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Human Factors Guidelines for Road Systems: Second Edition (2012)

Chapter: Chapter 7 - Grades (Vertical Alignment)

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Page 59
Suggested Citation:"Chapter 7 - Grades (Vertical Alignment)." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 7 - Grades (Vertical Alignment)." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Page 61
Suggested Citation:"Chapter 7 - Grades (Vertical Alignment)." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Page 61
Page 62
Suggested Citation:"Chapter 7 - Grades (Vertical Alignment)." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Page 62
Page 63
Suggested Citation:"Chapter 7 - Grades (Vertical Alignment)." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Page 63
Page 64
Suggested Citation:"Chapter 7 - Grades (Vertical Alignment)." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
×
Page 64
Page 65
Suggested Citation:"Chapter 7 - Grades (Vertical Alignment)." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Page 65

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Design Considerations for Turnouts on Grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 Geometric and Signing Considerations to Support Effective Use of Truck Escape Ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-4 Preview Sight Distance and Grade Perception at Vertical Curves . . . . . . . . . . . . . . . . . . . . . . .7-6 7-1 C H A P T E R 7 Grades (Vertical Alignment)

D ES IG N C ON SI DE RA TI ON S FO R T UR NO UT S ON G RA DE S In tr od uc ti on Turnouts are widened, unobstructed shoulder areas that allow slow-m oving vehicles to pull out of the through lane to give passing opportunities to following vehicles ( 1 ). This guideline provides design recommendations that support safe and appropriate use of Turnouts on Grade s . Turnouts are also beneficial for two-lane highways in m ountainous terrain. To promote safe use, turnouts should be designed with elements that inform drivers of the presence of the turnout, encourage drivers to enter at safe speeds, encourage users to allow all trailing vehicles in the platoon to pass, and provide adequate sight distance of the lane (behind the vehicle) to safely me rge back onto the roadway. De si gn Gu id e lin es The following turnout design recomm endations should be considered in order to prom ote driver behavior that is consistent with safe use of turnouts. Entry/Exit Topic Recommendations Turnout Entry Signage • Use signs at turnouts to: − Notify drivers of an upco mi ng se ries of turnouts ( 2 ). − Notify drivers of a specific turnout ( 2, 3 ). − Remind drivers of the legal requirements for turnout use ( 3 ). − Identify the beginning of a specific turnout ( 3 ). • If a turnout is to be signed, it is recommended that both an advance sign and a turnout sign be used ( 2 ). • Do not place an advance sign too far in advance of the turnout. One source suggests 500 to 800 ft may be appropriate ( 2 ). • Include an upward-sloping arrow (e.g., MUTCD sign R4-14) to indicate that slow-m oving vehicles should m ove to the right ( 3, 4 ). Sight Distance • Locate turnouts with adequate sight distance to the entrance to allow time to decelerate to a safe entry speed ( 1 ). • Avoid locating a turnout on or adjacent to a horizontal or vertical curve that limits sight distance in either direction. The available sight distance should be at least 300 m (1,000 ft) on the approach to the turnout ( 1 ). Entry Speed • Approach speeds are a function of grade and horizontal alignm ent. Turnouts on downgrades with gentle curves should be lo nger than turnouts on steep upgrades with continuous curves ( 2 ). • When possible, avoid placing turnouts on fill slopes or drop-offs, particularly on the outside of curves. Turnouts on these geometries can appear very small to high-speed drivers, discouraging their use ( 2 ). Turnout Exit Sight Distance • Locate turnouts with adequate sight distance to the exit so that approaching drivers can see low speed vehicles leaving the turnout ( 2 ). • Use the Guideline in Chapter 10: Sight Distance at Right-Skewed Intersections for design considerations related to site distance for drivers exiting the turnout. Exit Behavior • Avoid excessively long turnouts to discourage drivers fro m using it as a passing lane and cutting back into the platoon ( 1, 2 ). Ba sed Primarily on Ex pert Jud g ment Based Equally on Expert Judgment and Empirical Dat a Based Primarily on Em pirical Da ta HFG GRADES (VERTICAL ALIGNMENT) Version 2.0 7-2

Discussion Turnouts may be utilized more by recreational vehicles and slower passenger cars, while heavy truck drivers are not likely to use them at all (2). Turnouts are most frequently used on lower volume roadways where long platoons are rare and in difficult terrain with steep grades where placement of additional lanes is not cost-effective. Over 80% of all following vehicles in platoons immediately behind a turnout user are generally able to pass the turnout user. A pass completed because of a turnout maneuver may not provide as much operational benefit as a pass completed in a passing lane (4). In a passing lane, the passing vehicles are self-selected, with higher desired speeds than their immediate platoon leader. By contrast, turnout users (rather than the passing vehicles) are self-selected, and the passing drivers may or may not have higher desired speeds. The passing vehicles at a turnout may simply continue downstream as a new platoon leader. Therefore, it is expected that a turnout may not provide as much reduction in platooning per passing maneuver as a passing lane. Design Issues Turnouts are most effective if their purpose is clearly conveyed to roadway users by appropriate signing and turnout placement. In one study (4), the signage that provided the greatest degree of positive guidance included an upward- sloping arrow indicating that slow-moving vehicles are to move to the right. Signage may also be effective for deterring drivers from utilizing turnouts as rest areas or scenic stopping points. When warning drivers of an upcoming turnout, it is recommended that two signs be placed: an advance warning sign and a location sign; however, the advance sign should not be placed too far in advance of the turnout. In one study (2), some vehicles stopped at areas other than the turnout when the advance sign was one-quarter mile from the turnout. Vehicle drivers that lead long platoons drive more slowly into the turnout when compared to those leading short platoons (4). In contrast, short platoon leaders that utilize turnouts enter with higher speeds that can be dangerous to any vehicle that is occupying the turnout. Designers should realize that vehicles could enter the turnout at speeds up to 50 mi/h and design the length of the turnout accordingly, accounting for grade and curvature. Turnouts on fill slopes or drop-offs can appear very small to high-speed drivers (particularly when located on the outside of a curve), which may discourage their use. Turnouts of inadequate length may be used only infrequently. In one study, turnouts with low safe-entry speed due to their short length of 200 ft were never used (4). However, if the turnout is too long, it may be used as a passing lane rather than a turnout. Average speeds of 26 to 31 mi/h have been observed within some longer turnouts (2). In that study, turnout lengths of 200 to 250 ft appeared to be suitable for low-speed roads (30 mi/h or less), while lengths of about 400 to 450 ft appeared suitable for high-speed roads (e.g., 50 mi/h). These lengths are generally consistent with those found in the Green Book (1). It is customary to find turnouts in mountainous regions but they should not be located where horizontal or vertical curves limit sight distance. Turnouts can be added to rural roadways to increase passing opportunities where available sight distance is at least 300 m (approximately 985 ft) on the approach to the turnout. In any case, the turnout must be designed with sufficient sight distance for approaching drivers to see and react to vehicles leaving the turnout at slow speeds. Exiting from a turnout is similar to navigating a right-skewed intersection. The guideline “Sight Distance at Right - Skewed Intersections” (page 10-8) can be used to determine the available sight distance for drivers who are leaving the turnout. However, any horizontal curvature in the roadway approaching the turnout should be considered when determining the skew angle. Cross References Sight Distance at Right-Skewed Intersections, 10-8 Key References 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. 2. Rooney, F. (1976). Turnouts, Traffic Operational Report Number 2. Sacramento: California Department of Transportation. 3. FHWA (2009). Manual on Uniform Traffic Control Devices for Streets and Highways. Washington, DC. 4. Harwood, D. & St. John, A. (1985). Passing Lanes and Other Operational Improvements on Two-Lane Highways (FHWA-RD-85-028). McLean, VA: FHWA. 7-3 HFG GRADES (VERTICAL ALIGNMENT) Version 2.0

G EO ME TR IC AN D S IG NI NG C ON SI DE RA TI ON S TO S U PPO RT E FFE CT IV E U SE OF T RU CK E SC AP E R AM PS In tr od uc ti on A Truck Escape Ramp (TER) is a facility designed and constructed to provide a location for out-of-control trucks (though other vehicles can use them as well), to slow and stop away from the ma in traffic stream . Out-of-control vehicles are generally caused by a dr iv er losing the ability to brake, either through overheating of the brakes due to mechanical failure or failure to downshift at the appropriate time. Multiple terms can be used to describe this family of ram ps including truck escape ram ps, em ergency escape ra mp s, safety ra mp s, runaway lanes, arrester beds, or gravity lanes. TERs typically slow out-of-control vehicles by dissi pating their energy through gravitational deceleration, rolling resistance, or both. AASHTO ( 1 ) provides co mp rehensive guidance regarding the design and location of emergency escape ra mp s; this guidance is provided in the “Element of Design” chapter. De si gn Gu id e lin es Consideration should be given to the following geom etric and signage aspects of TER design to prom ote driver behavior that is consistent with safe use of escape ram ps : Topic Guideline Geom etric • A TER should be designed such that the driver of a runaway truck can see the entire ramp (or at least a significant portion of it). • If a service road is developed adjacent to an escape ra mp , the design of the ra mp and service road should be distinct so that drivers of out-of-control vehicles will not mistake the service road for the ramp. Providing sufficient sight distance will help eliminate any possible confusion ( 2 ). • When truck drivers have the option of two escape ram ps on a given downgrade, the lower ramp is typically preferred over the first and higher elevation ramp. • Ensure that the ram p cannot mistakenly lead other mo torists from the mainline ( 3 ). • Operators lose their steering capability upon entering an arrester bed, thus escape ramps should be straight and their angle to the roadway should be as flat as possible ( 2 ). Signing • Advance signing is necessary to inform drivers of the existence of the ra mp , and access to the ramp should be made obvious by exit signing ( 1 ). • “Runaway Vehicles Only” and “No Parking” signs adjacent to escape ramps may be useful in ensuring other vehicles do not block escape ramps by entering or parking in front of the ram p. • Generic me ssage signs located at the roadside typically have less im pact than advisory signs that target specific at-risk vehicles ( 4 ). • Weight-Specific-Speed (WSS) signs should have no more than five weight classes posted; this will limit driver confusion from too much information ( 5 ). • Minimal, standard, or briefing signs can lead drivers to underestimate the severity of the severe grades and overestimate the severity of the benign ones. These estimates can result in potentially dangerous brake over-heating on se vere grades in addition to ov erly cautious and slower driving on non-severe grades ( 6 ). Ba sed Primarily on Ex pert Jud g ment Based Equally on Expert Judgment and Empirical Dat a Based Primarily on Em pirical Da ta HFG GRADES (VERTICAL ALIGNMENT) Version 2.0 7-4

Di scu ssi on As noted on the previous page, AASHTO ( 1 ) provides comprehensive guidance regarding the design and location of em ergency escape ram ps in the “Elem ent of Design” chapter. This guideline merely emphasizes key aspects of the Green Book ( 1 ) guidance and adds guidance from additional sources. De si gn Is su es On existing roadways, a field review, crash experience, and/or docum entation from law enforcem ent agencies can be used to assess the need for a TER. Another tool that can be used to assess the need for TERs on new and existing roadways is the Grade Severity Rating System , w hich is a si mu lation model that establishes a safe descent speed for the grade based upon a predetermined brake te mp erature limit. Where brake temperatures exceed the predetermined limit, the potential for brake loss exists, indicating that a TER may be necessary. The Arizona Department of Transportation ( 7 ) provides the following graph in their Roadway Design Guidelines to use to determ ine if a TER is needed. C ON SI DE RA TI ON OF TER VS . LENGTH AN D PER CE NT AG E OF D OW NGRADE Source: Adapted from Arizona Department of Transportation ( 7 ) Cr os s Re fe re nc es Sight Distance Guidelines, 5-1 Special Considerations for Rural Environments Guidelines, 16-1 Speed Perception, Speed Choice, and Speed Control Guidelines, 17-1 Signing Guidelines, 18-1 Markings Guidelines, 20-1 Ke y Re fe re nc es 1. AASHTO (2011). A Policy on Geometric Design of Highways and Streets . Washington, DC. 2. Witheford, D.K. (1992). NCHRP Synthesis of Highway Practice 178: Truck Escape Ramps . Washington, DC: Transportation Research Board. 3. Ballard, A.J. (1983). Current state of truck escape-ra mp technology. Transportation Research Record, 923 , 35-42. 4. Bush ma n, R., & Lindsay, C. (2002). Im pr oving safety with dynam ic warning system s. In Z.G. Zacharia (Ed.). Proceedings: International Truck and Bus Safety Research and Policy Symposium (pp. 451-459). April 3-5, 2002, Knoxville, TN. 5. Firestine, M., McGee, H., & Cunningham , D. (1989). Reducing Runaway Truck Accidents through Weight-Based Advisory Speeds (FHWA - IP-89-023). McLean, VA: FHWA. 6. Stein, A.C., & Johnson, W.A. (1984). Effective signing to reduce truck downgrade runaways. Proceedings of the 28th Annual Conference of the American Association for Automotive Medicine , 77- 89. 7. Arizona Depar tm ent of Transportation (2007). Roadway Design Guidelines . Retrieved January 13, 2012 fr om http://www.azdot.gov/Inside_ADOT/Misc/PDF/Publications.pdf. 0 1 2 3 4 5 6 7 4 5 6 7 Mi les of Do wngr ad e P e r c e nt D ow ng ra d e Co ns id er Tr uc k Esc ap e Ra mp 7-5 HFG GRADES (VERTICAL ALIGNMENT) Version 2.0

PREVIEW SIGHT DISTANCE AND GRADE PERCEPTION AT VERTICAL CURVES Introduction The preview sight distance (PVSD) is a measure of driver sight distance based on the assumption that “the driver views or previews the roadway surface and other cues that lie ahead to obtain the information needed for vehicular control and guidance” ( 1 ). It i s based on the assumption that a driver requires a minimum PVSD to perceive and respond to upcoming alignment cues. The PVSD applies directly to horizontal curves near the top of crest vertical curves (or at the bottom of sag vertical curves), in which th e horizontal curve is initially out of the driver’s line of sight. The AASHTO Green B ook ( 2 ) recommends avoiding these situations but provi des no specific design values. Design Guidelines Design values of required PVSD are shown in the table below for horizontal curves of different radii on level grades ( for a simple curve and with spiral transitions of different “flatness , ” i.e., spiral parameter A) . S T indicates the PVSD on the road section prior to the curve section (i.e., tangent section immediatel y preceding a simple curve) , and S C indicates PVSD on the curve section (see figure) . Horizontal Curve Radius (m) (1) Required PVSD (m)* * Values rounded to next integer; ** A is the square root of the product of radius and distance from the beginning of a spiral † Minimum value; †† Maximum value Simple Curve Spiraled Curve A** = 100 m A = 200 m A = 300 m ST (2) SC (3) ST (4) SC (5) ST (6) SC (7) ST (8) SC (9) 400 131 50 107 57 66 93 †† 66 119 †† 600 110 62 94 63 66 88 66 119 †† 800 99 70 87 70 † 66 86 66 117 1,000 93 76 83 76 † 66 84 66 109 1,200 88 80 80 80 † 66 83 66 103 1,400 85 83 78 83 † 66 83 † 66 98 1,600 83 83 77 83 † 66 83 † 66 92 1,800 81 83 76 83 † 66 83 † 66 86 2,000 80 81 75 81 † 66 81 † 66 81 † Illustration of PVSD on a crest vertical curve (left). S T is the driver’s reaction distance (PRT) that falls on the roadway section prior to the point of curvature (PC) of the horizontal curve , and S C is the amount of horizontal curvature required to make the curve detectable to the driver . Calculated minimum values of S C are shown on the right. BVC=Beginning of Vertical Curve; EVC=End of Vertical Curve Figure s recreated from Hassan and Easa ( 3 ) . Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG GRADES (VERTICAL ALIGNMENT) Version 2.0 7-6

Discussion The PVSD reflects sight distance needs apart from the typical sight distance requirements, such as stopping sight distance, decision sight distance, passing sight distance, and intersection sight distance. In particular, it addresses the adequate view of the roadway alignment, pavement surface, and other features that provide the vehicle control and guidance cues that drivers need to have a relaxed, comfortable, and safe ride (1). The PVSD can be considered a special case of the decision sight distance (3). In addition to horizontal curves following vertical curves, the PVSD can also apply to horizontal curves that are obscured by the surrounding topography, such as rock cuts, and to road segments directly preceding elevated freeway exit ramps. Conceptually, PVSD is the length of roadway traveled while the driver perceives and reacts to upcoming roadway guidance cues. It has practical importance with regard to horizontal curves that follow vertical curves because tangents usually have higher operating speeds than horizontal curves, and drivers may have to decelerate before entering the horizontal curve. The PVSD helps ensure that drivers have sufficient time to perceive the horizontal curve and to lower their speed by accounting for the sight distance restrictions caused by the vertical curvature. The PVSD has two components. The first is the distance associated with drivers’ perception-reaction time that falls on the roadway prior to the point of curvature of the horizontal curve (ST), and the second is the amount of the curve that must be visible for drivers to detect the horizontal curve (SC). If the tangent section transitions directly before the curve without a spiral, then the PRT distance should be accommodated fully within the tangent section and end before the curve. If the curve is preceded by a spiral transition, then the PRT distance can lie on the spiral and extend to the tangent if necessary. This leads to a trade-off. In particular, sharp curves (i.e., simple curves) require that a shorter segment be visible for the curvature to be detectable (smaller SC); however, more deceleration distance may be necessary to accommodate a larger speed change (longer ST). In contrast, since spiral curves are more gradual, they should require that more of the curve be visible for it to be detectable (longer SC), but the deceleration distance can be included in the spiral length, which leads to a shorter ST distance. Under daylight conditions, the PVSD is calculated as the line of sight from the driver’s eye to a point that intercepts and is tangent to the curvature of the pavement surface. The guideline information is based on analytical modeling of the PVSD (3, 4), and the guideline table shows PVSD calculations for ST and SC for various values of curvature radius and spiral parameter A (a measure of the flatness of the spiral). At low values of A and high radius values (R), the analytical modeling yielded non-applicable results since spiral curves should always be flatter than simple curves and consequently have PVSD values that are at least as long as for a simple curve (i.e., the curvature of a spiral curve should be more difficult to detect; see figure on the previous page). Accordingly, the required SC for spiral curves (bottom entries in columns 5, 7, & 9) are assigned the value of SC for the corresponding simple curves in these cases (i.e., column 3). Speed was modeled assuming a tangent speed of approximately 60 mi/h, with curve operating speed calculated based on the indicated curve radius (R). The deceleration level was assumed to be 0.85 m/s2. Note that the analytical work conducted to develop the guideline table was limited to separate horizontal curves on level grades. Also, the authors caution that the design values for the table are only applicable to the range of horizontal radii investigated in their experiment (i.e., those shown in the table). Design Issues Time of day is an important consideration. In particular, daytime driving should use the line of sight from the driver’s eye to the pavement marking. However, at night, sight distance is limited by headlamp illumination of the pavement markings, which means that the line of sight should be to the reference vehicles’ headlamp height. Cross References Sight Distance Guidelines, 5-1 Curves (Horizontal Alignment), 6-1 Nighttime Driving, 21-4 Key References 1. Gattis, J.L., & Duncan, J. (1995). Geometric design for adequate operational preview of road ahead. Transportation Research Record, 1500, 139-145. 2. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. 3. Hassan, Y., & Easa, S.M. (2000). Modeling of required preview sight distance. Journal of Transportation Engineering, 126(1), 13-20. 4. Hassan, Y., & Easa, S.M. (1998). Design considerations of sight distance red zones on crest curves. Journal of Transportation Engineering, 124(4), 343-352. 7-7 HFG GRADES (VERTICAL ALIGNMENT) Version 2.0

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 600: Human Factors Guidelines for Road Systems: Second Edition provides data and insights of the extent to which road users’ needs, capabilities, and limitations are influenced by the effects of age, visual demands, cognition, and influence of expectancies.

NCHRP Report 600 provides guidance for roadway location elements and traffic engineering elements. The report also provides tutorials on special design topics, an index, and a glossary of technical terms.

The second edition of NCHRP 600 completes and updates the first edition, which was published previously in three collections.

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