As the summer approaches and more families hit the road, many will take for granted the stability of the bridges that get them where they're going.

Keeping the 600,000 bridges in the U.S. safe is no small challenge. That’s why engineers are beginning to fit some of the most-traversed structures with sensors that can alert them to potential problems.

The collapse of Minneapolis's I-35 bridge across the Mississippi river in August 2007 served as a wake-up call to many. The bridge collapsed without warning during the evening rush hour, killing 13 people and injuring 145. It served a busy eight-lane highway, and had been built in 1967 with a life expectancy of 50 years. Investigators laid the blame on faulty steel plates connecting the beams of the bridge.

Sensors might prevent a repeat. The National Institute of Standards and Technology (NIST) and the Michigan Department of Transportation are funding a $19 million project to build next-generation monitoring systems for bridges. "There are quite a few bridges that get a D grade" for maintenance and safety, says Marc Stanley, director of the Advanced Technology Program at NIST, which is promoting sensor-laden "smart bridges."

See a slide show of smart bridges worldwide.

Sensor technology can make a big safety difference by giving a bridge a "physical" and then monitoring its health, says lead researcher Jerome Lynch, an assistant professor of civil and environmental engineering at the University of Michigan, Ann Arbor.

Lynch and his colleagues are exploring how to make a cement-based sensing skin—basically a mesh of steel, carbon or polymer fibers covered with cement—that can detect excessive strain. Individual sensors, he says, are limited in usefulness because the initial cracks in a bridge may not occur where the sensor is placed. On the other hand, a skin is ideal because a deformation can be detected wherever it happens.

In addition, Lynch and his colleagues are experimenting with a paint-like substance made of carbon nanotubes that can be applied to the surface of bridges to detect corrosion and cracks. Since carbon nanotubes conduct electricity by sending a current through the paint, he says, it is possible to detect structural weakness through changes in the electrical properties.

Also in development by the same partnership are sensors that would measure how a bridge sways or bounces when a heavy vehicle speeds across it. Such information is not available today because bridge engineers typically make approximations and assume that a truck is just standing (rather than moving) on a bridge. (This simplifies calculations but may not represent reality). Lynch says the new sensors would be housed on city buses or police cars or other vehicles that regularly travel on a bridge.

The next generation of sensors to monitor bridge health will be wireless, which will make installing them a lot cheaper (the miles of wire needed to connect sensors to the central computers can add significantly to cost). Already a few bridges in South Korea, China, and Taiwan have tested wireless sensors, and they have done extremely well, Lynch says.

The New York City Department of Transportation uses sensors to determine how the structures are responding to the weather, including which antifreeze to use in icy conditions. The technology is advancing rapidly, says Bojidar Yanev, the department's executive director of bridge inspection and bridge management. What is feasible today, he adds, "would not have been possible even five years ago."