No one knows what will happen when the toll lanes open on North MoPac Boulevard.

Yes, there will be an extra lane in each direction of MoPac for the 11-mile stretch north of Lady Bird Lake to help ease congestion. The northbound leg starting just south of Far West Boulevard and running to Parmer Lane should open later this summer, followed by the rest of the northbound lane and the entire southbound stretch by year’s end.

But with Austin getting its first taste of variable tolls, what will the prices look like and how many drivers will pay for the express experience?

Around the United States, there are 30 urban highways that have some form of variable tolls. But many of those roads have “time of day” tolls, charges that vary on a set schedule and typically are highest during rush hours.

North MoPac tolls, on the other hand, will have no such schedule and will change in response to traffic flow as often as every five minutes. And even among U.S. roads with “dynamic” tolls such as North MoPac, many have a maximum toll. North MoPac will have no toll ceiling.

On the vast majority of those 30 roads, including all but one in Texas, cars with two or three occupants may use the toll lanes for free or at discounted rates. Not on MoPac.

Most of those other roads have two or more toll lanes in each direction. Not here, where, because of space constraints, there will be just one MoPac toll lane northbound and one southbound.

Most projects elsewhere separate the toll lanes from the adjacent free lanes with concrete barriers. On North MoPac, though, pylons and stripes on the pavement will be all that stand between those who pay and those who don’t.

Officials last week said that partial opening of the northbound toll lane could happen as soon as August, pushing it back from June and July opening dates and adding to a series of delays that have plagued the project. Will tolls skyrocket at rush hour? And what will all this mean to traffic on the free lanes?

The Central Texas Regional Mobility Authority, which is building the toll lanes, has commissioned studies that tell it all will be well — that the system will nimbly toggle the tolls to keep traffic flowing in the express lanes at 45 mph or more, and that the levy will remain reasonable even at rush hour.

But officials can’t be sure. And what happens with North MoPac will weigh heavily as the area ponders other toll lane projects, such as MoPac south of Lady Bird Lake and Interstate 35, envisioned as having variable tolls.

“It’s still a crapshoot on how many people are going to want to get into that lane, and what are the behavior characteristics of Austinites,” said Mike Heiligenstein, executive director of the mobility authority since 2003. “How frustrated are they by going 5 mph, and what will they pay to go to 50 mph? Our models tell us it will be $3 or less to go the whole way, and I could see the Austin commuter saying, ‘I’m willing to pay more than that.’ I hear that all the time: ‘Just give us a lane.’ ”

A matter of time

Tim Reilly says it’s all about the value of time.

Reilly, the mobility authority’s operations director, said drivers have perceptions about how valuable saving a few minutes on a trip might be to them. But that varies, he said, from person to person, road to road, city to city.

Once a new toll lane opens and people try it, their perception evolves as they confront the cost — particularly if those tolls fluctuate, other toll operators have told Reilly.

“When you first open it, a lot of people start using it and say, ‘Wow, I love this thing,’ ” Reilly said in an interview at the authority’s traffic management center in Cedar Park along the 183-A toll road. That enthusiasm lasts “until they get their first bill and realize how much they’re spending and say, ‘Well, I can’t use it that much.’ ”

Others avoid it at first, he said, but then give it a try after witnessing people streaming by them in the toll lane.

“They use it and say, ‘This was worth it.’ So it takes months until people figure out what their actual value of time is,” Reilly said.

Monitoring the flow

That’s where Reilly’s small staff and the “algorithm” come in.

North MoPac traffic will be monitored at all times by 59 “wavetronic” sensors and 17 cameras scattered along those 11 miles. The sensors, Reilly and toll operations manager Tracie Brown said, will constantly record traffic volumes in all of the lanes: the one toll lane and three free lanes in each direction.

The algorithm crafted by consultant Kapsch TrafficComm, with what amounts to coaching by its human overseers, will use those volumes to meet the authority’s goal of keeping traffic flowing at a speed of at least 45 mph.

Toll rates will be both the carrot and the stick.

When traffic is light on North MoPac, both in the toll and free lanes, the toll rate will be at its minimum rate of 25 cents for each of the two northbound and two southbound sections, or 50 cents to go the whole 11 miles. Those rates are likely to pertain throughout the night, at midday and for long stretches on weekends.

But as the sensors detect traffic volumes thickening on the free lanes, which are a precursor to more people beginning to choose the toll option, the toll rate will begin to creep up. Overhead signs stationed a quarter-mile to a half-mile before each of the four entryways to the toll lanes will alert drivers to what the current toll is, giving each of them a chance to make that “value of time” decision.

As rush hour approaches and volumes begin to rise rapidly, the sensors and the underlying software will switch their attention to the toll lanes. The point, Reilly said, will be to maintain speeds between 46 mph and 54 mph in the toll lanes.

Entering traffic volumes of 1,550 to 1,850 vehicles per hour on each lane would probably trigger such speeds, and the tolls would rise to whatever it takes to discourage enough people from taking the toll lane to keep volumes in the express lane from rising above that level.

Then, as traffic ebbs at the end of rush hour, the tolls would go down to bring some other drivers back into the fold.

The algorithm, Reilly said, relies on a history of traffic volumes and toll rates, then adjusts to what is happening on any given day. That means, he said, the authority and Kapsch have had to construct a virtual history for the beginning of operations, a set of predictions. After a few months of human and computer tweaks (using not only the sensors but the camera images relayed to that Cedar Park center), and that adjustment period for the public, he said, “we’ll let the algorithm do the work” absent wrecks or other unusual disruptions of the toll lane traffic.

The Dallas experience

After a five-year construction project ended in September, Interstate 635 crossing North Dallas has had dynamic tolling on six added lanes over a 13-mile stretch from Interstate 35 to U.S. 75. Drivers with at least one passenger can also use the toll lanes at a discount.

A private consortium built the toll lanes (along with revamped free lanes and frontage roads) and will operate them for decades under an agreement with the Texas Department of Transportation.

According to a June report by the Metroplex’s Regional Transportation Council, average speeds in the free lanes have increased from a morning rush hour low of about 30 mph to more than 50 mph, and to more than 60 mph in the evening peak period. The toll lanes, meanwhile, are running at close 70 mph all day long, the report said.

Tony Hartzel, a spokesman for TxDOT’s Dallas district, said the public has accepted the varying tolls.

“We actually get a lot of appreciation,” Hartzel said. “What we always say is, it gives you an alternative; you don’t have to use them. But people are using them, and what it is showing is that it is improving traffic on both the regular lanes and the (toll) lanes.”

The great unknown

The delay in getting the MoPac project finished (it will be more than a year late) and the construction-related headaches have not put Austinites in an appreciative mood. And with just one added lane on each side of North MoPac, open only to toll payers, transit buses and Capital Metro registered van pools, sharp spikes in tolls could happen.

Toll lanes on Interstate 405 east of Seattle have hit a $10 maximum for 17 miles several times in the past year, and a 16-mile Atlanta toll project on Interstate 85 has seen prices as high as $11, according to news reports.

“That’s the great unknown: How much demand is there in the marketplace?” Heiligenstein said. “If it opens up and a week later we’re seeing $5 or $7 rates, that just means there are more people willing to pay that. But that express lane is going to free up capacity on the general purpose lanes. Until that fills back up with latent demand, I think the toll price is going to stay pretty reasonable.”