Q: What exactly is a Gridlock Alert Day? Why do they happen?

A: Simply put, Gridlock Alert Days are when the city expects more than 1 million cars to come into Manhattan. Roughly 930,000 cars drive into the borough on a normal workday, according to a Department of Transportation spokesman, Craig Chin. Because that’s about as many cars as Manhattan can hold, the 7.5% increase in traffic on alert days is enough to fill the grid of streets and avenues. If, at a light, cars don’t make it all the way through the intersection, they block cross-traffic. The crossing cars can’t move, blocking the street behind them, and the problems cascade up and down and across the island – gridlock.

The term was coined by a former DOT chief engineer, Sam Schwartz. In various published materials, Mr. Schwartz wrote that while he worked for the DOT under Mayor Lindsay in the mid-1970s, he and a colleague, Roy Cottam, were discussing their fears that the grid system would “lockup” during the holidays. The words were pushed together and a neologism was born.

The fact that it’s still with us shows what we all know: City traffic has only gotten worse in the intervening 30 years, and the Christmas season is the worst time of the year to drive. In addition to the sporadic gridlock days that popup throughout the year – presidential visits are guarantees of an alert – the holiday season produces no fewer than seven gridlock days between now and Christmas. Mr. Chin said New Year’s Eve and New Year’s Day are expected to be added to the list. To put that in context, 25% of the 36 days left in 2004 will be high-traffic days.

To keep everyone rolling, the city has just released some new temporary traffic patterns. On Monday, “thru lanes” will be established on the center lanes of Fifth and Sixth avenues. Traffic entering the center lane of Fifth Avenue at 56th Street will not be allowed to exit until 47th Street, and the same will be true on Sixth Avenue between 48th and 53rd streets. At Second Avenue and 34th Street, the DOT will install flexible plastic posts, called “bollards,” to divide the lanes so drivers won’t make turns from the center lanes of the avenue onto the street. The same technique will be used at Eighth Avenue and 42nd Street. Police officers and traffic agents will be on hand to keep traffic moving. As always, mass transit is your best bet to spend your weekends shopping and not cursing the gridlock.

Got a question about getting around New York? E-mail transit@nysun.com.