Because fog delayed George Rashid Sr. one day more than 30 years ago, it didn't delay Jerry Davis of Richmond one day early this spring when he was wheeling through a heavy dose of it along Interstate 80 in Pennsylvania for Preston Trucking Co. of Preston, Md.

Each time a special light on the dashboard of his truck was activated, Davis changed lanes, soon passing a previously hidden slow-moving or stopped vehicle. After this had happened a few times, another trucker who had started to follow Davis cranked up his citizens band radio and inquired:

"Hey, Preston, what you got in there -- radar? Haw! Haw! Haw!"

Davis's reply that this was the case was met first by silence and then by pungent expressions of disbelief until he had a chance to explain that his truck was equipped with a prototype radar unit invented by Rashid.

The Rashid Radar Safety Brake consists of a miniature radar unit near the truck's front bumper that transmits a narrow signal down the road in front of the truck. If the radar beam strikes another object and bounces back to the receiver-antenna, a computer under the hood determines whether the space between the truck and the object is narrowing so quickly that a collision could be imminent. If so, it activates a red dashboard light and then, if the driver does not slow the vehicle enough to widen the gap, a buzzer sounds a more strident warning, with the loudness of the noise depending upon the degree of danger.

Davis, who has been driving for Preston for seven years, and Don Cable of Fort Wayne, Ind., a 15-year veteran for the company, have been testing the trucks equipped with prototype units built to Preston's specifications by Rashid's Vehicle Radar Safety Systems Inc. of Mount Clemens, Mich.

On Friday an agreement was reached under which Rashid's company will manufacture 551 of the units for Preston. The Radar Safety Brake first was developed for automobiles and got its name because the original prototypes go one step further than the units Preston has tested and will acquire: In the for-autos model, if the driver doesn't react sufficiently to the buzzer, the unit will slow the car by easing up on the gas pedal and depressing the brake pedal.

Preston's safety director, Don Hansen, wanted warning-only units for the company's truckers. "I did not want to take the brake away from the professional truck driver," he said, citing extra dangers that truckers face, such as jack-knifing rigs.

The company hopes to begin production within 45 days and eventually will be able to produce up to 200,000 units a month at its plant in Kenosha, Wisc., according to Jack E. Rashid, a son of the inventor.

"We hope within 90 to 120 days to make this available to the general public" in Michigan, Illinois, Wisconsin and Massachusetts, while the company also hopes to make the devices available in the Washington D.C. area some time next year, he said. The Radar Safety Brake will be sold through car dealerships and car repair facilities that meet Vehicle Radar Safety Systems' standards, he added.

The units for automobiles will carry a retail price of about $400, plus an installation fee of between $35 and $50. The truck units will be priced at $800 because they have to cast longer beams to compensate for trucks' greater stopping distances.

Officials at Preston Trucking read about Rashid's prototypes in July 1980, arranged for a demonstration and then struck a deal for the two modified prototypes.

Driver Cable has put 120,000 miles on his truck testing the device over the past 18 months. "I'm very enthused about it," he said. "It does everything the inventor told us it would. It gives you a lot of peace of mind. It's my backup system: If that audio warning goes off, it bumps you back to reality" if a driver's attention has wandered.

The elder Rashid had come to the United States from outside Beirut at age 16 with his brother Carl. He studied electronics at Cass Tech in Detroit, and started out in business with a small grocery store. By the late 1940s, he owned eight grocery stores, three automobile dealerships, vast amounts of property in the Detroit area, a home in Gross Pointe Park and a farm in Anchorville, Mich. He was 39 years old.

When the elder Rashid was attempting to return home from the farm one day in the late 1940s, he hit fog so dense that he had to pull off the road and wait for a semitrailer to come by so that he could follow its lights home, son Jack recalls. The drive took about five hours instead of the normal hour and 45 minutes, and it was then that he decided there had to be a better way.

He took over the recreation room of his house, cut the basement in half and sealed the windows. "That was the first lab," Jack Rashid said. "Five years later, he came up with the first prototype that could fit on our kitchen table." Final development had to wait about 30 years for the necessary miniaturization -- for chips and microprocessors to replace bulky and fragile vaccuum tubes.

The company is a family affair. Son Jack is vice president for marketing, distribution and administration, his brother George Jr. is a vice president and treasurer, and his other brother, Charlie, is vice president for engineering.