NCHRP Report 730 (Review)

DESIGN GUIDANCE FOR FREEWAY MAINLINE RAMP TERMINALS

FOCUS OF RESEARCH1. Do AASHTO models describe all sets of design parameters

2. Is AASHTO SCLs model appropriate for todays driver population and vehicle fleet

3. Should truck be used as design vehicle on mainline terminals.

4. Is there any certain impact of tapered and parallel design on SCLs

5. Difference in drivers behavior in low and high speed ramps

6. Implication of any design on current roadway practice and existing design.

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INTRODUCTIONThis report is a brief test comparison of AASHTO green book models to the present conditionsof driver behaviors, accident analysis and the design of freeway main ramps.

Need of the research developed because some of the design values, assumptions were developed before the development of the study area, so while considering present scenario the values were supposed to be out dated.

Research was conducted in two phases i.e. Phase-1 and Phase-2

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STUDY AREA

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◦ Literature review◦ Geometric Design and safety parameters

◦ Vehicle performance

◦ Human factors

◦ Operational characteristics

PHASE-1

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◦ AASHTO & HCM models for freeway◦ Condition-1 : Free merge at LOS A to C

◦ Condition-2 : Constrained merge at LOS at D to E

◦ Condition-3 : Forced merge for LOS E to F

PHASE-1

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Analysis of Truck Related Crashes Near Freeway Main-line Ramp

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Ramp Truck Average Daily Traffic

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PHASE-2: Observational Study of freeway Mainline Ramp Terminals

◦ Objective:◦ Gather speed and distance data for

merging and diverging vehicles

◦ Generate speed profile

◦ Predict driver behaviors and vehicle performance.

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CONCLUSIONS FROM OBSERVATIONAL STUDYVehicles are entering at a higher speed than assumed speed in Green Book.

Acceleration rates of trucks are lower than for the passenger cars.

Vehicles are exiting at a lower speed than the assumed speed in Green Book.

Exit Ramps are generally longer than Green book minimum criteria,

Deceleration rates on loop are lower than those on straight ramps.

Parallel SCLs have higher Deceleration rate than tapered SCLs.

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The behavioral study was designed to collect detailed information on a limited number ofdrivers, in contrast to more generalized information gathered on a large number of vehicles inthe observational study.

By observing drivers in close detail, it is possible to identify behavioral patterns and influencesthat determine how drivers operate their vehicles on freeway ramps.

Thus providing further information to determine whether the assumptions and data used tosupport existing design guidelines are appropriate for current conditions.

PHASE-2: Behavioral Study of freeway Mainline Ramp Terminals

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The behavioral study took place in the Dallas-Ft. Worth Metroplex area of Texas, using a data collection protocol approved by institutional review boards at Texas A&M University and MRIGlobal. Flyers were distributed to various agencies/organizations to recruit 12 subjects to participate in the study.

The Texas A&M Transportation Institute (TTI) has developed an instrumented vehicle to facilitate robust data collection in both test-track and public road environments

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CONCLUSIONS FROM BEHAVIORAL STUDY (acceleration)

In uncongested or lightly congested conditions, a typical glance into mirror or over the shoulderby merging driver onto freeway was about 2.5 to 3.0s but it takes average 3 glance beforemerging. During typical glance driver travels 100 to 200ft with a increase in speed of 2.5 mi/hr.

In uncongested or lightly congested conditions, drivers uses only half the SCL length beforemerging

Observed acceleration rates are lower than the green books values.

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Drivers began to decelerate before entering the SCL lane

Most of the time drivers enter the SCL within first 50% of length.

Actual deceleration rates are lower than the Green Book but none of the drivers appeared to be uncomfortable .

CONCLUSIONS FROM BEHAVIORAL STUDY (Deceleration)

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RESEARCH FINDINGS (Conclusion for report)

1. Do AASHTO models describe all sets of design parameters◦ Here the answer for this question can be yes, The model which is in the green book and observational

and behavioral study presents all sort of parameter regarding geometric design, human behaviors, operational features are defined.

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2. Is AASHTO SCL model appropriate for today’s drivers population and vehicle fleet

◦ It is clear that under certain circumstances, current design policy does not sufficiently provide for theneeds of all drivers and all vehicle types. The merging and diverging tasks are complex and are difficultto accommodate within design, especially given changing driver populations and vehicle fleets. On theother hand, there is only limited evidence that current design policy values produce substantive safetyconcerns for the normal range of conditions. For this it need more future research to give in-depthvalues regarding it.

RESEARCH FINDINGS (Conclusion for report)

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3. Should trucks be used as design vehicle on freeway mainline ramp terminals for all ramps or any certain design conditions

◦ Passenger cars should remain the principal design vehicle for freeway mainline ramp terminals. This isconsistent with current AASHTO policy. This conclusion is based primarily upon the analysis of truck-related crashes, which indicates that truck crashes are not overrepresented near freeway mainlineramp terminals. In addition, during the observational study, no critical maneuvers were observed astrucks merged and diverged onto and off of the freeway without much difficulty. This suggests thattruck drivers and the general driver population adjust their behavior as necessary to accommodatelarger, heavier vehicles at freeway mainline ramp terminals.

◦ As expected, heavy vehicles do not perform as well as passenger cars at entrance ramps. Theiracceleration rates are lower, and they merge onto the freeway at lower speeds. However, at rampswith a small proportion of truck traffic, their merging behavior does not appear to negatively impactthe overall operation of the ramp terminals.

RESEARCH FINDINGS (Conclusion for report)

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4. Is there any certain impact of Tapered and parallel design on SCLs

◦ Vehicles are more likely to use the full length of a tapered SCL to accelerate to near freeway speedsbefore merging in contrast to parallel SCLs, where vehicles may merge earlier along the ramp and atlower speeds.

◦ A potential disbenefit of a parallel SCL is that vehicles from the freeway mainline may use theadjacent SCL to pass vehicles within the freeway lanes, particularly during congested conditions. Withthe geometry of a tapered SCL, such undesirable maneuvers by freeway vehicles are less likely.

RESEARCH FINDINGS (Conclusion for report)

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5. Drivers behaviors for Low speed ramps compared with high speed ramps

◦ In free-merge conditions, many vehicles choose to enter the freeway at speeds much lower than the speed of freeway traffic. It appears that drivers simply choose not to use the full length of the ramp and SCL for acceleration when gaps are abundant and merging is not difficult.

◦ There is no substantive difference in the operational performance between low-speed (loop) and high-speed (straight) ramps under free-merge conditions.

RESEARCH FINDINGS (Conclusion for report)

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6. Implications of any design on current roadway practice and existing design.

Referring to this research perform some changes were proposed

for consideration in next edition of the green book

RESEARCH FINDINGS (Conclusion for report)

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