California Highway Patrol said that a six-mile backup of southbound traffic occurred Thursday morning after three trucks tangled on Interstate 5 near the Carmel Valley Road exit shortly before 7 a.m. Traffic Officer Sheryl Garinger said that three of the four southbound lanes were blocked by the pileup until about 9:30 a.m., creating a massive traffic jam that backed up commuter traffic for more than six miles--as far north as Encinitas. The accident occurred when the driver of a large semi with two trailers pulled off the highway so he could consult a map. His rig was struck by a second truck whose driver apparently had fallen asleep at the wheel. A third truck then hit the second truck. The accident caused only minor injuries and no citations were issued.