Recurring Traffic Bottlenecks: A Primer
Focus on Low-Cost Operational Improvements (Fourth Edition)

Appendix A. Additional Principles on Traffic Flow and Bottlenecks

Shock Waves and the Accordion Effect: The Movement of Queues on Freeways

Queues are formed when the volume of traffic trying to use a highway section exceeds the section's capacity to carry it. This situation is familiar to all drivers—"stop-and-go traffic," characterized by very low speeds. Shock waves are byproducts of traffic congestion and queuing. They are transition zones between two traffic states that move through a traffic environment like, as their name states, a propagating wave. That is, they form both when a queue is forming and when it is dissipating. When a queue is forming, the shock wave is said to be "backward forming"—the cascading sequence of brake lights on a freeway gives a good indication of the spread of a backward forming shockwave.

Shock waves are abrupt transitions in traffic flow characteristics—speed, density, and volume past a point change quickly as the shock wave moves. In other words, they are boundary conditions in time and space that demark a discontinuity in traffic flow conditions. A rapidly growing backward forming shockwave presents a dangerous situation for drivers, as the unexpected queue can appear very quickly.

On the flip side, a "recovery" shock waves occur as queues begins to dissipate. There are two types of recovery shock waves. A forward recovery shock wave forms at physical bottleneck areas such as congested on-ramp areas and lane-drops, i.e., areas where capacity is constant. As demand at the bottleneck drops below its capacity, the queue will start to dissipate from the rear toward the front, and the shock wave—the boundary condition—will move forwards. A backward recovery shock wave forms when capacity has been lost but is then restored, such as when a lane-blocking incident is cleared. The queue will start to dissipate from the front, so the shock wave will be backwards.

The nature of shock waves and queuing often gives drivers the impression that the congestion they just experienced had no cause. In the incident example, by the time a vehicle gets from the back of the queue to the point of the original blockage, the incident may have been totally removed. Rest assured, a bottleneck of some type was the reason for the congestion—something caused volumes to exceed capacity (our own ramp example) or something lowered capacity to the point where volumes now exceeded it (our incident example).