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115 This section provides the conclusions and proposals based upon the findings of this research. General conclusions are first provided related to the design of both entrance and exit ramps. Then, conclusions are provided specifically related to entrance ramps, followed by conclusions specifically for exit ramps. Finally, potential changes proposed for consideration in the next edition of the Green Book are provided, followed by future research needs. General Conclusions â¢ Passenger cars should remain the principal design vehicle for freeway mainline ramp terminals. This is consistent with current AASHTO policy. This conclusion is based pri- marily upon the analysis of truck-related crashes, which indicates that truck crashes are not overrepresented near freeway mainline ramp terminals. In addition, during the observational study, no critical maneuvers were observed as trucks merged and diverged onto and off of the freeway without much difficulty. This suggests that truck drivers and the general driver population adjust their behavior as necessary to accommodate larger, heavier vehicles at free- way mainline ramp terminals. Ultimately, it comes down to assessing the risks asso- ciated with not providing the additional acceleration/ deceleration lengths to accommodate the performance capabilities of trucks and making a decision based upon benefits and costs given available resources. No economic analysis calculations were performed in this research because of the lack of data, but given the level of information currently available from the research findings, passenger cars appear to be the appropriate design vehicle for most freeway-ramp terminals. The exception to this rule is at freeway mainline ramp ter- minals where the truck volumes on the ramps are substan- tial, in which case further consideration should be given to more fully accommodating trucks within the design. A value of 1,000 ramp trucks/day appears to be a good threshold for defining when the truck volume is significant, but this value should not be considered an absolute. â¢ Considering the three potential freeway operational design conditions evaluated (i.e., free, constrained, and forced merge/diverge), freeway mainline ramp terminals should be designed based upon free-merge/diverge con- ditions (i.e., free-flow conditions). By designing for free- merge/diverge conditions, sufficient length is provided to accommodate merging/diverging behaviors during more congested operating conditions. This is consistent with current AASHTO policy. â¢ It is most appropriate to design freeway mainline ramp terminals based upon average operating speeds of vehi- cles, rather than design speeds. Again, this is consistent with current AASHTO policy. Designs based upon design speeds would provide terminals that are over-designed. Conclusions Specific to Entrance Ramps â¢ Merging vehicles do not accelerate at a constant rate along the length of the ramp and SCL. â¢ A clear view of the freeway and mainline traffic is impor- tant to accommodate merging vehicles. On average, driv- ers glance into the mirror or over their shoulder three times before merging onto the freeway. Drivers begin glancing at the freeway and mainline traffic prior to reaching the paint- ed nose. While drivers are taking a glance at the freeway traf- fic, they continue to accelerate along the ramp and SCL. â¢ Vehicles exit curves on ramps at speeds much higher than the values given for âinitial speedâ in Green Book Exhibit 10-70. This indicates that vehicle performance and driver prefer- ences have changed since these values were determined. â¢ 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 S e c t i o n 7 Conclusions and Proposals
116 full length of the ramp and SCL for acceleration when gaps are abundant and merging is not difficult. â¢ Constrained-merge conditions appear to be the most dif- ficult for drivers, as reaching freeway speeds becomes more critical since gaps are smaller and do not provide as much opportunity for accelerating in the freeway lane. Addition- ally, drivers have a more difficult task identifying appro- priate gaps. In these conditions, some drivers take the first available gap they find even if they have not reached an ideal merge speed, while other drivers use the full length of the SCL, and in some cases the taper, to reach an ideal speed near the speed of the freeway traffic before merging onto the freeway. â¢ As expected, heavy vehicles do not perform as well as pas- senger cars at entrance ramps. Their acceleration rates are lower, and they merge onto the freeway at lower speeds. However, at ramps with a small proportion of truck traf- fic, their merging behavior does not appear to negatively impact the overall operation of the ramp terminals. â¢ Vehicles are more likely to use the full length of a tapered SCL to accelerate to near freeway speeds before merging in contrast to parallel SCLs, where vehicles may merge earlier along the ramp and at lower speeds. â¢ A potential disbenefit of a parallel SCL is that vehicles from the freeway mainline may use the adjacent SCL to pass vehi- cles within the freeway lanes, particularly during congest- ed conditions. With the geometry of a tapered SCL, such undesirable maneuvers by freeway vehicles are less likely. â¢ There is no substantive difference in the operational perfor- mance between low-speed (loop) and high-speed (straight) ramps under free-merge conditions. â¢ Based upon vehicle capabilities and driver preferences, many vehicles are capable of accelerating at higher rates than the assumed acceleration rates used to determine minimum acceleration lane lengths for entrance terminals in the Green Book. However, because most situations do not require that vehicles accelerate to the speeds assumed with the design, many drivers choose to accelerate at lower rates than assumed within AASHTO policy. â¢ Only three of the 11 entrance ramps studied met or exceed- ed the Green Book recommendation for minimum accel- eration lane lengths. Despite this, all the ramps appeared to have acceptable operational performance. This is evidence that the current design guidance is conservative. â¢ Upgrades as steep as 3 to 4 percent do not impact the acceler- ation capabilities of passenger cars, at least over lengths nec- essary for entrance terminals. As grades increase to 5 or 6 per- cent, the acceleration rates of passenger cars tend to decrease. â¢ The conceptual approach that assumes constant accel- eration used in AASHTO policy is a reasonable approach for determining minimum acceleration lane lengths for design. In addition, the current values provided in Green Book Exhibit 10-70 are conservative estimates for mini- mum acceleration lane lengths, given the current vehicle fleet and driver population, and do not need to be modi- fied. In particular, they provide sufficient length for vehi- cles to merge onto the freeway under a range of freeway operating conditions. It is also concluded that in situa- tions where free-merge (i.e., free-flow) conditions are expected for the foreseeable future and constraints make it difficult to provide the recommended minimum accel- eration lengths, the minimum acceleration lane lengths can be reduced by 15 percent without causing expected operational problems. Reducing minimum acceleration lane lengths by 15 percent is even conservative considering that, based upon the median acceleration rates of free-flow passenger cars measured under free-merge conditions, minimum acceleration lane lengths could be reduced between 16 to 46 percent and still be sufficient for 50 percent of vehicles. Conclusions Specific to Exit Ramps â¢ Most diverge maneuvers begin before or within the taper or within the first or middle thirds of the SCL. Few diverge maneuvers take place in the final third of the SCL or beyond the painted nose. â¢ Vehicles that diverge earlier along the deceleration lane diverge closer to freeway speeds than vehicles that diverge later along the deceleration lane, closer to the painted nose. â¢ Deceleration rates of diverging vehicles along the SCL and ramp are not constant. â¢ Where the deceleration lane length is longer than the Green Bookârecommended minimum length, most vehicles decel- erate at rates lower than those assumed by the Green Book. This is due in varying degrees to the additional length pro- vided and to vehicles decelerating in the freeway lane prior to initiating the diverge maneuver. â¢ Deceleration rates of exiting vehicles are greater for vehi- cles that diverge closer to the painted nose than for vehicles that diverge further upstream from the painted nose. â¢ Free-flow vehicles decelerate at greater rates than pla- tooned vehicles. This is most likely due to higher initial diverge speeds for free-flow vehicles. â¢ When exiting the freeway, trucks decelerate at rates very comparable to those of passenger cars. In addition, trucks typically diverge from the freeway at lower speeds than passenger cars. â¢ Crash rates for trucks are higher at parclo, free-flow, and âotherâ ramp configurations than at diamond; outer con- nection; direct or semi-direct connection; and button hook, scissor, and slip ramp configurations. â¢ Drivers exiting on loop ramps tend to reduce their speed in the freeway lane more, and decelerate along the SCL
117 at a greater rate, than drivers exiting on straight ramps. This may be due to the visual perceptions of drivers as they approach the horizontal curvature of a loop ramp. â¢ The geometry of parallel deceleration lanes generally leads to substantially higher deceleration rates than on tapered deceleration lanes. This may be the result of vehicles diverg- ing slightly closer to freeway speeds along parallel decel- eration lanes than along tapered deceleration lanes. The disparity between deceleration rates is most apparent on straight ramps. â¢ For initial speeds above 30 mi/h, deceleration rates along the SCL and ramp generally increase as the speed increases, hav- ing a strong relationship with the natural log of initial speed. â¢ AASHTO policy assumes a two-step process for establish- ing design criteria for minimum deceleration lane lengths. Deceleration is accomplished first while coasting in gear without the use of brakes and then during the applica- tion of the brake. AASHTO assumes 3.0 s for the coast- ing period. This is consistent with the amount of time spent coasting during diverge maneuvers in the behavioral study, assuming the coasting time includes the time spent releasing the throttle until application of the brake. Thus, 3.0 s of coasting time is a valid assumption for describing the diverge maneuver. However, 2 s of the coasting time typically occurs prior to the diverge maneuver while in the freeway, and 1 s of the coasting time occurs within the SCL following the diverge maneuver. â¢ The conceptual approach used in AASHTO policy for the design of exit ramps, which assumes a two-step process of deceleration, is a reasonable approach for determining minimum deceleration lane lengths for design. In addi- tion, the current values provided in Green Book Exhibit 10-73 are conservative estimates for minimum decelera- tion lane lengths, given the current vehicle fleet and driver population, and do not need to be modified. No critical or unusual diverge maneuvers were observed at the study sites that met and exceeded the current design criteria. In addi- tion, at these study locations, vehicles decelerated at rates well within the capabilities of the vehicle fleet and driver preferences. This was in part due to some deceleration by diverging vehicles in the freeway mainline prior to the diverge maneuver. Given this last point, it is beneficial to have a conservative design process that does not assume that vehicles begin decelerating in the freeway mainline. Potential Changes Proposed for Consideration in the Next Edition of the Green Book â¢ Include a statement in the Green Book text that tapered SCLs are preferred over parallel SCLs at entrance ramps because vehicles tend to merge closer to freeway speeds at tapered SCLs, and, if parallel SCLs are used, they are most appropriate at ramps expected to experience constrained- or forced-merge conditions because they provide greater flexibility in selecting a merge location along the SCL. â¢ Include a statement in the text accompanying Green Book Exhibit 10-70 that the design values in the exhibit are conservative, and that in situations where free-merge (i.e., free-flow) conditions are expected for the foreseeable future and constraints make it difficult to provide the recommended minimum acceleration lengths presented in the Green Book, minimum acceleration lane lengths may be reduced by 15 percent without causing any expected operational problems. â¢ Include additional exhibits that provide speed-distance curves for trucks in a range from 140 to 200 lb/hp. This will provide the designer with more flexibility to select an appropriate heavy vehicle for design, and, in some cases, find a better compromise between designing for passenger cars and designing for trucks, especially at entrance ter- minals with substantial truck volumes. These additional exhibits should be provided in Chapter 3 of the Green Book because they could be used for more general purposes than the design of freeway mainline ramp terminals, rather than just for acceleration lanes. A reference to the new exhibits should be included in Chapter 10 of the Green Book. â¢ Modify Green Book Exhibit 10-70 to indicate that the ini- tial speed (or design speed) is based upon the controlling feature, which, in the case of a straight ramp, is the cross- road terminal rather than a horizontal curve, and within the exhibit, replace âSpeed reachedâ with âMerge speed.â In addition, an estimate should be provided in the text for the speed that vehicles enter the ramp at the crossroad terminal. â¢ Emphasize in the Green Book text that the values presented for minimum deceleration lane length on exit ramps are a conservative estimate that do not account for any decel- eration in the mainline freeway lanes. While some drivers do accomplish a considerable portion of their decelera- tion on the freeway, such that a shorter deceleration lane length would be operationally sufficient, it is prudent for the designer not to assume deceleration in the mainline freeway lanes in the design of an exit ramp. â¢ Mention within the text that providing deceleration lanes longer than the minimum values listed in Green Book Exhibit 10-73 may promote more casual deceleration by exiting drivers, particularly under uncongested or lightly congested conditions. This is not necessarily a negative result, but it does change the operational characteristics of the ramp, as those drivers will maintain higher speeds further into the SCL and possibly into the ramp proper. â¢ Modify Green Book Exhibit 10-73 and accompanying text to indicate that the final speed (or design speed) of the exit ramp is based upon the controlling feature, which, in the case of a straight ramp, is the crossroad terminal rather
118 than a horizontal curve. Also within the exhibit, replace âSpeed reachedâ with âDiverge speed.â â¢ Modify Green Book Exhibits 10-70 and 10-73 to be more consistent in format. Exhibit 10-70 provides the desired taper rate as a note, and Exhibit 10-73 does not. Exhibit 10-73 provides the definitions of each speed term, and Exhibit 10-70 does not. Future Research Needs â¢ The data set collected and analyzed for this study indicated that the âinitial speedâ and âspeed reachedâ values shown in Green Book Exhibit 10-70 are lower than the actual speeds for both. In many cases, vehicles exit the control- ling feature on the ramp at speeds greater than the design speed of the ramp, indicating that vehicle and driver per- formance have evolved from what they were when both the design speeds were determined and when the exhibit was created. Additional research is recommended to determine more appropriate values for the âinitial speedâ row and the âspeed reachedâ column in the exhibit. â¢ A study similar to the one performed in this project should be conducted to cover a wider range of design conditions in Green Book Exhibit 10-70 than was possible in this study. In particular, a future study should include ramps with entrance curve design speeds in the range of 15 to 25 mi/h. Also, the study should measure speeds of vehicles beginning at the crossroad terminal. â¢ A study on the âspeed reachedâ (i.e., diverge speed) for given values of freeway design speed in Green Book Exhibit 10-73 would be beneficial to determine whether these val- ues reflect not only current vehicle performance but also common driver behavior. In particular, a future study should include ramps with highway design speeds in the range of 30 to 60 mi/h. â¢ The study on diverge speed should also include a review of higher running speeds on the ramp proper, as straight ramps with no controlling curve allow drivers to maintain higher speeds as they approach the crossroad terminal, particularly where that terminal does not require a stop or yield (e.g., intersection with a one-way frontage road). â¢ The results of this study suggest that deceleration length, occurrence of freeway deceleration, and diverge speed may be influencing factors that have measurable effects on deceleration rate. Research to quantify these effects, as well as to identify possible relationships between and among those influencing factors, is recommended. â¢ This research focuses on single-lane freeway mainline ramp terminals that are outside the influence of upstream or downstream operations of other freeway mainline ramp terminals. This research does not specifically address free- way mainline ramp terminals with two or more lanes, weaving areas where a continuous auxiliary lane is provid- ed between entrance and exit terminals, nor issues specifi- cally related to ramp metering. Additional research should be conducted to further investigate these topics.