Mayor Dianne Watts is concerned that the tolls on the new Port Mann Bridge are driving away truckers, prompting them to take the Pattullo Bridge instead.

Watts, who is also the chair of Metro Vancouver's transportation committee, cited studies conducted by the B.C. Trucking Association and the City of Surrey on traffic across the Pattullo that show significant traffic increases from 18-wheelers. The City of Surrey counted the number of vehicles that entered the bridge over the last several months and found that more trucks are taking the narrow structure than before the new Port Mann's completion.

She noted that December, January and February saw truck traffic increases of 28 per cent, 30 per cent and 36 per cent, respectively.

"We're doing an analysis and we're looking at what has changed from last year to this year, in terms of different contributing factors," said Watts. "What has changed is that the tolls have been put on the Port Mann, that a section of the South Fraser Perimeter Road has been opened to the Pattullo, and that there's been some expansion at the Deltaport."

Watts said that while the numbers fluctuate, the Pattullo has steadily seen more volume from big rigs connecting to the bridge from King George Boulevard and Scott Road. That increase in congestion can affect Surrey economically by slowing the movement of goods throughout the city.

"What we've heard is that during rush hour, they will take the Port Mann Bridge and pay the toll because it's a lot more efficient," she said. "On off-peak hours, they will use alternate routes to avoid the tolls."

However, Transportation Investment Corporation, the public Crown Corporation that implemented the Port Mann/Highway 1 Improvement Project, is investigating the accuracy of the city's findings.

"Those numbers are not consistent with what we've seen on the Port Mann Bridge in the first three months," said Max Logan, director of communications for TI Corp. "We're seeing traffic volumes that are very similar to what we saw this time last year."

Logan said that an estimated 2.9 million vehicles crossed the Port Mann in January on par with estimates from January 2012. He said about 100,000 vehicles cross the Port Mann every day, with an estimated 10,000 to 15,000 being semi-trucks.

The B.C. Trucking Association is currently looking at different options to encourage truckers to use the Port Mann instead of the Pattullo. Watts mentioned a monthly pass with discounts for large trucks could help truckers who currently pay $9 per crossing.

But Logan said TI Corp has no plans to change the toll rates this year. He noted that truck drivers can already register with TReO for a 50-per-cent discount on overnight crossings between 9 p.m. and 5 a.m.

"We have heard from the B.C. Trucking Association that they would like us to consider additional discounts for trucks," he said. "If we did extend additional discounts to large trucks - over and above the 50-per-cent discount that we've already provided - that would likely mean we'd have to reconsider the other discounts we've given to cars."

He added that smaller vehicles get smaller tolls and bigger discounts because they make up the vast majority of traffic across the Port Mann.