Diane K. Steed has been running the National Highway Traffic Safety Administration since administrator Raymond A. Peck Jr. left May 21. But Steed has been in the driver's seat with a temporary license.

Steed, who served as Peck's deputy administrator, took over as acting administrator the day he left. She served 30 days in the acting post and then transferred all of the administrator's duties to the deputy's slot, which she now fills.

The bureaucratic lane change was caused by an unexplained delay in her nomination. But despite the stall, Steed's supporters, including many agency regulars, say they are not worried.

"We don't know what the holdup is, but most people here expect her to make it okay. She is the boss around here, and that's the way it's been," one of them said yesterday.

Steed began her federal career in 1965 as a congressional intern with Robert J. Dole (R-Kan.), then a representative. Dole is now a senator and his wife, Elizabeth Hanford Dole, is Transportation secretary, with jurisdiction over the NHTSA. Steed worked for Mrs. Dole shortly before Peck resigned.

Peck's legacy has kept Steed busy. For example, the NHTSA is working on a response to the Supreme Court's June 24 ruling that overturned its move to scrap rules requiring new cars to be equipped with air bags or automatic seat belts.

The passive restraint regulations were supposed to take effect for 1982-model cars. But Peck withdrew the rules in October, 1981, as part of the Reagan administration's deregulatory drive.

The court remanded the case to the U.S. appeals court here and ordered the NHTSA to review the rules and either let them stand or come up with a better reason for junking them. NHTSA officials yesterday said their review will take at least another week. * * *

THE FRONT END . . . The effects of one of the NHTSA's regulations will be seen on the front ends of several 1984-model domestic luxury cars. Domestic auto makers for years have been trying to get a variance in federal headlight standards, which required the use of rectangular or round headlights in a sealed housing. The rule stemmed from safety and political considerations.

Sealed lamps keep moisture and other light-deflecting substances out of the lens housing. Largely because domestic suppliers made only rectangular and round sealed headlights, the government ruled that domestic auto makers had to incorporate them in their designs to make them easy to replace.

But European suppliers had developed replaceable-bulb headlights that could be shaped to fit the varying contours of their cars, particularly the sleek, aerodynamic models. And this year Ford Motor Co., Chrysler Corp. and General Motors Corp. petitioned the agency to allow curved or angled headlights.

There was some concern that approving the change would open the door to the foreign manufacturers. But the NHTSA approved the petitions in May, taking into consideration that headlights that were more aerodymnamic would improve the fuel efficiency of domestic cars.