rampmeter

A rampmeter is a traffic control device installed on an on-ramp to regulate the rate at which vehicles enter a freeway or expressway. Its primary purpose is to manage merging, reduce bottlenecks, and improve safety and reliability of mainline traffic by smoothing inflow during congested periods.

Rampmeters typically operate with signals at the ramp, displaying red, yellow, and green phases. The metering rate determines how many vehicles are allowed to enter the freeway per cycle. Control systems use sensors on the ramp and along the mainline to monitor traffic conditions and adjust the release rate to achieve targets such as a desired mainline speed, density, or queue length. Some rampmeters use fixed-time schedules, while others employ adaptive or feedback-based algorithms, such as local controllers like ALINEA, to respond to real-time conditions.

Variants exist for different roadway settings, including single-lane and multi-lane ramps. Metering can cause queues on the ramp, which may spill back to arterial streets if there is insufficient ramp storage or upstream congestion. Effective ramp metering requires careful design of cycle lengths, green splits, ramp geometries, and coordination with adjacent signals and freeway ramp terminals.

Evidence suggests ramp metering can reduce peak-period mainline congestion, improve travel time reliability, and lower crash rates when properly implemented. Limitations include increased travel time for ramp users during metering, the need for maintenance, and potential public acceptance concerns. The approach is a component of broader intelligent transportation systems and freeway-management strategies.

See also: intelligent transportation systems, freeway management, traffic signal coordination, adaptive traffic control.