For what its worth, lower speeds are one of the most straightforwardly effective way to reduce congestion. Road capacity is higher at lower speeds. Errors are less likely to cause serious incidents at lower speeds. Traffic controls don't need to be so aggressive, causing you to spend less of your trip fully-stopped. For most trips, going a bit slower has a completely negligible effect on drive times, especially when you can get most of traffic to do it leading to more laminar flow.
The problem is, only road design is effective to lower speeds. You can't just ask drivers to slow down or change the posted signs, you have to re-engineer roads. People tend to just drive at whatever speed feels comfortable on the road.