Domino effect creates havoc for New York commuters

Trucks entering New York City underwent security checks on Wednesday

NEW YORK (CNN) -- Stepped-up vehicle inspections added to traffic backups on the routes into Manhattan Wednesday, and prompted an order for citizens to start carpooling amid heightened security concerns in the wake of the terrorist attacks two weeks ago.

The police checkpoints have been in place since the World Trade Center attacks but were "modified" Tuesday at some bridges and tunnels, said Police Chief Joseph Esposito. "As a result," he said, "there was increased traffic."

Neither the police department nor the Port Authority revealed the reason for the modified inspections. New York Mayor Rudy Giuliani said it had to do with heightened "security concerns."

Police were seen stopping truck drivers and checking their cargo. The inspections lasted a few minutes for each truck.

The massive traffic jams at the entrances to Manhattan -- a result of some of those entrances remaining closed, more and more commuters returning to work and some Lower Manhattan streets still being blocked off -- prompted the mayor's office to order that all passenger vehicles must carry more than one person starting Thursday morning.

The order will be in effect from 6 a.m. to noon every day until further notice, Giuliani said Wednesday. Taxis, delivery vehicles, and emergency vehicles will be exempt.

The action affects all traffic entering Manhattan from 62nd Street south, which includes all the East River bridges and the two open tunnels, the Lincoln Tunnel, a major crossing point from New Jersey, and the Queens-Midtown Tunnel on the east side.

The other tunnels, the Holland Tunnel to New Jersey and the Brooklyn-Battery Tunnel at the southern point of Manhattan, have been closed since the attacks to all unofficial traffic.

"The reality is that we're in a situation where we assess threats a couple of times a day," Giuliani said. "Sometimes they're deemed credible, sometimes they're not."

Esposito said the vehicle checks are necessary because the entire city remains in a heightened state of alert.

City officials said the system will be reviewed each day to see how it is working.

"This is a trial-and-error thing," Giuliani said, adding that because Thursday is a Jewish holiday, traffic will be lighter, giving officials a good opportunity to assess the situation. "It'll be a good test of whether it works or not."

Giuliani said city officials are still not sure how they would punish those in violation of the one-per-vehicle rule. Possibilities include warnings, tickets, or turning back the vehicle.

He again urged commuters to take public transportation to alleviate the traffic mess.