00750059

Monograph

http://onlinepubs.trb.org...nepubs/nchrp/nchrp_w12.pdf

This research study was conducted to develop and validate an appropriate methodology for determining capacity and level of service at signalized interchanges. The study considered a wide variety of field characteristics that might affect saturation flow, including turning radius, traffic volume, lane use and downstream queue spillback. A large program of field studies was conducted at 12 interchanges located in 5 states. A sizeable data base of over 51,000 queued vehicles covering 3,800 signal cycles of operations was electronically recorded. These data were used in some cases to directly develop analytic models of a statistical nature; whereas in other cases, the field data were used to calibrate, test, and verify various analytic models having been developed from a more theoretical basis. As an example, the Prosser-Dunne model was enhanced and tested extensively using a NETSIM-based experimental process. Other prototype software (INTERCHANGE) was developed to illustrate how a large group of interchanges might be systematically analyzed for capacity and level of service. The research found that mobility at signalized interchanges is dependent on many factors that influence capacity. The importance of the prevailing factors may change depending on the general volume level and degree of existing congestion. Traditional prevailing factors affecting capacity include the interchange geometry, traffic mix, and signal green splits. The effect of queue spillback on upstream saturation flow was modeled and verified with field data. This model could prove very useful for providing reliable operational analysis of traffic conditions near capacity-flow levels. During periods of oversaturation where the arterial links are filled with queues, additional non-traditional factors come into play. During oversaturation, the upstream input capacity becomes highly dependent on the downstream signal timing and capacity to keep the output link flowing. Upstream input flow cannot exceed the total downstream capacity, and may be even less if demand starvation occurs. Signal offset during oversaturation was found to be the most important factor in determining which upstream signal phases can use the downstream capacity. This offset is not the offset that provides optimal traffic progression during uncongested flow periods.

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NCHRP Proj 3-47 FY '94,
Final Report,
TAMURF 7241

English

355 p.

1997-4

NCHRP Web-Only Document

Digital/other

Appendices (6) ; Figures; References; Tables

Data collection; Field studies; Highway capacity; Highway design; Interchanges; Interchanges and intersections; Lane distribution; Level of service; Mathematical models; Offsets (Traffic signal timing); Queuing; Signalized intersections; Traffic congestion; Traffic flow; Traffic signal timing; Traffic volume; Turning radius; Vehicle mix

Analytical models

Highways; Operations and Traffic Management; I71: Traffic Theory

TRIS, TRB

Jun 25 1998 12:00AM