Guidelines for Spacing Between Freeway Ramps (0-5860-1)

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.