The National Transportation Safety Board has issued a report blaming both the 18-year-old driver of a dump truck and the signal system at an intersection on Rte. 50 for an accident last summer in which four automobile passengers were killed.

The accident occurred last July when the driver of the dump truck, which was carrying 17 tons of stone, ran a red light at Rte. 50 and Virginia Rte. 28 and collided with a northbound car.

The board report recommends to the Virginia Division of Motor Vehicles that it change current regulations to require a road test for heavy truck operators. According to the recommendations, an operator's license should be issued only if operators pass a road test, using the vehicle they expect to be driving. The recommendations, issued last week, were agreed to by all four members of the safety board.

According to the report, Virginia and many other states do not require such a test if the driver is 18 years old, has a chauffeur's license and states that he has driven at least 500 miles in the type of vehicle he expects to operate.

The driver of the dump truck in the accident filed such a report and did not take a road test on the type of vehicle he was operating. "It is doubtful that the driver could have accumulated the heavy vehicle experience he claimed since Virginia does not issue a learner's permit to drive heavy trucks and he did not drive such a vehicle on his previous job," the safety board report states.

As for the signal system, the safety board recommends adjusting it so that the length of time of the green light takes into account the high speed that many trucks use to approach the intersection from Rte. 50.

The current system automatically adjusts to changing traffic conditions, with the length of the green light varying from 15.2 seconds to 58 seconds. "The variable cycle length," the report states, "could have caused some motorists to think that the signal was malfunctioning . . . Such changes often caused driver indecison and may lead to 'maladaptive' behaviour, such as running a red light."

Apparently that was just what the driver of the dump truck thought. although he was familiar with the intersection and had passed it several times a day during his one-month employment period, he said after the crash that the green light had remained on for a short time and that he thought it was not working well.

If lengthening the time of green lights is not desired, the report says, additional traffic detectors could be placed farther away from the intersection so the signal would remain green until fast-moving traffic passes the intersection.