When the Secret Service first described to us its proposal to eliminate vehicular traffic from two busy blocks of Pennsylvania Avenue, I and the five other persons serving as outside advisers to the Treasury Department's White House Security Review were dead set against it. We were all well aware that the presidency carries with it inevitable risks: Certainly, this president has been far more vulnerable on his two trips to the Middle East than he would ever be in the White House.

Moreover, as longtime Washington area residents and commuters, we were concerned about the effects on the city. We were also mindful of the public's possible reaction to restricting access to the people's house, and with this in mind, we consulted three of the four living former presidents.

But in the final analysis -- and unfortunately much of that analysis cannot be made public because it concerns sensitive security matters -- it became clear to us: The evidence unequivocally established that the No. 1 threat to the president in the White House, and to all those who work and visit there, would be an explosive-lad\en truck driven right up to the White House gates. A limousine, a large car, a station wagon, a bus would also have the capacity to carry such dangerous devices. And in fact all of these vehicles have been used to deliver explosives in one place or another in the world.

Surely those clamoring for the reopening of Pennsylvania Avenue to vehicular traffic cannot believe that the risks are imaginary [editorial, May 22; op-ed, June 8]. The increase in fanatical terrorism, foreign and domestic, the availability of powerful explosives and the proliferation of information explaining how to build explosive devic\es yield a potent mix that can no longer be ignored.

The recommendation we finally made to the Treasury Department was based on the realization that failure to adopt the Secret Service's proposal would undercut the service's responsibility to protect the first family and the government's responsibility to protect the people who visit or work in or near the White House.

Eliminating vehicular traffic from those two blocks of Pennsylvania Avenue was not a response to any of the specific events that precipitated the review. That is to say it was not intended simply to prevent another plane crash or an assault by a gunman. Our mandate from the beginning was to review all aspects of White House security. In fact our recommendation and Secretary Robert Rubin's decision were made prior to the tragic incident in Oklahoma City. But that tragedy, as well as the earlier bombing of the World Trade Center, painfully underscored the reality we must face.

Having served as secretary of transportation in the Ford administration, I was especially concerned about the transit implications of this act. So were the other advisers. All six of us racked our brains, our imaginations and our experience to come up with a solution that would keep some vehicular traffic on that segment of Pennsylvania Avenue. In the end, however, we determined that there was no feasible way to do it.

Nevertheless, the White House remains one of the most accessible executive residences and offices in the Western World. While the avenue is closed to motor vehicles, it is more open than ever to pedestrians. (And I do sense a weakness in the critics' argument that barring vehicles limits or thwarts the chances of out-of-town visitors to see the White House. I doubt that many who visit Washington to see the president's home content themselves with merely passing by in a car, taxi or bus.)

The security situation changes, and not always for the worse. American schoolchildren, for example, no longer have to go through drills to prepare for nuclear attack. On the other hand, we all now take for granted metal detectors at airports, and are becoming accustomed, reluctantly, to presenting photographic identification before boarding a plane. In the 1980s, access to the Capitol, the home of the people's Congress, was restricted to pedestrians in response to threats of Libyan-sponsored terrorism. Then as now, many Washingtonians grumbled about the traffic disruption, and complained that the deployment of Jersey barriers created a concrete perimeter around the Capitol grounds. We now take that change for granted.

The Jersey barriers currently blocking Pennsylvania Avenue are indeed unsightly. But they are temporary measures, to be employed only until a permanent redesign can be accomplished. The Park Service's proposed design shows that protecting the White House will not require unsightly barricades. The federal government should move quickly to implement a permanent plan.

Although only a handful of individuals will know the specific facts underlying our recommendation, anyone who reads the newspapers or watches television news will recognize that Secretary Rubin made the right decision.

The writer, a Washington lawyer, was transportation secretary in the Ford administration and is chairman of the NAACP Legal Defense and Educational Fund.