The history of left turn signals in Los Angeles:
Though more left-turn signals could have been added at this time in Los Angeles, the city chose not to, fearing the adverse effects the extra signal cycles could have on traffic flow. Adding left-turn signals to L.A.'s grid would have required significantly longer light cycles. Where L.A.'s traffic signals today are computer-controlled, and "know" how many cars there are at an intersection, signals in the '70s were timer controlled. Making left-turn signals ubiquitous would require motorists to spend even more time at increasingly clogged intersections, potentially waiting for a left-turn light to complete its cycle for cars that might not even be there. A philosophical decision was made by city planners to maintain the status quo, and prioritize through-traffic over traffic-making left-hand turns.
Planners maintained this philosophy until 1991, when the Los Angeles City Council forced the installation of a left-turn signal at the intersection of Temple and Main. Turns out, a judge working in a downtown courthouse had grown frustrated waiting to turn left without a signal, and convinced a friend on the City Council to pass a motion.