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Selecting Ramp Design Speeds, Volume 1: Guide (2021)

Chapter: Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed

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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
×
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
×
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
×
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
×
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
×
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Suggested Citation:"Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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40 Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed Interchange design and the design and orientation of each individual interchange ramp is an art form that requires consideration of a wide range of factors including: • Number of intersecting legs. • Classification of intersecting facilities. • Volume and pattern of existing and future traffic. • Environmental requirements. • Local access and circulation considerations. • Physical constraints and right-of-way considerations. • Construction and maintenance costs. • Road-user costs (i.e., costs related to safety and operations). • Design controls. • Signing. The decision on selecting an appropriate ramp design speed and then designing the horizontal and vertical alignments to allow for appropriate speed transitions along a ramp is still left to the engineering judgement of the designer. The design guidelines presented in this section can be used during the design process to help design a ramp in a consistent manner based on the selected ramp design speed. First, several general design guidelines are presented followed by design controls and criteria for entrance and exit ramps. 3.1 Design Guidelines Several guidelines for designing an interchange ramp in a consistent manner based on the selected ramp design speed are as follows: 1. The ramp design speed is typically associated with an individual segment of a ramp near the upstream or downstream end of the ramp. For an entrance ramp, the ramp design speed is normally associated with a downstream portion of the ramp; and with an exit ramp, the ramp design speed is usually associated with a segment on the upstream portion of the ramp. It is desirable (but not essential) that the design speeds of the contiguous segments along the ramp proper increase or decrease in a stepwise manner; and unless a segment is rather long, the change in design speed between adjoining sections should be limited to no more than 10 to 15 mph. For entrance ramps, this means that the design speeds of the contiguous segments along the ramp proper should typically increase in a stepwise manner; and for exit ramps the design speeds of the contiguous segments along the ramp proper should decrease in a stepwise manner. Decreasing design speeds sequentially along a ramp in a stepwise manner is more critical for exit ramps than increasing design speeds along a ramp for entrance ramps, as unexpected and abrupt changes in design speeds along an exit ramp are more likely to be associated with potential lane keeping and/or loss of control issues.

41 The design speed of the controlling feature of the ramp (i.e., the ramp design speed) is first selected, and then the design speeds of the other elements or segments of the ramp proper should be determined accordingly. 2. Each component of the ramp should be designed such that a vehicle has sufficient distance to accelerate or decelerate within the performance capabilities of the vehicle and driver expectations given the design speeds and features of the individual components of the ramp. Several design controls and criteria can be used in the design of entrance and exit ramps so that the individual components and features of the ramps are designed in a consistent manner, accounting for sequential speed transitions from one component or section to the next. 3. Each ramp component should be designed in a consistent manner based on the design speed of the component and anticipated operating speed along the component. The design speed of each individual curve and tangent along the ramp proper should be consistent with the speeds that drivers are likely to be traveling along the respective curve and tangent. If the design speed of an individual curve or tangent (or multiple curves and tangents) along the ramp proper is not consistent with the speeds that drivers are likely to be traveling along the respective curve and tangent, consider potential options so that the design speed would be more consistent with operating speeds. For example, if the anticipated operating speed exceeds the design speed, consider changing (e.g., raising or potentially decreasing) the design speed for the respective section. If changing the design speed of an individual section means that the design speeds of the contiguous segments along the ramp proper no longer increase or decrease in a stepwise manner, consider changing the ramp design speed. If the design speed or ramp design speed cannot be changed (e.g., due to physical site constraints, environmental or social impacts, economic constraints, etc.), consider implementing speed-control measures so that the design speed or ramp design speed would be more consistent with operating speeds. 3.2 Design Controls and Criteria for Ramps This section describes several design controls and criteria that could be used to design a ramp in a consistent manner based on the selected ramp design speed. Design controls are attributes, values, or qualities that influence discrete geometric element dimensions or considerations. Design criteria are dimensions and values that meet design control needs, such as curve radius, acceleration lengths, and deceleration lengths. Section 3.2.1 addresses design controls and criteria for entrance ramps, followed by Section 3.2.2 which addresses design controls and criteria for exit ramps. Section 3.2.3 addresses design controls and criterial for freeway-to- freeway ramps. Guidelines for Entrance Ramps With entrance ramps, vehicle acceleration is a primary design control. Individual components and features of the ramp should be designed with sufficient length such that vehicles may accelerate in a comfortable and controlled manner from an initial speed to a final speed over the length of the ramp so vehicles can merge onto the through lanes of the primary highway or freeway at a speed approximately equal to the operating speed of the primary highway or freeway. The design controls and criteria discussed below for entrance ramps are presented as

42 appropriate in order of the primary components of an entrance ramp based on the direction of travel. As such, applicable design controls and criteria are first discussed for the crossroad ramp terminal, then the ramp proper, and finally the freeway mainline ramp terminal. The first component of an entrance ramp is the crossroad ramp terminal. The crossroad ramp terminal does not have an associated design speed assigned to it. Therefore, the design of the entrance ramp is based on the operating speed of vehicles entering the ramp proper from the crossroad ramp terminal. As vehicles enter from the crossroad ramp terminal onto the ramp proper, typically they enter the ramp proper at lower speeds depending upon the design and type of traffic control of the crossroad ramp terminal. Where stop-, yield-, or signal control are implemented at a crossroad ramp terminal, vehicles are expected to enter the ramp proper at speeds around 15 mph (AASHTO, 2014). If free-flow operations are provided at the crossroad ramp terminal, vehicle speeds may be higher entering the ramp proper. For the ramp proper of an entrance ramp, AASHTO policy does not provide specific design criteria for determining the minimum lengths of individual tangent and curve sections to allow for vehicles to accelerate to an assumed or defined speed prior to entering the freeway mainline ramp terminal. However, Green Book Figure 2-33 (Acceleration of Passenger Cars, Level Conditions) (see Figure 15) may be used as guide for determining minimum practical lengths of individual tangent and curve sections along the ramp proper (AASHTO, 2018). Green Book Figure 2-33 depicts acceleration capabilities of lower performance vehicles which may be used for design. Table 5 presents acceleration distances from Green Book Figure 2-33 in tabular form for entering and exiting speeds that would be consistent for the design of entrance ramps. Figure 15. Acceleration of Passenger Cars, Level Condition (AASHTO, 2018)

43 Table 5. Minimum Lengths for Tangents and Curves on Ramp Proper of Entrance Ramps Speed Exiting the Tangent or Curve on the Ramp Proper (Final Speed) (mph) Recommended Minimum Length of Tangent or Curve on Ramp Proper to Provide for Vehicle Acceleration1 (ft) Initial Speed Entering Tangent or Curve on the Ramp Proper (Initial Speed) (mph) 15 20 25 30 35 40 45 50 55 20 50 25 110 70 30 175 140 75 35 275 245 175 100 40 380 355 300 230 105 45 540 500 445 365 265 140 50 735 685 615 560 445 355 200 55 945 875 825 775 670 575 415 210 60 1230 1170 1125 1050 965 860 730 500 285 1 Lengths based on acceleration capabilities of passenger cars on level conditions as depicted in Green Book Figure 2-33. Green Book Figure 2-33 may be used when referring to both initial and final design speeds and initial and final anticipated operating speeds. The speed distance relationships depicted in Green Book Figure 2-33 do not account for the effects of horizontal or vertical alignments. The lengths should be adjusted accordingly to account for the effects of horizontal curvature and/or grade. The selected design speed of an individual tangent section should be consistent with the speeds that drivers are likely to be traveling at the end of the tangent. With curves on entrance ramps, it may also be assumed that vehicle speeds will increase slightly over the length of the curve depending on the curve radius, so the speeds of vehicles exiting a curve may be slightly higher than the speeds of vehicles entering the curve. Speeds associated with the midpoint of the curve are to be used to design the curve rather than the speeds at the beginning or end of the curve. Again, the design speed of the curve should be consistent with the speeds that drivers are likely to be traveling at the midpoint of the curve. For consistency, the anticipated operating speed exiting a tangent section or curve should be equivalent to the anticipated operating speed entering the adjoining downstream tangent section or curve. As operating speeds are expected to increase along an entrance ramp from section to section, subsequently the design speeds of the individual sections should also increase. As a general rule, the change in design speed between adjoining sections should be limited to no more than 10 to 15 mph. This general rule is not as critical for longer sections of ramps. As vehicles transition from the ramp proper to the freeway mainline ramp terminal of an entrance ramp, the primary design criterion provided in AASHTO policy addresses minimum acceleration lengths (see Green Book Table 10-4) along the freeway mainline ramp terminal. The freeway mainline ramp terminal portion of a ramp does not have an associated design speed. Green Book Table 10-4 is presented below as Table 6 with a few adaptions. The headings were adapted to be more consistent with the design guidelines presented herein. In addition, the design values were expanded to include additional design conditions for higher ramp design speeds and higher highway (or freeway) design speeds. The expanded design values were estimated for use with the case studies in Section 5 and recognizing the limitations of Table 10-4 in the 2018

44 edition of the Green Book to be used in the design of higher speed facilities. The new design values (shown in red text in Table 6) were estimated through numeric extrapolation of the minimum acceleration lengths for the lower speed design conditions and to be consistent with the trends and acceleration rates of the design values in the original table. They were not developed based on field data or vehicle dynamics models that estimate vehicle performance. It is recommended that additional research be conducted to verify and/or modify these design values for higher speed facilities. With Green Book Table 10-4 and Table 6 presented below, the design speed of the final section of the ramp proper upstream of the gore point and the design speed of the freeway are used to determine the minimum acceleration length for the freeway mainline ramp terminal. Both tables also provide anticipated speeds entering the freeway mainline ramp terminal and merging with freeway traffic for given design speeds. For the freeway mainline ramp terminal to be designed in a consistent manner with the upstream components of the ramp, the anticipated speed of vehicles exiting the ramp proper should be equivalent to the speed of vehicles entering the freeway mainline ramp terminal [i.e., the initial speed of vehicles entering the acceleration length (VAcc Length(i))]. Thus, although the freeway mainline ramp terminal does not have an associated design speed, the operational and design conditions of the freeway mainline ramp terminal are related to and coordinated with the operational and design conditions of the individual sections of the upstream components of the ramp.

45 Table 6. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Less Than 3 Percent (adapted from AASHTO, 2018) Highway Acceleration Length, LAcc Length (ft), for Design Speed (VDS) of Controlling Feature on the Ramp Proper (or the Final Section of the Ramp Proper Upstream of the Gore Point) Stop Condition 15 20 25 30 35 40 45 50 55 1 601 651 Design Speed VDS Hwy (mph) Merge Speed VAcc Length(f) (mph) Average Running Speed (i.e., Initial Speed) Exiting the Controlling Feature on the Ramp Proper (or the Final Section of the Ramp Proper Upstream of the Gore Point), VAcc Length(i) 0 14 18 22 26 30 36 40 44 48 52 56 30 23 180 140 35 27 280 220 160 40 31 360 300 270 210 120 45 35 560 490 440 380 280 160 50 39 720 660 610 550 450 350 130 55 43 960 900 810 780 670 550 320 150 60 47 1200 1140 1100 1020 910 800 550 420 180 65 50 1410 1350 1310 1220 1120 1000 770 600 370 135 70 53 1620 1560 1520 1420 1350 1230 1000 820 580 350 135 75 55 1790 1730 1630 1580 1510 1420 1160 1040 780 550 300 80 57 2000 1900 1800 1750 1680 1600 1340 1240 980 725 500 200 851 59 2230 2090 1985 1930 1865 1790 1525 1455 1250 940 700 500 NOTE: Uniform 50:1 to 70:1 tapers are recommended where acceleration lengths exceed 1,300 ft. 1 Acceleration lengths estimated based on extrapolation. The minimum acceleration lengths associated with freeway mainline ramp terminals of entrance ramps are calculated using a simple kinematic equation (see Equation 2) with constant acceleration. The minimum acceleration length is based on: A. The speeds at which drivers enter the freeway mainline ramp terminal (i.e., the speed at the end of the ramp proper’s controlling feature). B. An assumed acceleration rate. C. The speeds at which drivers merge with through traffic (i.e., the speed at the end of the acceleration length). 𝐿 = 1.47𝑉 ( ) − 1.47𝑉 ( ) 2𝑎 (2) where: LAcc Length = acceleration length, ft.

46 VAcc Length(f) = merge speed (i.e., the speed at which a driver merges with through traffic at the end of the acceleration length), mph. VAcc Length(i) = initial speed entering the freeway mainline ramp terminal of an entrance ramp from the ramp proper (i.e., the speed exiting the controlling feature of the ramp proper), mph. A = acceleration rate, ft/s2. Green Book Table 10-4 (and corresponding Table 6) provides minimum acceleration lengths for various combinations of design conditions for both taper- and parallel-type exit terminals on level grades. The minimum acceleration lengths are to be adjusted in accordance to Green Book Table 10-5 where grades are present. Several points are worth noting concerning the basis for determining the minimum acceleration lengths within AASHTO policy: • The acceleration rates used to determine the acceleration distances are based on “normal acceleration” for passenger vehicles on level grade. • The speed of vehicles entering the primary highway or freeway from the freeway mainline ramp terminal should approximate that of the through traffic on the highway or freeway. In addition, it is satisfactory and does not unduly inconvenience through traffic if vehicles enter the highway at a speed approximately 5 mph less the speed of traffic on the highway. Figure 16 illustrates the primary components associated with the freeway mainline ramp terminal of an entrance ramp, including but not limited to: • A speed-change lane or auxiliary lane, including tapered area, is provided primarily for acceleration of vehicles entering through traffic. The speed-change lane joins the traveled way of the turning roadway to the highway and does not necessarily imply a definite lane of uniform width. For an entrance ramp, the length of the speed-change lane is measured from the gore point to the end of the taper. The speed-change lane should have sufficient length to enable a driver to make the appropriate change in speed between the ramp proper and intersecting highway. • The gap acceptance length is that portion of the speed-change lane necessary to allow motorists to evaluate gaps in the freeway traffic and position their vehicles to use the gap. The gap acceptance length begins at the gore point and ends where the width of the speed-change lane reduces to less than 12 ft. The gap acceptance length should be a minimum of 300 to 500 ft depending on the nose width. The gap acceptance length is always less than or equal to the acceleration length. • The acceleration length is the distance needed for acceleration upon exiting the controlling feature of the ramp proper or the final section of the ramp proper upstream of the gore point to the point of convergence with the freeway. The acceleration length begins at Point A and ends where the gap acceptance length ends and the taper begins. Point A represents the location where the ramp proper ends and the freeway mainline ramp terminal begins. Conceptually, the acceleration length begins at the end of the controlling curve or feature of the ramp proper; however, if the radius of the final curve of the ramp is greater than 1,000 ft and drivers on the ramp have a clear view of traffic in

47 the right lane of the freeway, the beginning of the acceleration length may be located somewhere along the curve. This means the beginning of the acceleration length may be located upstream of the gore point. Notes: 1. LAcc Length is the recommended acceleration length as shown in Table 6 or adjusted due to grade. 2. LGap Acpt is the gap acceptance length and should be a minimum of 300 to 500 ft depending on the nose width. 3. The value of LAcc Length or LGap Acpt whichever produces the greater distance downstream from the gore point is suggested for use in the design of the freeway mainline ramp terminal. Figure 16. Primary Components of Freeway Mainline Ramp Terminal of an Entrance Ramp (adapted from AASHTO, 2018)

48 Guidelines for Exit Ramps With exit ramps, a primary design control is vehicle deceleration. Individual components and features of the ramp should be designed with sufficient length such that vehicles decelerate in a controlled manner from an initial to final speed over the length of the section and do not exceed an assumed maximum deceleration. The design controls and criteria discussed below for exit ramps are presented as appropriate in order of the primary components of an exit ramp based on the direction of travel. As such, applicable design controls and criteria are first discussed for the freeway mainline ramp terminal, then the ramp proper, and finally the crossroad ramp terminal. As vehicles transition from the primary highway or freeway to the freeway mainline ramp terminal of an exit ramp and then to the ramp proper, the primary design criterion associated with vehicle deceleration addresses the minimum deceleration lengths along the freeway mainline ramp terminal (see Green Book Table 10-6). Green Book Table 10-6 is presented below as Table 7 with a few adaptions. The headings were adapted to be more consistent with the design guidelines presented herein. In addition, the design values were expanded to include additional design conditions for higher ramp design speeds and higher highway (or freeway) design speeds. The expanded design values were estimated recognizing the limitations of Table 10-6 in the 2018 edition of the Green Book to be used in the design of higher speed facilities. The new design values (shown in red text in Table 7) were estimated through numeric extrapolation of the minimum deceleration lengths for the lower speed design conditions and to be consistent with the trends and acceleration rates of the design values in the original table. They were not developed based on field data or vehicle dynamics models that estimate vehicle performance. It is recommended that additional research be conducted to verify and/or modify these design values for higher speed facilities. With Green Book Table 10-6 (Table 7), the minimum deceleration length for the freeway mainline ramp terminal is determined based on the design speed of the freeway and the design speed of the controlling feature of the ramp proper or the first section of the ramp proper downstream of the gore point. Green Book Table 10-6 (Table 7) also provides anticipated speeds of vehicles diverging from the freeway and exiting the freeway mainline ramp terminal. For the freeway mainline ramp terminal to be designed in a consistent manner with the downstream components of the ramp, the anticipated speed of vehicles exiting the freeway mainline ramp terminal [i.e., the final speed of vehicles exiting the deceleration length (VDec Length(f))] should be equivalent to the speed of vehicles entering the ramp proper. In this way, the operational and design conditions of the freeway mainline ramp terminal are related to and coordinated with the operational and design conditions of the individual sections of the ramp proper.

49 Table 7. Minimum Deceleration Lengths for Exit Terminals with Flat Grades of Two Percent or Less (adapted from AASHTO, 2018) Highway Design Speed (VDS Hwy) (mph) Diverge Speed (VOS Hwy) (VDec Lenth (i)) (mph) Deceleration Length, LDec Length (ft), for Design Speed (VDS) of Controlling Feature on the Ramp Proper (or the Initial Section of the Ramp Proper Downstream of the Gore Point) (mph) Stop Condition 15 20 25 30 35 40 45 50 Average Running Speed at the End of the Deceleration Length, VDec Length (f) (mph), Entering the Controlling Feature on the Ramp Proper (or the Initial Section of the Ramp Proper Downstream of the Gore Point) 0 14 18 22 26 30 36 40 44 30 28 235 200 170 140 - - - - - 35 32 280 250 210 185 150 - - - - 40 36 320 295 265 235 185 155 - - - 45 40 385 350 325 295 250 220 - - - 50 44 435 405 385 355 315 285 225 175 - 55 48 480 455 440 410 380 350 285 235 - 60 52 530 500 480 460 430 405 350 300 240 65 55 570 540 520 500 470 440 390 340 280 70 58 615 590 570 550 520 490 440 390 340 75 61 660 635 620 600 575 535 490 440 390 80 64 705 680 665 645 620 580 535 490 440 851 67 750 725 710 690 665 625 580 540 485 1 Deceleration lengths estimated based on extrapolation. The basis for the recommended minimum deceleration lengths in the Green Book is as follows. It is assumed that a vehicle maintains its speed in the freeway lanes and first decelerates after exiting the freeway mainline and upon entering the deceleration length portion of the speed- change lane (see Figure 17). It is assumed that a vehicle completely diverges from the freeway mainline lanes where the width of the auxiliary lane increases to 12 ft or greater. This point is the end of taper and the beginning of both the divergence zone and the deceleration length. The divergence zone allows for deceleration after clearing the through-traffic lane of the freeway and extends to the gore point. For a taper-type exit ramp, the divergence zone may be relatively short. The deceleration length is the distance needed for deceleration after clearing the through- traffic lane of the freeway and before reaching the first location that significantly limits vehicle speeds on the ramp proper or the first section of the ramp proper downstream of the gore point. As illustrated in Figure 17, the deceleration length ends at Point D, which represents the location where the freeway mainline ramp terminal ends and the ramp proper begins. Point D cannot be located upstream of the gore point on the speed-change lane but may be located downstream of the gore point.

50 Figure 17. Freeway Mainline Ramp Terminal Elements of an Exit Ramp AASHTO policy uses a basic two-step process for establishing the minimum deceleration length criteria for exit ramps. Deceleration is accomplished first as the driver removes his or her foot from the accelerator pedal and the vehicle slows in gear for a period of time (assumed to be 3 sec) without the use of brakes, and then as the driver applies the brakes and decelerates at a comfortable rate. Equations 3 and 4 represent this two-step process. The minimum deceleration length is based on the combination of the following inputs: A. The speed at which drivers maneuver onto the auxiliary lane. B. The speed at which drivers turn after traversing the deceleration lane. C. The manner of deceleration. 12 ft Tapered Design Taper Length (LTaper) Divergence Zone Length (LDiv Zone) Speed-Change Lane Length (LSCL) Deceleration Length (LDec Length) D Through Lanes Painted Nose Gore Point (2 ft) 12 ft Taper Length (LTaper) Divergence Zone Length (LDiv Zone) Speed-Change Lane Length (LSCL) Deceleration Length (LDec Length) D Parallel Design Through Lanes Painted Nose Gore Point (2 ft) Freeway Mainline Ramp Terminal Freeway Mainline Ramp Terminal

51 𝐿 = 1.47𝑉 ( )𝑡 − 0.5𝑑 (𝑡 ) + 1.47𝑉 ( ) − (1.47𝑉 )2𝑑 (3) 𝑉 = 1.47𝑉 ( ) − 𝑑 𝑡1.47 (4) where: LDec Length = Deceleration length, ft. VDec Length (i) = Speed at the beginning of the deceleration length, mph. Va = Speed after tn sec of deceleration without brakes, mph. VDec Length (f) = Speed at the end of the deceleration length, mph. tn = Deceleration time without brakes (assumed to be 3 sec), sec. dn = Deceleration rate without brakes, ft/s2. dwb = Deceleration rate with brakes, ft/s2. Green Book Table 10-6 (Table 8) provides recommended minimum deceleration lengths for various combinations of design conditions for both taper- and parallel-type exit terminals. The minimum recommended deceleration lengths are to be adjusted in accordance to Green Book Table 10-5 where grades are present. The speed-change lane for an exit ramp should meet the recommended minimum deceleration lengths based on the operating speed of the highway and the anticipated speed entering the controlling feature on the ramp proper or the initial section of the ramp proper downstream of the gore point. A key factor in designing the freeway mainline ramp terminal of an exit ramp is determining where the deceleration length and the freeway mainline ramp terminal end (i.e., location of Point D). The deceleration length and freeway mainline ramp terminal may end at the gore point; however, the deceleration length and freeway mainline ramp terminal may also extend beyond the gore point. To determine the boundary between the freeway mainline ramp terminal and the ramp proper, the ramp beyond the gore point should be divided into individual tangent and curve sections. If the horizontal alignment of the ramp is curvilinear, the beginning of the first curve encountered at the gore point or downstream of the gore point that significantly affects vehicle speed is the boundary between the freeway mainline ramp terminal and the ramp proper. It should be assumed that any curve radius less than or equal to 1,000 ft will significantly affect vehicle speed. If the first curve that significantly affects vehicle speed begins at the gore point, then the boundary between the freeway mainline ramp terminal and the ramp proper is at the gore point; however, if the beginning of the first curve that significantly affects vehicle speed is downstream of the gore point, then the boundary between the freeway mainline ramp terminal and the ramp proper may be downstream of the gore point. Thus, in some situations the portion of the ramp immediately downstream of the gore point is part of the ramp proper, while in other situations the portion of the ramp immediately downstream of the gore point is part of the freeway mainline ramp terminal for some extended length. If the horizontal alignment of the ramp is relatively straight and has little or no influence

52 on vehicle speeds, then conceptually the freeway mainline ramp terminal may extend beyond the gore point all the way to the beginning of the functional area of the crossroad ramp terminal. No design criteria exist for specifying minimum length requirements of the individual tangent and curve sections of the ramp beyond the gore point; but the deceleration distances depicted in Green Book Figure 2-34 (Deceleration Distances for Passenger Vehicles Approaching Intersections) (see Figure 18) may be used as guide for determining the minimum practical lengths of the individual sections. These deceleration distances depict performance capabilities of lower performance vehicles decelerating with steady breaking suitable for design application. For each individual section, entering and exiting speeds should be assumed for the design, consistent with the speed exiting the freeway mainline ramp terminal and decreasing sequentially along the adjoining sections of the ramp. The individual component or section of the ramp should at least be as long as the deceleration distances depicted in Green Book Figure 2-34, based on the assumed entering and exiting speeds. Table 8 presents the deceleration distances in Green Book Figure 2-34 in tabular form. Figure 18. Deceleration Distances for Passenger Vehicles Approaching Intersections (AASHTO, 2018)

53 Table 8. Deceleration Distances for Passenger Vehicles Approaching Intersections based on Steady Braking Initial Speeds (mph) Deceleration Distances for Passenger Vehicles Approaching Intersections (ft) Final Speeds (mph) 0 20 30 40 50 20 75 30 167 108 40 268 204 115 50 360 302 233 127 60 460 404 346 264 127 70 567 512 458 383 275 80 675 620 567 500 413 Green Book Figure 2-34 can be used when referring to both initial and final design speeds and anticipated operating speeds. The speed distance relationships depicted in Green Book Figure 2- 34 do not account for the effects of horizontal or vertical alignments. The lengths should be adjusted accordingly to account for the effects of horizontal curvature and/or grade. When it comes to the ramp design at the end of the ramp proper near the crossroad ramp terminal, the operational characteristics of the crossroad ramp terminal control the design. The primary determinant in the design of the crossroad ramp terminal is the type of traffic control (i.e., whether operation onto the crossroad is stop-, yield-, or signal-controlled or free-flow). Where the crossroad ramp terminal is stop-, yield-, or signal-controlled, the ramp design should account for queue storage as depicted in Figure 19 which illustrates the functional area of a crossroad ramp terminal. The ramp should be designed to provide sufficient distance for a vehicle to come to a complete stop at the end of the queue storage length near the crossroad. The ramp should be designed so that vehicles enter the functional area of the crossroad ramp terminal at relatively low speeds (e.g., between 15 and 20 mph). Figure 19. Elements of the Functional Area of a Crossroad Ramp Terminal (adapted from AASHTO, 2018) The length from the beginning of the crossroad ramp terminal to the end of the storage length (i.e., LLength to Q) consists of the decision length (LDecis Length) and the maneuver length (LMan Length).

54 Similar to the brake reaction time used for stopping sight distance (SSD), the decision length is the distance covered by a vehicle from the instant that the driver recognizes the presence of a queue of vehicles stopped ahead until the instant that the driver actually applies the breaks. For design purposes, the decision length is based on an assumed perception reaction time of 2.5 seconds. The maneuver length approximates the distance that a vehicle travels after the brakes are applied. For design purposes, the maneuver length is based on deceleration distances for passenger vehicles approaching an intersection based on steady braking. Green Book Figure 2-34 (Figure 18) and Table 8 can again be used to determine the maneuver length to the rear of the queue. Table 9 provides the recommended minimum length from the beginning of the crossroad ramp terminal to the end of the storage length (i.e., LLength to Q) based on the decision length (LDecis Length) and the maneuver length (LMan Length). Table 9. Recommended Minimum Length to Queue at a Crosroad Ramp Terminal Initial Speeds (mph) Decision Length (LDecis Length) (ft) Maneuver Length (LMan Length) (ft) Length to Queue Calculated (LLength to Q) (ft) Length to Queue Design (LLength to Q) (ft) 15 55 45 90 90 20 74 75 149 150 25 92 125 217 220 30 110 167 277 280 35 129 220 349 350 Where the crossroad ramp terminal provides for free-flow operations onto the crossroad, the speed at the downstream end of the final tangent or curve of the ramp proper should be equivalent to the initial speed entering the acceleration length connecting the ramp proper to the crossroad. Green Book Table 10-4 could be used to design the acceleration length for the crossroad ramp terminal. Thus, Green Book Tables 10-6 and 10-5 serve to establish the minimum requirements for the deceleration length of a freeway mainline ramp terminal; Green Book Figure 2-34 provides guidance on the minimum lengths of the individual tangent and curve sections of the ramp proper; and the operational characteristics near the crossroad ramp terminal control the design of the terminal. The key element in coordinating the design of the individual components and features of an exit ramp is the ramp design speed. The ramp design speed is used to coordinate the design of adjacent sections of the ramp to accommodate sequential deceleration from one component or section to the next. For each tangent section on the ramp proper, the speed at the downstream end is associated with the design speed of the tangent. The selected design speed of a tangent section should be consistent with the speeds that drivers are likely to be traveling at the end of the tangent. With horizontal curves on exit ramps, it is assumed that vehicle speeds will decrease slightly over the length of the curve, so the speeds of vehicles exiting a curve will be slightly lower than the speeds of vehicles entering the curve. Speeds associated with the midpoint of the curve should be used to design the curve rather than speeds at the beginning or end of the curve, and the design

55 speed of the curve should be consistent with the speeds that drivers are likely to be traveling at the midpoint of the curve. Guidelines for Freeway-to-Freeway Ramps The same design controls and criteria for freeway mainline ramp terminals and the ramp proper for entrance and exit ramps are also applicable to freeway-to-freeway ramps. In other words, the same design controls and criteria used to coordinate the design of individual components and features of entrance and exit ramps based on the ramp design speed may also be used to design freeway-to-freeway ramps. This means that design criteria such as Green Book Tables 10-4, 10-5, and 10-6 may be used to determine the minimum acceleration and deceleration lengths for the freeway mainline ramp terminals of freeway-to-freeway ramps, and Green Book Figures 2-33 and 2-34 may be used to determine the minimum practical lengths for individual tangents and curves along the ramp proper of freeway-to-freeway ramps. For freeway-to-freeway ramps, the sharpest curve on the ramp proper is considered the controlling curve, from which the ramp design speed for the entire ramp is based. The ramp design speed is used to coordinate the design of adjacent sections of the ramp to accommodate sequential acceleration or deceleration from one component or section to the next. For each tangent section on the ramp proper, the speed at the downstream end is associated with the design speed of the tangent. The selected design speed of a tangent section should be consistent with the speeds that drivers are likely to be traveling at the end of the tangent. With horizontal curves, speeds associated with the midpoint of the curve should be used to design the curve rather than speeds at the beginning or end of the curve, and the design speed of the curve should be consistent with the speeds that drivers are likely to be traveling at the midpoint of the curve. Although the freeway mainline ramp terminals do not have associated design speeds, the operational and design conditions of the freeway mainline ramp terminals are related to and coordinated with the operational and design conditions of the downstream and upstream components of the ramp. For freeway-to-freeway ramps, rather than the design speeds of the contiguous segments along the ramp proper continuously increase or decrease in a stepwise manner; design speeds of the contiguous segments along the ramp proper should initially decrease and then increase. It is still desirable that the change in design speed between adjoining sections be limited to no more than 10 to 15 mph. 3.3 Measures for Reducing Speeds on Exit Ramps On exit ramps, where anticipated operating speeds exceed the ramp design speed or design speed of an individual section, and changing the ramp design speed (or other design options) cannot resolve the issue, the following speed-control measures may be considered (Harwood and Mason, 1993): • Provide signing with an appropriate advisory speed for the ramp and place the advisory speed sign so that drivers have sufficient length to reduce their speed prior to the most critical curve. • Increase the deceleration length.

56 • Realign the ramp to increase the distance from the gore point to the controlling curve. • Supplement the standard advisory speed signs to make the signing more conspicuous, increase the distance from the signing to the controlling curve and draw attention of truck drivers to the signing. These objectives may be accomplished through: - Using more than one advisory speed sign. - Placing an advisory speed sign on the mainline highway in advance of the ramp. - Incorporating an exit speed panel in the guide signing for the exit ramp. - Using overhead signs. - Using a TRUCK SPEED advisory sign. - Using flashing beacons to call attention to the advisory speed. • Avoid designs where the presence of the controlling curve is not obvious (e.g., where a tight horizontal curve follows a larger-radius curve). • Consider use of C-D roads at the interchange to introduce an intermediate-speed roadway between the mainline freeway and the ramp.

Next: Section 4. Design Tool to Evaluate Ramp Designs for Consistency with the Selected Ramp Design Speed »
Selecting Ramp Design Speeds, Volume 1: Guide Get This Book
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Selection of a design speed should be based upon the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the roadway.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 313: Selecting Ramp Design Speeds, Volume 1: Guide provides further detail for selecting an appropriate ramp design speed than presented in the 2018 Green Book, to address several overarching challenges that may lead to confusion or inconsistent interpretation of existing AASHTO guidance for selecting an appropriate ramp design speed.

Supplemental to the document are NCHRP Web-Only Document 313: Selecting Ramp Design Speeds,Volume 2: Conduct of Research Report and Ramp Speed Profile Model worksheets.

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