He added, “We were more concerned with that than an attack on the motorcade.”

The spokesman for the Secret Service, Ed Donovan, pointed out that the motorcade does not move in normal traffic conditions, saying there is typically “no other traffic on the road at the time the presidential motorcade is moving.”

The so-called secure package of vehicles that includes the president’s limousine and the agents traveling with him could easily detach from the vans and take off, Mr. Donovan said.

“As far as the ambulance being at the back of the motorcade, the doctor is in the secure package in the event they are needed,” he said.

Privately, Secret Service officials said they did not use agents or uniformed personnel to drive the vans because it was not the agency’s responsibility to protect the White House staff members or journalists. White House officials said they were forced to use volunteers because staff members needed to be with the president at all times, and reporters demand that they travel with the president wherever he goes.

Often, the White House reaches out to campaign volunteers or friends to be drivers. One woman who drove a van during Mr. Obama’s trip to Arkansas in May said that a mass email had gone out to students at the Clinton School of Public Service at the University of Arkansas in Little Rock asking whether anyone wanted to drive.

There is a long history of accidents in presidential motorcades dating back more than a century.

In 1902, a trolley car in Lenox, Mass., struck and killed a Secret Service agent in President Theodore Roosevelt’s protective detail. And during the 2008 Democratic primaries, a motorcycle police officer escorting Hillary Rodham Clinton was killed in Dallas after performing a “leapfrog” maneuver to speed from one intersection to the next, sometimes done at over 100 m.p.h.