The three-phase traffic theory was developed in the late 90s and overtook the existing theory which had only two phases. The older model considered free flow and congested to be the only two states that traffic could have. The three-phase model adds a very important intermediate phase called synchronized flow and renames the last phase a wide moving jam.
The first of the three phases is free flow, which represents what most people would consider a normal road. While in free flow the flow rate of vehicles is proportional to the density of vehicles. This means that there is plenty of space on the road and more vehicles can join the pack without affecting trip times and the flow of vehicles will increase since you have more vehicles but they’re all traveling at the same speed as when there were fewer vehicles.
The next phase is synchronized flow, which is harder to define than free flow or congestion. Synchronized flow is defined as the point at which speeds begin to decrease drastically; however, the flow of traffic remains constant. This is possible because as the density of vehicles on the road continues to increase the speed only decrease enough to keep overall traffic flow constant. Synchronized flows are broken down into two major subgroups, localized synchronized flow and widening synchronized flow.
Localized cases are usually caused by a bottleneck in the design of the road such as a sharp bend that forces people to slow down. Localized synchronized flow has a fixed front end and remains at a constant length. Widening synchronized flow is a synchronized flow that is growing with time, which usually indicates a need for new infrastructure and can lead to the final phase of traffic.
The final phase of traffic in the three-phase theory is wide moving jams. Wide moving jams are defined as a situation in which both the velocity and flow rate is decreasing. This means there is either no room left for a greater density of vehicles to be accommodated or that any increase in the number of vehicles results in a decrease in velocity much greater than would be needed to keep the overall flow constant. Wide moving jams can be spontaneous because of an accident or inclement weather or can be a chronic symptom of aging architecture.