Bus- and train-ridership numbers show mistakes stretching back to 1995, said UTA Chief Operating Officer Jerry Benson. The problem is due to inherent shortcomings in all three counting systems UTA uses, none of which can measure ridership numbers with absolute certainty, he said.

But it wasn't until 2004, when federally required random sampling seemed to show about 8,000 fewer riders on weekday buses, that UTA managers realized something was very wrong - and perhaps expensive.

It turned out that a UTA employee responsible for tracking the random samples failed to follow the rules and reported falsely, Benson said. That employee, who was not named, has since retired.

The numbers gathered for the feds go into the National Transit Database, which contains public information used by transportation agencies, academics and others. But they also figure into calculations of how much federal money UTA may receive.

Benson said he didn't know how much UTA might have missed out on because the miscounts came at a time the agency was expanding.

But UTA General Manager John Inglish promised a deeper probe to find out. "We probably lost some money," he said.

Keeping accurate count of transit riders is necessary for UTA to manage its budget and plan for the future. The agency has relied on bus drivers and hired surveyors, who count heads on the first Tuesday and first Saturday and Sunday of every month, to gather numbers that statisticians use to reach their conclusions. Electronic counting systems also contribute.

Benson said those ridership numbers, though not absolutely accurate, appeared consistent, with none of the spikes that showed up in the purportedly random counts.

The federal numbers are compiled annually from random counts taken throughout the year. The first downward plunge showed up in 2000. Ridership appeared to spike up and down for the next three years then plunged to the nadir of about 15,000 average weekday bus riders in 2004, down from a peak of about 23,000 in 1997.

No one, however, could explain why it took so long to figure out there was a problem.

Benson said all of the monthly ridership reports for this year will be corrected. UTA also will hire an outside consultant to re-examine the reports that an accounting firm and state legislative analysts already have examined and found wanting.

TRAX trains are equipped with electronic counting technology that also is being installed in FrontRunner commuter rail trains. UTA has installed new mileage counters on buses that will help clear up inaccuracies logged between 1999 and 2005, Benson said.

Keeping track of riders is of paramount importance as the biggest bus-route redesign in UTA history is set to start up on Sunday, said UTA board member Keith Bartholomew.

"We should expect to see a drop" in ridership in the coming months, he said. But accurate numbers will help UTA evaluate how the redesign is working and how well buses are connecting with TRAX, he said.