A University of Maryland researcher has developed inexpensive wireless sensors that could avert the kind of bridge collapse that killed 13 and injured 145 along I-35W in Minneapolis four years ago.

"One of every four U.S. highway bridges has known structural problems or exceeded its intended life span. Most only get inspected once every one or two years. That's a bad mix," Mehdi Kalantari, an electrical engineering researcher at the university, said in a statement late last month.

Kalantari's tiny wireless sensors monitor a bridge's structural health, such as strain, vibration, flexibility and cracking. The sensors transmit minute-by-minute data to a central computer that analyzes the data and instantly warns officials of possible trouble. The professor founded a start-up, Resensys LLC, to ramp up production of the devices, which is slated to begin in September.

The sensors, which cost about $20 each, are rugged and could last more than a decade, Kalantari said. An average-size highway bridge would need about 500 sensors, for a total cost of about $10,000.

Newer "smart bridges," such as the I-35W replacement in Minneapolis, have wired networks of sensors. But Kalantari said the cost of wired systems is too high for use on older spans.

"A wired network will cost at least 100 times more than a wireless alternative, and that's simply unaffordable given the strain on local, state and federal budgets," he said.