Commercial trucks and motor homes - put together in the same classification this year by the Mackinac Bridge Authority - have similar impacts on the structure because of their 7,200-pound to 30,000-pound weight, Mr. Sweeney said.

"RVs are heavy vehicles," he said. "Anything above 7,200 pounds, the bridge is going to feel it going across."

The change in classification, and the way the recreational vehicles were checked through the booths by toll collectors, made it virtually impossible for the authority to accurately track the number of motor homes crossing the bridge this year. The change was made to correct an earlier error that drew some customer complaints - charging a motor home towing a car the same rate for the car as for the multiple axle motor home. (Almost half of the motor homes that do cross are towing cars, Mr. Sweeney said.) Now that oversight has been corrected, but discrepancies in hitting the "recreational vehicle" tally button in the booth among toll collectors skewed this summer's record of that traffic category.

A look at campground occupancy rates on each side of the Straits suggests that some campers stopped short of crossing the Mackinac Bridge. Several private campground owners have speculated that high gas prices this summer and higher tolls on the bridge may have kept the big campers out of the U.P.

In Mackinaw City, Wilderness State Park reported only a slight drop in occupancy of 2%, and no discernible drop-off in motor home stays this summer, while on the St. Ignace side, Straits State Park reported a 14% decline in business.

"They just seemed to stop at the bridge," said Wayne Burnett, who manages both campgrounds. "In July and August, every day Wilderness had 20 or 30 more campsites filled than Straits."

High gas prices and high bridge fares were among the most frequent complaints he heard from customers, he said. While the two parks on each side of the Straits of Mackinac had been "about equal" in occupancy in past years, this was the first year he noted the drop off.

In the commercial trucking sector, bridge traffic fell 3.5% for the year to date. Trucking companies were among those who voiced opposition to the toll increases at public hearings a year ago. There is little if anything they can do to avoid paying the higher prices. Now, almost a year later, while individual truckers are bearing higher costs that eat into their profits, said Tom Barnes of the Michigan Association of Timbermen, commercial truck companies are still crossing, as they are locked into their delivery commitments by contract.

The industry has appealed to the Mackinacd Bridge Authority for a commuter-type discount, without success.

"They're penalizing the users who bring commerce across the bridge...," he told The St. Ignace News. "About all we can do is voice our displeasure."

Commercial truck tolls rose this year from $3 per axle to $3.50 and will be increased every two years to $6 in 2014. The giant vehicles hauling Upper Peninsula logs and wood products, for one example, typically have 11 axles, paying $38.50 per crossing.

Motor homes took the largest initial hit in this year's price increase, jumping from $2 per axle to the $3.50 truck rate March 1, and in the future will rise the same as trucks.