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, 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,'' Dan Adams, vice president of operations for Tele Atlas, which provides data for about half of all Internet map searches worldwide. ''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.''

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On the Net:

Google Maps: http://maps.google.com/

Yahoo Maps: http://maps.yahoo.com/

MapQuest: http://www.mapquest.com/