In the event of the most likely type of nuclear attack, people would be better off ignoring the instinct to flee and remain inside their homes, security experts said Tuesday.

“The natural inclination is to flee and what they’re going to do is they’re going to get caught in gridlock,” said Cham E. Dallas, director of the Institute for Health Management and Mass Destruction Defense at the University of Georgia. “They’re going to get caught in gridlock because everyone else will be fleeing, and that is not viable. Most people should not flee” because they won’t be affected.

Dallas made his comments following a Senate Homeland Security and Governmental Affairs Committee hearing on confronting challenges a day after a nuclear attack.

Ashton B. Carter, co-director of the Preventive Defense Project at the John F. Kennedy School of Government, echoed the sentiment. “Certainly most of my friends and family who don’t do this, they think that a nuclear weapon levels an entire city and that anybody nearby better get out of the way or they’re going to get covered in radiation,” Carter said after the hearing. “That’s just not true.”

In fact, Dallas’ written testimony said, “The highest impacts of radiation generally occur when people are caught in the open, or are tied up in traffic jams trying to escape in vehicles which provide little protection against fallout.”

As part of his testimony, Dallas presented visuals of the impact of a 10-kiloton (kt) detonation near the White House. He said the bombs dropped on Hiroshima and Nagasaki were only slightly larger. Both Dallas and Carter said 10 kt is the standard reference.

“It could be expected that there would be at least 150,000 serious injuries and that at least 70 percent of these could involve fatalities,” Dallas said in his written testimony. “Depending on the resources made available at the time, it is likely that there would be an attempt to evacuate as many as 500,000 people from the area, though the efficacy of such an attempt is dubious.”

“This would be the worst disaster to befall the country, but it’s not the case, as many believe, that the entire city would be destroyed,” Carter said after the hearing. “Most of the people in this city would be unaffected — and those that were affected but not killed would have a great chance to save themselves.”

Carter said most Washington residents wouldn’t need to leave their homes even if the nuclear weapon went off downtown “because the radioactive debris would be carried downwind from them.”

In response to a question from Chairman Joseph I. Lieberman, I-Conn., about what the most important thing the federal government can do to be prepared to respond the day after a nuclear attack, Dallas said his priority would be to get the truth out about the limited, but serious, impact of such a scenario.

“The misconceptions of the public about radiation are incredible,” he said.

The panel’s ranking Republican, Susan Collins of Maine, said she thinks the public has “gone backwards” since the 1950s and 1960s in terms of understanding what to do in the event of a nuclear attack after listening to witnesses “predict that most members of the public would immediately begin fleeing when, in fact, sheltering in place for many of them is the better option.”

After the hearing, Lieberman said, “Rather than attempt an evacuation, which might be futile given gridlocked roads, the vast majority of people beyond the blast zone need to listen carefully for official instructions through the media and shelter in place. A mass evacuation would disrupt rescue and response efforts and possibly expose evacuees themselves to lethal radiation.”

Daniel Fowler can be reached at dfowler@cq.com.