WASHINGTON (AP) _ The federal government has moved to require that tractor-trailer trucks carry reflectors that outline the bottom edge of their trailers.

The Federal Highway Administration said Friday the change should reduce the chances that vehicles will smash into or drive beneath trailers. The agency estimated the change will save more than 100 lives and prevent 1,700 injuries in the next decade.

Since January 1, 1993, the government has required the bottom edge on all new trailers be rimmed with red-and-white reflective tape or with so-called reflex reflectors, like the red plastic devices used on bicycles.

Once the rule announced Friday takes effect in two months, truckers using trailers made prior to 1993 will have two years to install similar warnings. Truckers who have outlined their rigs with tape of a different color must switch to the new color scheme within 10 years.

The order marked the first time the Federal Highway Administration has required trucking companies to retrofit older vehicles to meet a new-vehicle standard.

The change was welcome news to Beth Hall, of Allentown, Pa.

Her father died in 1993 on Route 313 in Pennsylvania when a tractor-trailer jackknifed and blocked all lanes on a highway. Driving at night, Carl Hall apparently spotted the lights on the cab and swerved left to avoid it.

Instead, he smashed under the darkened trailer he could not see and died instantly.

Beth Hall began a campaign to put reflective tape on truck trailers manufactured before the 1993 rule change. She estimated the cost per truck at $50.

In a previous interview with The Associated Press, Hall recalled meeting with then-Transportation Secretary Federico Pena to push for the new federal rule.

``He thought reflective tape would be easy. I'd meet with (his deputies) every few months. They kept saying, `Yes, reflective tape is going.' They thought it would be out by this spring,'' Hall said during the summer of 1997.

On Friday, members of the Hall family were on Capitol Hill with Rep. James Greenwood, R-Pa., to announce the new rule.

A spokesman for the federal Office of Motor Carriers, which oversees truck safety, said the delay was due to negotiations over what turned out to be a nearly 90-page rule.

Among the parties to the negotiations was the American Trucking Associations, which represents major trucking firms in Washington.

``There were a lot of comments and different points of view on how to do it,'' said OMC spokesman David Longo. ``Everyone agreed that it should be done, but there was discussion about the timing.''