Baghdad, Iraq ? A popular anti-terrorism commercial in Iraq shows a lone man rigging his car with explosives, chaining his hands to the steering wheel and speeding toward a market packed with women and children.

It is a chilling scene that focuses on a truth about terrorists: Suicide bombers attack alone.

This month, Iraqi authorities began cracking down on solo drivers, a measure that underscores the alarming ability of insurgents to strike at will. More than 400 Iraqis have been killed over the past two weeks, the majority of them blown apart in car bombings.

There’s no official order from the central government to ban driving alone — the tactic comes from local police chiefs struggling to battle crafty insurgents.

In Tikrit, Saddam Hussein’s hometown north of the capital, police in early May introduced a temporary ban on all solo drivers. In Baghdad and other cities, men alone in their cars are routinely singled out as suspicious and questioned at checkpoints. While some Iraqis said they support any attempt to stop the barrage of bombings, others complain that authorities have gone too far.

“This multiple-passenger law will paralyze life inside Baghdad,” said Jassim al-Wazzan, who owns a car-rental agency.

The crackdown on solitary drivers even worked its way onto a recent call-in show with a top Iraqi security official. An irate resident yelled into the phone: “What comes next? There’s nothing left. You’re even stopping people from driving their cars alone!”

In Tikrit, police said, the crackdown is necessary. Two days before the ban on solo drivers, a car bomber detonated on a bridge, killing seven Iraqis and wounding a dozen. Rumors abounded that insurgents had packed 25 cars with explosives and would set them off, one by one, throughout the city.

Tikrit Police Col. Majid Ahmed responded to residents’ fears by blocking some roads and seizing cars driven by lone men. He instituted a temporary ban on solo drivers that drew mixed reactions.

“It was a successful experiment because of the great assistance and cooperation by the people of Tikrit, who were happy with the ban despite the difficulties it caused them,” Ahmed said.

But, by another measure, the idea was not a success. A car bomber struck two days into the ban, authorities shot and killed a solo driver in an apparent mistake, and residents grumbled about yet another disruption to their already war-ravaged lives. Even a woman driver was stopped and warned to pick up passengers or get off the road, police said.

In Baghdad’s jammed streets, there are simply too many cars to stop everyone driving alone, police at several checkpoints said. Still, they conceded, it’s lone men driving fast who most catch their eyes.

“Every car brings suspicion these days, whether with a single driver or multiple people inside,” said Capt. Ahmed Salam, the commander of a patrol police unit in Baghdad. “We’re using our own judgment.”