Ramp Meters
Ramp_Meter.png 
Ramp meters are traffic signals on freeway entrance ramps that release one or two vehicles at a time. By regulating flow of traffic onto the freeway, ramp meters can break up large platoons of vehicles to make it easier and safer for motorists to merge onto the freeway. Ramp meters have been shown to improve travel times by up to 60% and reduce crashes by up to 50%.
 
References
FHWA Ramp Management and Control Handbook
The purpose of this handbook is to improve the operation of freeways and their associated ramps by providing support, information, guidance, and recommended practice to practitioners responsible for freeway management and operations. 
FHWA Ramp Management and Control Handbook

Minnesota Ramp Meter Research Study
In 2001, the Minnesota Department of Transportation released the results of an independent study on ramp metering in the Twin Cities metro area. Cambridge Systematics of Cambridge, Mass., evaluated the traffic flow and safety impacts associated with turning off all 430 ramp meters for six weeks as mandated by the 2000 Legislature.
Minnesota Ramp Meter Research Study

Washington State DOT
WSDOT uses ramp meters to reduce accidents and decrease travel times for commuters. Most ramp meters allow only one vehicle through each green light, creating a 4 to 15 second delay between cars entering the highway. This delay helps reduce disruptions to freeway traffic and reduces accidents that occur when vehicles merge onto the highway.
Washington State DOT

Georgia DOT Ramp Meter Program
In an effort to alleviate congestion and emphasize motorist safety, the Georgia Department of Transportation continues to implement its ramp metering program.
Georgia DOT Ramp Meter Program

California DOT Ramp Meter Design Manual
Caltrans is committed to using ramp metering as an effective traffic management strategy to maintain an efficient freeway system and protect the investment made in constructing freeways by keeping them operating at or near capacity.
California DOT Ramp Meter Design Manual

I-74 Ramp Metering Study
The purpose of this document is to summarize HNTB Ohio, Inc.'s work on the I-74 ramp metering project, specifically the impact ramp meters may have on inbound (eastbound) I-74 safety and traffic flow. Interstate 74, in Hamilton County, Ohio, is approximately 8.5 miles from I-275, the western terminus, to I-75, the eastern terminus.
I-74 Ramp Metering Study.pdfI-74 Ramp Metering Study.pdf