

The goal is to keep the managed lanes moving at a good clip. If traffic slows below 50 mph, adding more cars will only make it worse. So by charging more, engineers hope to discourage some drivers to steer clear of the managed lanes.

That’s why for the next 30 days, engineers are testing high-tech sensors that ultimately will determine how much motorists will pay.

“We have counters that count the number of cars in each lane and count the speed that they’re traveling, so we can post the toll on the sign before you go in,” said Rittler.

Tolls are likely to range anywhere from 15 cents a mile to 55 cents a mile, depending on how fast traffic is moving. The sensors can send back real-time data so the cost is likely to fluctuate as traffic slows or speeds up.

Managed lanes are not like HOV lanes where drivers can cheat the system by simply crossing over the white lines. There will only be five entry and exit points along the 13-mile stretch of LBJ.

Traveling the entire stretch could cost you as little as $2 to as much as $7, once again, depending on the speed of traffic.