Q: If you drive a sporty two-seat coupe, like, say, a Nissan 350Z, with only yourself and one passenger, how legal would you be in a carpool lane that requires at least three people? "I heard that driving in a three-person carpool is legal with only two people if your car only seats two," wrote Aaron Brooks of Duvall. "Is this true?"

A: The truth is, no, it is not legal for a two-seater vehicle to use HOV lanes that require at least three people, such as westbound on Highway 520, says Leah Bolotin, senior HOV planner for the state Department of Transportation.

The three-plus requirement applies to 520's westbound carpool lane between Interstate 405 and the Evergreen Point Floating Bridge, and the state says the reason is to improve traffic flow.

Motorcycles are permitted by federal law to travel in all carpool lanes because it is considered safer to keep two-wheeled vehicles moving than to have them traveling in start-and-stop traffic. But that has nothing to do with how many people the motorcycle is designed to carry, says Bolotin.

The state has concluded that the HOV lane on that limited stretch of Highway 520 cannot safely carry the kind of traffic that other carpool lanes do. (Bolotin says the lane was created in the 1970s from the outside freeway shoulder in order to allow buses to bypass toll booths on the bridge at that time.)

The lane is narrow, has no shoulder and handles merging traffic from on- and off-ramps. "To keep drivers safe, we limit the amount of traffic in that lane by only allowing vehicles carrying three or more people," Bolotin said. The restriction also helps reduce congestion at the point where the carpool lane merges back into general purpose lanes just before the bridge.

That location is a real bottleneck, says Bolotin, "and every additional vehicle forced to merge into the two lanes crossing the bridge causes longer backups for everyone."

So, the DOT weighed having a consistent 3-plus carpool lane for the length of Highway 520, or allowing 2-person carpools in places. "After consulting with local communities, we agreed that it would be better to get full use of the HOV lanes east of I-405 by allowing 2-plus carpools where it is safe to do so," she said.

As part of the Highway 520 bridge replacement and carpool project (www.wsdot.wa.gov/projects/SR520Bridge), the DOT says it will rethink the carpool situation. "Once this project is complete, we may be able to reduce the occupancy requirement to two people," said Bolotin. "We will decide whether to make a policy change after looking at the amount of traffic and the conditions on the rest of the freeway system."