Until four years ago, I worked in Los Angeles as a traffic engineer. The ATSAC system is used on more than 4,000 intersections, is interconnected, and makes adjustments to signal timing either manually or automatically. There aren't cameras at all of the monitored intersections, but you don't need cameras to measure traffic volumes and speed, they're just an additional tool.
Further, the systems that use cameras for vehicle detection are falling out of favor. There are too many conditions, such as rain, snow, fog and bright sunshine that can befuddle the systems and cause them to fall back to pre-set timing rather than relying on vehicle detection. Where inductive loops aren't used, radar is proving to be more reliable than cameras.
Even where there's no interconnection, most intersections have controllers that are considerably more sophisticated than simple timers.