EVEN BEFORE ITS scheduled opening later this month, the last leg of Northern Virginia's most loved and hated highway -- I66 -- has commuters and other would-be users in a new tizzy: not everyone will be eligible to drive on this stretch in rush hours, which has caused the latest furor. Unless you are in a car with at least three other passengers or are on your way to or from Dulles Airport, you won't be permitted to take advantage of inbound lanes between 6:30 a.m. and 9 a.m. or of outbound lanes between 3:30 p.m. and 6:30 p.m. (Heavy trucks need not apply at any hour.) Is this fair to taxpayers who may not be able to muster the requisite number for a car pool? Does the policy make practical sense?

We had doubts -- but after checking with highway, police and other traffic experts from Richmond to the banks of the Potomac, we think the restrictions deserve a fair trial. Besides, the limitations, if not the precise hours, are as close to being engraved in stone as they could be -- the product of an agreement between former transportation secretary William T. Coleman, former Virginia governor Mills Godwin and a host of regional transportation agencies that freed federal money for this route.

But even if the restrictions could be dropped easily, the question is whether the policy serves the public interest fairly and actually improves the traffic flows. A decade of experience with express lanes on Shirley Highway points to success: police and highway officials report that at the peak of morning and evening rush hours -- at around 8 a.m. or 5 p.m. -- the express lanes are at capacity, filled with carpoolers, and that the express lanes carry far more people than do the outer lanes.

So why wasn't I66 built to accommodate a similar multiple choice? Because part of the reason for the I66 agreement and its restrictions was to reduce the highway from six or eight lanes to four. In any event, those who are not eligible to use I66 in rush hour can continue to drive their present routes and may find traffic lighter because other cars are on the new route.

If the Shirley Highway plan is any indication, in time drivers will find it worth their while to pick up passengers at bus stops. Police say enforcement is not complicated or costly -- though the fines and court costs for violators can be; they also warn against posing as Dulles Airport traffic, because they have ways of checking.

To those who question the precise hours of the restrictions (for example, is 3:30 too early?), officials say wait and see; the hours could be altered if traffic patterns prove different from what they've estimated.

Like it or not, I66 is here. The job now is to find out how to make the best of it.