United States Department of Transportation
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Guidelines for spacing between freeway ramps.
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2009-11-01
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[PDF-1.26 MB]
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Edition:Technical Report; Sept. 2007¿Aug. 2009.
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Abstract:Existing geometric design guidance related to interchange ramp spacing in the Texas Roadway Design
Manual and the AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) is not
speed-dependent even though intuition indicates spacing and speed are related. Understanding the
relationship between interchange ramp spacing, speed, and freeway operations is important, especially in
developing potential design values for higher speeds (e.g., 85 to 100 mph). The objectives of this project
were to: (a) investigate relationships between weaving length, speed, and overall vehicle operations on
Texas freeways and (b) propose updates to current Texas Department of Transportation guidance on
recommended distances between ramps. Within the research tasks several methods were utilized to assist in
developing guidance on ramp spacing lengths. The methods or resources used to generate potential lengths
included: guidance provided in Design Manual for Roads and Bridges published by the Highways Agency
in England, minimum deceleration and acceleration length for freeway conditions, decision sight distance,
sign spacing needs, NCHRP project 3-75 findings, findings from field studies at seven study sites, findings
from simulation conducted as part of this research, and safety relationships identified in the literature.
Suggested ramp spacings were developed for the entrance ramp to exit ramp and exit ramp to exit ramp
conditions.
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