The concept "bridge freezes before road surface" has been demonstrated often on the Parkway West at the Carnegie interchange, below the towering railroad trestle.

A 622-foot bridge, one of three near the interchange, freezes up before the rest of the parkway. Drivers go from wet pavement to a skating rink.

But no more.

After recording 132 crashes at the site in a 12-year span, the Pennsylvania Department of Transportation is installing an automatic anti-icing system on the bridge this summer, part of a $5.4 million project to rehabilitate the three parkway bridges near the Carnegie exit.

The system has sensors that measure the temperature of the bridge deck and the air and, when needed, activates a pump that sends brine solution through a network of spray disks embedded in the bridge, said Todd Kravits, PennDOT district traffic engineer.

"It just puts out enough of the mist to keep the bridge deck from freezing. It holds us over until we get a plow or salt truck out there," he said. "We see this as a very effective means to try to prevent some of these accidents."

The bridge is especially prone to accidents because it is sloped and is on a slight bend. A crash there often produces a chain reaction involving multiple vehicles, Mr. Kravits said.

PennDOT has installed similar systems on a Route 28 bridge near the Butler County line and the new ramps at the Parkway West/Interstate 79 interchange. It hopes to install another on the Parkway North spans over McKnight Road.

The system will have a pump house building next to the outbound off-ramp. When it is activated, the mist from the embedded spray nozzles will shoot no more than 15 inches high, but flashing lights will warn approaching motorists that the system is operating.

Mascaro Construction Co. of Manchester has begun work on improvements to the interchange bridges, which were built in the early 1950s and are rated structurally deficient. At present, work is being done under the bridges without impacting traffic.

That will change later in the season, PennDOT spokesman Jim Struzzi said.

In June, the contractor will start to replace bearings and pedestals on the bridges, which will require jacking the spans to remove existing bearings. During those operations, traffic will be halted for up to 15 minutes at a time during off-peak hours.

In late July or later, expansion dam replacement on the bridge over Bell Avenue and Arch Street will require lane closures on four weekends.

Installation of the spray nozzles for the anti-icing system is expected to cause lane restrictions during off-peak hours in August, Mr. Struzzi said.

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