The DOT planning department I worked for (in the 80s) had studies showing that synchronizing the lights 10-15mph ABOVE the speed limit kept traffic flowing "optimally" to minimize congestion without having to expand the infrastructure (mostly: lanes). Even today I see city traffic lights synchronized mostly +10mph over the posted limit in the direction of highest flow on major arteries - you can tell when you "make" one yellow light, then lock in cruise control at limit+10 and "make" the next 3+ lights at exactly the same flash of yellow as you approach.

They do this to delay improvement projects like additional lanes, straightening curves, etc. by showing a higher "level of service" for the roadway than they would have with the lights synchronized at, say: speed limit -10mph. If you're talking about a 45mph roadway, moving the cars at 55mph instead of 35mph means 157% as many vehicles per hour at the same spacing, and we all know that people don't actually increase following distances at higher speeds, so... it's a real, and significant difference. Not as significant as getting people to carpool, or take public transit, but that's about as likely to happen as increasing following distances.