Traffic accidents resulting in injuries decreased slightly on rural interstate highways in California, where the speed limit was raised to 65 m.p.h. four months ago, despite a heavy increase in traffic, the California Highway Patrol reported. The number of speeding tickets on those highways also decreased, CHP spokeswoman Susan Cowan-Scott said. Fatal accidents on major highways increased from 432 during the four-month period in 1986 to 464 accidents during the same period this year. The number of all highway accidents for that period increased from 31,720 to 31,835. But the number of injury accidents related to speed dropped from 111 in the 1986 period to 110 in the same period of 1987.