Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report (2021) / Chapter Skim
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8 Section 2. Literature Review and Survey of Practice This section summarizes the state of practice and other information related to ramp design speed selection.
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9 vehicle speeds. Thus, when a change is made in design speed, many elements of the highway design will change accordingly." AASHTO policy continues to explain that the selected design speed should be consistent with the speeds that drivers are likely to expect on a given highway facility and should fit the travel desires and habits of all drivers expected to use the particular facility.
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10 The Green Book also states that Green Book Table 10-1 is not applicable to the portion of ramps near at-grade intersections, because stop signs or signals typically control those locations. The sight distance along a ramp should be at least as great as the design stopping sight distance.
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11 Entrance Ramp Terminals A taper-type entrance, as shown in Figure 2A, merges into the freeway with a long, uniform taper. The geometrics of a taper-type entrance should be designed in a way to allow drivers to attain a speed within 5 mph of the operating speed of the freeway by the time they reach the point where the left edge of the SCL joins the traveled way of the freeway.
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12 Table 4. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Two Percent or Less (AASHTO, 2011)
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13 the ramp proper and the operating speed of the highway. Similar to a taper-type entrance, Green Book Table 10-3 (Table 4)
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14 Table 6. Minimum Deceleration Lengths for Exit Terminals with Flat Grades of Two Percent or Less (AASHTO, 2011)
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15 essentially the same as the Green Book and/or specifically referred the reader to the Green Book. For the remaining five states, much of their guidance was also very similar to the Green Book but contained some unique features.
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16 crashes. The crash modification factors (CMFs)
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17 Step 2 -- Compute Limiting Curve Speed: The limiting curve speed is computed for each curve on the ramp using Equation 2. vmax,i = 3.24 (32.2 Ri)
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18 Step 2 -- Compute Limiting Curve Speed: This step is the same as Step 2 for the entrance ramp procedure. A lower curve speed than that obtained from Equation 2 is possible as deceleration may occur along the ramp as the driver transitions from the freeway speed to the crossroad speed.
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19 tkisosllMCcent IWWWRIv 333.4682.0912.0313.0040.0978.1359.8 2,, −+++++= (11) tkoslPTcent IWRIv 051.4079.1054.0444.1276.16 2,, −+++= (12)
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20 freeway exits. In the process of this evaluation, Guo et al.
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21 Xia et al.
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22 Their dataset included speed and acceleration profiles of hundreds of vehicles at 16 freeway entrances in Ottawa, Canada, where the SCL ended before reaching the next downstream exit ramp. They developed the following models to predict the 85th-percentile speeds of merging (entering)
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23 • Exit ramps have higher crash rates than entrance ramps (Twomey et al., 1993; Khaoshadi, 1998)
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24 Figure 4. Aggregated Ramp Entrance CMF (Bonneson and Pratt, 2009)
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25 Figure 5. Ramp Horizontal Curve CMF for FI Crashes (Bonneson et al., 2012)
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26 1. Average ramp speeds on all observed entry ramps are consistently greater than 50 percent of the freeway design speed, even where the freeway design speed is 70 mph.
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27 ITE Freeway and Interchange Geometric Design Handbook This handbook presents fundamental concepts and practices related to freeway and interchange geometric design, often referencing the AASHTO Green Book (Leisch, 2005)
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28 The ramp design speeds given in Table 1 as a function of the mainline highway design speed are applicable to ramps on arterials and collectors, as well as freeway ramps. For arterials and collectors, the design speed of the arterial or collector is used as the highway design speed.
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29 Table 11. Minimum Radii for Interchange Ramp Curves (Leisch, 2005)
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30 5. Supplement standard advisory speed signs to make the signing more conspicuous, increase the distance from the signing to the most critical curve, and draw attention of truck drivers to the signing.
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31 Table 14 provides suggested design speeds for exit and entrance vertical curves based on highway design speeds. These values are 80 to 90 percent of the highway design speed.
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32 angle of convergence is 1 degree, or a departure rate between 50:1 and 70:1. For highway design speeds of 50 mph or less and ramps, a rate of convergence of 40:1 may be appropriate.
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33 NCHRP Report 730 (Design Guidance for Freeway Mainline Ramp Terminals) The objective of this research was to develop improved design guidance for freeway mainline ramp terminals based on modern driver behavior and vehicle performance capabilities (Torbic et al., 2012)
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34 • Vehicles are more likely to use the full length of a tapered SCL to accelerate to nearfreeway speeds before merging in contrast to a parallel SCL, where vehicles may merge earlier along the ramp and at lower speeds. • 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, and 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.
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35 • Drivers exiting on loop ramps tend to reduce their speed in the freeway lane more, and decelerate along the speed-change lane at a greater rate, than drivers exiting on straight ramps. This may be due the visual perceptions of drivers as they approach the loop ramp.
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36 • Sufficient length should be provided for a vehicle stopped at the meter to accelerate to freeway merging speed. When metering is implemented on an existing ramp, it is necessary to verify that sufficient acceleration length can still be provided after a portion of the ramp is allocated for deceleration and queue storage needs.
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37 The purpose of the surveys was to learn about the experiences of professionals related to ramp design speed strategies, including: • What factors most influence the choice of design speed for various ramp types. • To what extent design practices conform to or differ from Green Book guidance.
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38 Organization type Number of responses City or County 3 Consultant 12 University -- US 5 University -- International 2 DOT (retired staff) 2 None given 8 Total 32 Summary of Responses to Web-Based Survey Responses to both surveys are summarized below.
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39 Entrance Ramps Answer Options High Importance Medium Importance Low Importance N/A Ramp configuration (e.g., diamond, loop, directional)
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40 The factor most frequently identified as being not applicable to the selection of ramp design speed was the presence of a ramp meter, although ramp metering was considered to be slightly more important on entrance ramps than exit or freeway-to-freeway ramps. The use of spiral transition into the controlling curve was also considered to have little importance to the selection of ramp design speed by most respondents.
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41 Question: Specifically, does your agency use the Table 10-1 from the 2011 AASHTO Green Book in selecting ramp design speeds? (State DOT survey)
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42 typical of an urban loop ramp. It would also be desirable to expand the highway design speeds shown to higher values, as many states now have a 75 mph speed limit.
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43 Additional comments from state DOT respondents include: • Ramp proper design speed should be from 50 to 85 % of mainline design speed (70%-85% is preferred)
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44 Question: Does your agency's policy for selection of ramp design speed differ between ramp types, ramp configurations, or some other factor? (State DOT survey)
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45 • Freeway-to-freeway ramps are often intended to maintain mainline speeds, while entrance or exit ramps are near mainline speeds. Exit and entrance ramps also may have a perceived design speed much higher than intended, creating safety concerns.
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46 Question: Should a single design speed be specified for an entire ramp or should the design speed vary along the ramp? (local agencies, university researchers, and private sector survey)
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47 Question: Is your agency considering any change in its policy for selection of ramp design speed? (State DOT survey)
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48 Comments from local agencies, university researchers, and private-sector practitioners included the following: • Generally do not use this information from the Green Book. • I teach highway design, and have always had difficulty effectively conveying the ramp design speed procedure to students, largely because I'm not sure if I fully understand it myself.
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49 Comments included the following: • Sometime it is based on a field check. • We utilize the MUTCD and PA Publication 236.
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50 Question: Has your agency/organization used the safety predictive models from Chapter 19 of the 2014 Supplement to the AASHTO HSM in the design of ramps and/or the selection of ramp design speeds? (both surveys)
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51 Answer Options State DOT Responses Local Agencies, University Researchers, and Private Sector Responses None 38.9% (7)
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52 • Our office has not been involved with projects that required a federal design exception from the AASHTO criteria for ramp design speed. Several projects have required exceptions to the project criteria particularly as it relates to horizontal stopping sight distance.
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53 Comments from local agencies, university researchers, and private-sector practitioners included: • Toll plazas. • Design exceptions.
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54 Discussion Points from Web-Based Survey Several key points that can be drawn from the web-based survey are as follows: • Committee members, local agencies, university researchers, and private-sector practitioners overwhelmingly think policy should differ by facility type and ramp configuration, but most states do not have policies that vary by those factors. • Most state DOTs use Green Book or similar policies and have no plans to change, but most academics and private-sector practitioners would like to see changes to the policy.
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55 - The table needs to be expanded to show different ramp types (exit, entrance, freewayto-freeway) and ramp configurations (loops, diamonds)
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56 The vertical alignments are also checked against the horizontal speed profiles to ensure consistency in vertical/horizontal design. • Ramp safety and use of the HSM: - When using the HSM in ramp design, a few anomalies have been identified that do not seem logical.
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57 Table 10-1 is insufficient for selecting ramp design speeds. Rather than Green Book Table 10-1 addressing entrance and exit ramps separately and the various types of ramp configurations, the table provides upper, middle, and lower range values of ramp design speeds.
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58 Figure 7. Elements of the Functional Area of an Intersection (AASHTO, 2011)
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59 liability concerns related to ramps designed using existing design policy for selecting ramp design speeds. These key issues and limitations were considered in developing the Phase II work plan for this research and the guidelines that were developed.
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