If you went to Google, Yahoo, AOL or another mapping site to plot a route from San Francisco to Oakland in the hours after an oil tanker exploded, they would have sent you driving over a collapsed overpass engulfed in flames.

But within 48 hours of Sunday's accident, engineers at the major mapping sites had reprogrammed with alternate routes that added only a couple minutes to estimated drive times. That's a big improvement from a couple years ago, when routing algorithms were only updated sporadically.

Digital cartographers say the response to the Oakland disaster was a promising sign of what's to come—up-to-the-minute detours and routing technology that takes into account not only major disasters but fender benders and traffic jams.

''I'd say overall we were very successful—we fixed the routing but didn't break anything in the process,'' Yahoo Maps director Jeremy Kreitler said Wednesday. ''This has been an interesting challenge—an example of how we put into practice our attempts to provide real-time traffic information.''

Well, not exactly real time. It took Yahoo nearly 39 hours to suggest new routes. Google Maps was updated late Monday night. MapQuest Inc., a subsidiary of Time Warner Inc.'s AOL, began suggesting detours late Tuesday night.

Mapping experts say they'll continue to shave those response times. Eventually they may be able to reroute drivers before drivers themselves learn of problems—no more listening to radio traffic reports or sitting in stalled traffic.

''The goal is to fix the problem and have people say, 'Wow, Mapquest has already taken this into account,''' said Christian Dwyer, vice president of operations for Denver-based MapQuest, which serves up 16 million maps and driving directions daily.

But real-time rerouting presents a computational dilemma. The fastest solution—letting the computer pick an alternate route—could be dangerous: The machine might suggest the shortest route by mileage or drive time, even if that sends freight trucks through a residential neighborhood or steers commuters down a commercial artery with stoplights.

So mapping companies insist on a degree of human intervention that's unusual in the technology industry. Engineers don't approve detours until they prove it doesn't create new problems—and in many cases, that means sending field workers on a test run.

Engineers at Mountain View-based Google Inc. and Santa Clara-based Yahoo Inc. were quick to update the database after the highway collapse in part because the disaster happened only a few dozen miles from their Silicon Valley headquarters.

Employees already knew the best detours—some had even taken them to work Monday.

While Google, Yahoo and other Internet companies employ engineers who customize maps for their site, nearly all big sites buy their geographic data from one of two companies: Navteq Corp. and Tele Atlas NV.

At Tele Atlas' North American headquarters in Lebanon, N.H., Web crawling software began compiling news articles, online postings and transportation department bulletins about the Oakland disaster moments after it happened.

Staff editors called contacts in the California Department of Transportation, local police and fire stations. The company also has 80 field analysts who look at aerial photos to determine how traffic flow might be affected.

On Monday, Tele Atlas asked one of its local field agents to drive to the collapsed highway and take digital photos. When Tele Atlas confirmed which stretch of highway was out of commission, it fixed the database.

''We never act on just a news article itself,'' said Dan Adams, vice president of operations for map-data provider Tele Atlas. ''We need to go through a series of questions to understand the impact, the closure time, the target dates for reopening and a lot of other details.''