Former Senate sergeant-at-arms Terry Gainer, right, shown here with President Barack Obama in March, believes the Capitol complex should be expanded. (Chip Somodevilla/Getty Images)

Nobody knows better than Terry Gainer how to make the U.S. Capitol secure. He’s the only person to have served as both chief of the Capitol Police and sergeant at arms of the Senate.

So it’s worth heeding Gainer’s views, now that he’s free to speak out after retiring last month from the Senate job at the end of 47 distinguished years in law enforcement.

Gainer says he thinks that the threat of terrorist vehicle bombs obliges Congress and the District to substantially expand the Capitol complex by gradually buying up surrounding blocks and banning traffic there.

Basically, he wants to create a spacious, pedestrian-only campus at the east end of the Mall. It would ban vehicles from more than a dozen square blocks where they’re now allowed and extend from the Capitol north to Union Station and east to Second Street.

Many in the city are sure to object. His plan would wreak havoc with traffic patterns, especially by blocking through traffic along Constitution and Independence avenues. Hill residents already moan about the lack of parking.

But Gainer said last week in a wide-ranging interview that safety must come first.

“First Street and Second Street are dangerous areas to the buildings, to the Supreme Court, the House and the Senate, because of what truck bombs and car bombs can do,” he said.

“There are those who would like to exploit vulnerabilities,” he said. “Action after something happens is fighting the last war.”

Gainer would also put a fence around Capitol Square, similar to what he called the “functional yet tasteful enclosure” around the National Gallery of Art Sculpture Garden.

These aren’t idle musings. The Capitol Police Board, which he chaired, has commissioned studies on such proposals. None has been shared with the city yet.

I support Gainer’s vision, for the sake of both security and expanding green space downtown. To minimize disruption, he would stretch out the project over decades, perhaps taking up to 50 years.

“The Mall gets larger, greener [and the] air cleaner, and safety abounds,” Gainer said.

The authorities would have to guarantee that any changes wouldn’t infringe on the constitutional right to peaceably assemble.

Gainer drew criticism in 2011 for potentially reducing space for protesters when the Capitol Complex annexed Union Square, just west of the Capitol, from the Park Service.

In the interview, Gainer also urged our region to prepare more for a possible evacuation and recalled a career that amounts to a highlights reel of U.S. police work.

He graduated from the Chicago police academy a week early so his class could serve during the infamous protests at the 1968 Democratic convention. (He said he wasn’t involved in any of the cops’ misbehavior, which led an official investigation to condemn what it called “a police riot.”)

Later, Gainer was No. 2 in the D.C. police force during the Sept. 11, 2001, terrorist attacks and the sniper slayings the next year. He dealt with anthrax and ricin threats at the Capitol, as well as playing host at presidential inaugurations and State of the Union addresses.

One major misstep was poor crowd control that caused hundreds of people to miss the 2009 swearing in of President Obama because they were trapped in a pedestrian backup in the Third Street tunnel.

It was instantly nicknamed the Purple Tunnel of Doom because of the color codes on their inauguration tickets.

“I took a tongue lashing for that” from Senate and House committees, Gainer said. “It was probably a well-deserved beating.” He added that the problem was solved for the next inauguration.

Gainer, 66, who lives in Annapolis, is working as a part-time senior adviser to the private security company Securitas.

As part of that job, he’d like to see greater cooperation between government and business in our area to prepare for terrorist attacks or other emergencies.

In particular, Gainer urged more exercises in which authorities practice sending people home from work.

“The public-private partnership has to be better on that. Everybody can’t dump their buildings at the same time,” Gainer said.

“Maybe the first reaction, when we don’t know what’s going on, is shelter in place,” he continued. “Then, in order not to overload the streets or the Metros, it would be nice if people were patient and we practiced, ‘Okay, all buildings between L [Street] and N [Street], you leave now.’”

Fifteen minutes later, another set of buildings could evacuate, and so on.

Better to be overprepared than sorry later. We should listen to this voice of experience.

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