Superelevation Criteria for Sharp Horizontal Curves on Steep Grades (2014)

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C-1 A P P E N D I X C This appendix provides potential changes recommended for consideration in the next edi- tions of the Green Book and Manual on Uniform Traffic Control Devices (MUTCD), based on findings and conclusions of this research. The recommendations are based upon a review of the 2011 edition of the Green Book and the 2009 edition of the MUTCD (with Revision Numbers 1 and 2 incorporated, dated May 2012). Recommended text is specified for selected sections of the documents as follows. Text beginning on pg. 3-20 of 2011 Green Book (1) Side Friction Factor The side friction factor represents the vehicle’s need for side friction, also called the side friction demand; it also represents the lateral acceleration af that acts on the vehicle. This acceleration can be computed as the product of the side friction demand factor f and the gravitational constant g (i.e., af =  fg). Note that the lateral acceleration actually experienced by vehicle occupants tends to be slightly larger than predicted by the product fg due to vehicle body roll angle. With the wide variation in vehicle speeds on curves, there usually is an unbalanced force whether the curve is superelevated or not. This force results in tire side thrust, which is counter- balanced by friction between the tires and the pavement surface. This frictional counterforce is developed by distortion of the contact area of the tire. The coefficient of friction f is the friction force divided by the component of the weight per- pendicular to the pavement surface and is expressed as a simplification of the basic curve formula shown as Equation (3-6). The value of the product ef in this formula is always small. As a result, the 1-0.01ef term is nearly equal to 1.0 and is normally omitted in highway design. Omission of this term yields the following basic side friction equation: U.S. CustomaryMetric R Vf 0.01e 127 2 −= R Vf 0.01e 15 2 −= (3-7) This equation is referred to as the simplified curve formula and yields slightly larger (and, thus, more conservative) estimates of friction demand than would be obtained using the basic curve formula. The coefficient f has been called lateral ratio, cornering ratio, unbalanced centrifugal ratio, friction factor, and side friction factor. Because of its widespread use, the term “side friction fac- tor” is used in this discussion. The upper limit of the side friction factor is the point at which the tire would begin to skid; this is known as the point of impending skid. Because highway curves Potential Changes Recommended for Consideration in the Next Editions of the Green Book and MUTCD

C-2 are designed so vehicles can avoid skidding with a margin of safety, the f values used in design should be substantially less than the coefficient of friction at impending skid. The side friction factor at impending skid depends on a number of other factors, among which the most important are the speed of the vehicle, the type and condition of the roadway surface, and the type and condition of the vehicle tires. Different observers have recorded dif- ferent maximum side friction factors at the same speeds for pavements of similar composition, and logically so, because of the inherent variability in pavement texture, weather conditions, and tire condition. In general, research studies from the 1930s and 1940s show that the maximum side friction factors developed between new tires and wet concrete pavements range from about 0.5 at 30 km/h [20 mph] to approximately 0.35 at 100 km/h [60 mph]. For normal wet concrete pavements and smooth tires the maximum side friction factor at impending skid is about 0.35 at 70 km/h [45 mph]. More recent field measurements indicate that average peak side friction factors (representing the point of impending skid) range from about 0.50 at 140 km/h [85 mph] to 0.60 at 40 km/h [25 mph] for passenger cars on well-maintained roadways with high-type pavements and wet surfaces. For trucks, on well-maintained roadways with high-type pavements and wet surfaces, average peak side friction factors range from 0.50 at 140 km/h [85 mph] to 0.55 at 40 km/h [25 mph] (69). In all cases, the studies show a decrease in friction values as speeds increase (46, 47, 60, 69). Horizontal curves should not be designed directly on the basis of the maximum available side friction factor. Rather, the maximum side friction factor used in design should be that portion of the maximum available side friction that can be used with comfort, and without likelihood of skidding, by the vast majority of drivers. Side friction levels that represent pavements that are glazed, bleeding, or otherwise lacking in reasonable skid-resistant properties should not control design because such conditions are avoidable and geometric design should be based on accept- able surface conditions attainable at reasonable cost. A key consideration in selecting maximum side friction factors for use in design is the level of acceleration that is sufficient to cause drivers to experience a feeling of discomfort and to react instinctively to avoid higher speed. The speed on a curve at which discomfort due to the lateral acceleration is evident to drivers is used as a design control for the maximum side friction factor on high-speed streets and highways. At low speeds, drivers are more tolerant of discomfort, thus permitting employment of an increased amount of side friction for use in design of horizontal curves. The ball-bank indicator has been widely used by research groups, local agencies, and highway departments as a uniform measure of lateral acceleration to set speeds on curves that avoid driver discomfort. It consists of a steel ball in a sealed glass tube; except for the damping effect of the liquid in the tube, the ball is free to roll. Its simplicity of construction and operation has led to widespread acceptance as a guide for determination of appropriate curve speeds. With such a device mounted in a vehicle in motion, the ball-bank reading at any time is indicative of the com- bined effect of body roll, lateral acceleration angle, and superelevation as shown in Figure 3-3. The lateral acceleration developed as a vehicle travels at uniform speed on a curve causes the ball to roll out to a fixed angle position as shown in Figure 3-3. A correction should be made for that portion of the force taken up in the small body-roll angle. The indicated side force perceived by the vehicle occupants is thus on the order of F ≈ tan (a - r). In a series of definitive tests (47), it was concluded that speeds on curves that avoid driver discomfort are indicated by ball-bank readings of 14 degrees for speeds of 30 km/h [20 mph] or less, 12 degrees for speeds of 40 and 50 km/h [25 and 30 mph], and 10 degrees for speeds of 55 through 80 km/h [35 through 50 mph]. These ball-bank readings are indicative of side friction factors of 0.21, 0.18, and 0.15, respectively, for the test body roll angles and provide ample margin of safety against skidding or vehicle rollover. From other tests (11), a maximum side friction factor of 0.16 for speeds up to 100 km/h [60 mph] was recommended. For higher speeds, the incremental reduction of this factor was recommended. Speed studies on the Pennsylvania Turnpike (60) led to a conclusion that the

C-3 side friction factor should not exceed 0.10 for design speeds of 110 km/h [70 mph] and higher. A recent study (13) re-examined previously published findings and analyzed new data collected at numerous horizontal curves. The side friction demand factors developed in that study are generally consistent with the side friction factors reported above. An electronic accelerometer provides an alternative to the ball-bank indicator for use in deter- mining advisory speeds for horizontal curves and ramps. An accelerometer is a gravity-sensitive electronic device that can measure the lateral forces and accelerations that drivers experience while traversing a highway curve (20). It should be recognized that other factors influence driver speed choice under conditions of high friction demand. Swerving becomes perceptible, drift angle increases, and increased steer- ing effort is needed to avoid involuntary lane line violations. Under these conditions, the cone of vision narrows and is accompanied by an increasing sense of concentration and intensity considered undesirable by most drivers. These factors are more apparent to a driver under open- road conditions. Where practical, the maximum side friction factors used in design should be conservative for dry pavements and should provide an ample margin of safety against skidding on pave- ments that are wet as well as ice or snow covered and against vehicle rollover. The need to pro- vide skid-resistant pavement surfacing for these conditions cannot be overemphasized because superimposed on the frictional demands resulting from roadway geometry are those that result from driving maneuvers such as braking, sudden lane changes, and minor changes in direction within a lane. In these short-term maneuvers, high friction demand can exist but the discomfort threshold may not be perceived in time for the driver to take corrective action. Figure 3-4 summarizes the findings of the cited tests relating to side friction factors recom- mended for curve design. Although some variation in the test results is noted, all are in agree- ment that the side friction factor should be lower for high-speed design than for low-speed design. A recent study Recent studies (13, 69) have reaffirmed the appropriateness of these side friction factors. To illustrate the difference between side friction factors for design and available side friction supply during cornering, Figure 3-4 also includes friction supply curves for passen- ger vehicle and truck tires for the skidding condition on wet pavement during cornering. [Comment: Recommend adding supply friction curves from Figures 30 and 32 in the main body of the report, which are representative of more current research, to Green Book Figure 3-4. The data points for the curves to be added to Figure 3-4 are as follows, and the curves should be labeled accordingly. Figure 3-3. Geometry for ball-bank indicator.

C-4 Speed (km/h) 40 48 56 64 72 80 89 97 105 113 121 129 137 Speed (mph) 25 30 35 40 45 50 55 60 65 70 75 80 85 Passenger vehicle re measurements of skidding wet-re fricon in lateral (cornering) direcon 0.59 0.58 0.57 0.56 0.55 0.54 0.53 0.52 0.51 0.50 0.49 0.49 0.48 Truck re measurements of skidding wet-re fricon in lateral (cornering) direcon 0.52 0.49 0.45 0.42 0.40 0.38 0.36 0.34 0.32 0.31 0.30 0.29 0.28 Note, the scale of Figure 3-4 will need to be adjusted to include the curves.] Figure 3-4. Side friction factors for high-speed streets and highways.

C-5 The maximum allowable side friction factors for low-speed streets and highways are shown in Figure 3-5. For travel on sharper curves, superelevation is needed. The curves are based on several studies (14, 16, 23) conducted to determine the side friction factor for low-speed inter- section curves. A 95th percentile curve speed was used since it closely represents the 85th percentile tangent speed and provides a reasonable margin of safety against skidding (13). These curves also approximated the assumed values for low-speed urban design based on driver comfort. Figure 3-5 also includes friction supply curves for passenger vehicle and truck tires for the skidding condition on wet pavement during cornering (69). Comparisons of the side friction supply and the side friction factor The curves provide an sense of the appropriate margin of safety against skidding based upon the given side friction factors for designand a cost-effective limitation on superelevation. [Comment: Recommend adding supply friction curves from Figures 30 and 32 in the main body of the report, which are representative of more current research, to Green Book Figure 3-5. The data points for the curves to be added to Figure 3-5 are as follows, and the curves should be labeled accordingly. Figure 3-5. Side friction factors for low-speed streets and highways.

C-6 Passenger vehicle re measurements of skidding wet-re fricon in lateral Truck re measurements of skidding wet-re fricon in lateral (cornering) Speed (km/h) Speed (mph) (cornering) direcon direcon 40 48 25 30 0.59 0.58 0.52 0.49 56 64 35 40 0.57 0.56 0 0.45 0.42 0 72 80 89 45 50 55 .55 0.54 0.53 .40 0.38 0.36 97 105 1 60 65 7 0.52 0.51 0 0.34 0.32 0 13 121 129 0 75 80 .50 0.49 0.49 .31 0.30 0.29 137 85 0.48 0.28 Note, the scale of Figure 3-5 will need to be adjusted to include the curves.] The side friction factors vary with the design speed from 0.40 at 15 km/h [0.38 at 10 mph] to about 0.15 at 70 km/h [45 mph], with 70 km/h [45 mph] being the upper limit for low speed established in the design speed discussion in Section 2.3.6. Figure 3-6 should be referred to for the values of the side friction factor recommended for use in horizontal curve design. Figure 3-6. Side friction factors assumed for design.

C-7 Text beginning on pg. 3-33 of 2011 Green Book (2) Effects of Grades On long or fairly steep grades, drivers tend to travel faster in the downgrade than in the upgrade direction. Additionally, research (13, 69) has shown that the side friction demand is greater on both downgrades (due to braking forces) and steep upgrades (due to the tractive forces). Research (69) has also shown that, for simple horizontal curves, the maximum super- elevation rate on steep downgrades of 4 percent or more should not exceed 12 percent. If considering a maximum superelevation rate on a horizontal curve in excess of 12 percent, a spiral curve transition is recommended to increase the margins of safety against skidding or rollover between the approach tangent and horizontal curve. Sharp horizontal curves (or near minimum-radius curves) on downgrades of 4 percent or more should not be designed using low design speeds (i.e., 50 km/h [30 mph] or less). In the event that such situations cannot be avoided, warning signs to reduce speeds well in advance of the start of the horizontal curve should be used. On upgrades of 4 percent or more, the maximum superelevation rate should be limited to 9 percent for minimum-radius curves with design speeds of 90 km/h [55 mph] and higher, to minimize the potential for wheel-lift events on tractor semi-trailer trucks. Alternatively, if it can be verified that the available sight distance is such that deceleration at the rate assumed in stop- ping sight distance design criteria, 3.4 m/s2 [11.2 ft/s2], is unlikely to be required on upgrades of 4 percent or more, emax values up to 12 percent may be used for minimum-radius curves. Vehicle dynamics simulations have shown (69) that sharp horizontal curves with near or min- imum radii for given design speeds on downgrades of 4 percent or more could lead to skidding or rollover for a range of vehicle types if a driver is simultaneously braking and changing lanes on the curve. For this reason, it may be desirable to provide a “STAY IN LANE” sign (R4-9) in advance of sharp horizontal curves on steep grades on multilane highways (22). Consideration may also be given to using single solid white lane line markings to supplement the “STAY IN LANE” sign and discourage motorists from changing lanes. Some adjustment in superelevation rates should be considered for grades steeper than 5 per- cent. This adjustment is particularly important on facilities with high truck volumes and on low-speed facilities with intermediate curves using high levels of side friction demand. In the case of a divided highway with each roadway independently superelevated, or on a one-way ramp, such an adjustment can be readily made. In the simplest practical form, val- ues from Exhibits 3-21 to 3-25, presented in Section 3.3.5, can be used directly by assuming a slightly higher design speed for the downgrade. Since vehicles tend to slow on steep upgrades, the superelevation adjustment can be made without reducing the design speed for the upgrade. The appropriate variation in speed depends on the particular conditions, especially the rate and length of grade and the magnitude of the curve radius compared to other curves on the approach highway section. On two-lane and multilane undivided roadways, the adjustment for grade can be made by assuming a slightly higher design speed for the downgrade and applying it to the whole traveled way (both upgrade and downgrade sides). The added superelevation for the upgrade can help counter the loss of available side friction due to tractive forces. On long upgrades, the additional superelevation may cause negative side friction for slow moving vehicles (such as large trucks). This effect is mitigated by the slow speed of the vehicle, allowing time to counter steer, and the increased experience and training for truck drivers. Text beginning on pg. 3-66 of 2011 Green Book Location with respect to end of curve. In the tangent-to-curve design, the location of the superelevation runoff length with respect to the Point of Curvature (PC) needs to be determined. Normal practice is to divide the runoff length between the tangent and curved sections and to avoid placing the entire runoff length on either the tangent or the curve. With full superelevation

C-8 attained at the PC, the runoff lies entirely on the approach tangent, where theoretically no super- elevation is needed. At the other extreme, placement of the runoff entirely on the circular curve results in the initial portion of the curve having less than the desired amount of superelevation. Both of these extremes tend to be associated with a large peak lateral acceleration. Experience indicates that locating a portion of the runoff on the tangent, in advance of the PC, is preferable, since this tends to minimize the peak lateral acceleration and the resulting side fric- tion demand. The magnitude of side friction demand incurred during travel through the runoff can vary with the actual vehicle travel path. Observations indicate that a spiral path results from a driver’s natural steering behavior during curve entry or exit. This natural spiral usually begins on the tangent and ends beyond the beginning of the circular curve. Most evidence indicates that the length of this natural spiral ranges from 2- to 4-s travel time; however, its length may also be affected by lane width and the presence of other vehicles. Based on the preceding discussion, locating a portion of the runoff on the tangent is consistent with the natural spiral path adopted by the driver during curve entry. In this manner, the gradual introduction of superelevation prior to the curve compensates for the gradual increase in lateral acceleration associated with the spiral path. As a result, the peak lateral acceleration incurred at the PC should theoretically be about equal to 50 percent of the lateral acceleration associated with the circular curve. To achieve this balance in lateral acceleration, most agencies locate a portion of the runoff length on the tangent prior to the curve. The proportion of runoff length placed on the tangent varies from 0.6 to 0.8 (i.e., 60 to 80 percent) with a large majority of agencies using 0.67 (i.e., 67 percent). Most agencies consistently use a single value of this proportion for all street and highway curves. Theoretical considerations confirm the desirability of placing a larger portion of the runoff length on the approach tangent rather than on the circular curve. Such considerations are based on analysis of the acceleration acting laterally on the vehicle while it travels through the transition section. This lateral acceleration can induce a lateral velocity and lane shift that could lead to operational problems. Specifically, a lateral velocity in an outward direction (relative to the curve) results in a driver making a corrective steering maneuver that produces a path radius sharper than that of the roadway curve. Such a critical radius produces an undesir- able increase in peak side friction demand. Moreover, a lateral velocity of sufficient magnitude to shift the vehicle into an adjacent lane (without corrective steering) is also undesirable for safety reasons. Analysis of the aforementioned theoretical considerations has led to the conclusion that an appropriate allocation of runoff length between the tangent and the curve can minimize the aforementioned operational problems (12). The values obtained from the analysis are listed in Table 3-18. If used in design, the values listed in Table 3-18 should minimize lateral accelera- tion and the vehicle’s lateral motion. Values smaller than those listed tend to be associated with larger outward lateral velocities. Values larger than those listed tend to be associated with larger lateral shifts. Theoretical considerations indicate that values for the proportion of runoff length on the tangent in the range of 0.7 to 0.9 (i.e., 70 to 90 percent) offer the best operating conditions yramotsuC SU cirteM Design speed (km/h) Portion of runoff located prior to the curve Design speed (mph) Portion of runoff located prior to the curve detator senal fo .oN detator senal fo .oN 1.0 1.5 2.0−2.5 3.0−3.5 1.0 1.5 2.0−2.5 3.0−3.5 20−70 0.80 0.85 0.90 0.90 15−45 0.80 0.85 0.90 0.90 80−130 0.70 0.75 0.80 0.85 50−80 0.70 0.75 0.80 0.85 Table 3-18. Runoff locations that minimize the vehicle’s lateral motion.

C-9 on roadways with downgrades of less than 4 percent; the specific value in this range should be dependent on design speed and rotated width. Experience obtained from existing practice indi- cates that deviation from the values in Table 3-18 by 10 percent should not lead to measurable operational problems. In this regard, use of a single value for the proportion of runoff length on the tangent in the range of 0.6 to 0.9 (60 to 90 percent) for all speeds and rotated widths is con- sidered acceptable. However, refinement of this value, based on the trends shown in Table 3-18 is desirable when conditions allow. Research that considered minimum-radius horizontal curves on downgrades of 4 percent or more indicates that application of the proportion of runoff length values shown in Table 3-18 are acceptable to the design of curves using the maximum rate of superelevation and minimum curve radius for design speeds of 40 km/h [25 mph] or more (69). However, when designing above-mini- mum-radius curves for specific design speeds, and applying the runoff location proportions shown in Table 3-18, the curve-radius/design-superelevation-rate combinations shown in Tables 3-8 through 3-12 may produce margins of safety against skidding or rollover that are lower on the approach tangent than within the limits of the simple horizontal curve. This is undesirable and should be checked using the following condition: where: e ptangent V g R e 1100 + < = superelevation at PC of horizontal curve = proportion of the maximum superelevation attained at the PC of horizontal curve = design speed, km/h = gravitational constant, 9.81 m/s = radius of horizontal curve, m Metric g V ptangent 077.0 × R 2 2 where: U. e 100 < e = superelevation at PC of horizontal cur ptangent = proportion of the maximum PC of horizontal curve V = design speed, mph g = gravitational constant, 32.2 ft/s R = radius of horizzontal curve, ft S. Customary p genttan1 15.2 × + superelevation attained at the gR V 2 2 If the condition presented above is met, engineers can proceed with the superelevation transi- tion as designed using the guidance included in this section. If the condition presented above is not met, designers should reduce the proportion of the maximum superelevation attained at the PC of the horizontal curve, or introduce a spiral transition curve between the approach tangent and simple horizontal curve. Based on theoretical considerations, the condition above is satisfied for maximum-superelevation/minimum-radius curves for all design speeds when applying the propor- tion of superelevation runoff values in Table 3-18. However, the condition above may be violated when using design superelevation rates that are approximately 50 percent or less than the maximum superelevation rate for a given design speed-minimum radius combination. In these cases, locating 70 percent of the superelevation runoff prior to the horizontal curve will increase the margins of safety on the approach tangent relative to the simple horizontal curve. Limiting superelevation rates. Theoretical considerations indicate that, when a vehicle is traveling through a tangent-to-curve transition, large superelevation rates are associated with large shifts in the vehicle’s lateral position. In general, such shifts in lateral position can be mini- mized by the proper location of the superelevation runoff section, as described above. However, large lateral shifts must be compensated by the driver through steering action. In recognition of the potential adverse effect that large shifts in lateral position may have on vehicle control, the threshold superelevation rates associated with a lateral shift of 1.0 m [3.0 ft] are identified in Table 3-19. These limiting superelevation rates do not apply for speeds of 80 km/h [50 mph] or more when combined with superelevation rates of 12 percent or less. Designs that incorporate superelevation in excess of the limiting rates may be associated with excessive lateral shift. Therefore, it is recommended that such superelevation rates be avoided. However, if they are used, consideration should be given to increasing the width of the traveled way along the curve to reduce the potential for vehicle encroachment into the adjacent lane.

C-10 On upgrades of 4 percent or more, the maximum superelevation rate should be limited to 9 percent for minimum-radius curves with design speeds of 90 km/h [55 mph] and higher, to minimize the potential for wheel-lift events on tractor semi-trailer trucks. Alternatively, if it can be verified that the available sight distance is such that deceleration at the rate assumed in stop- ping sight distance design criteria, 3.4 m/s2 [11.2 ft/s2], is unlikely to be required on upgrades of 4 percent or more, emax values up to 12 percent may be used for minimum-radius curves. New Reference for Green Book pg. 3-184 69. Torbic, D. T., M. O’Laughlin, D. W. Harwood, K. Bauer, C. Bokenkroger, L. Lucas, J. Ronchetto, S. N. Brennan, E. T. Donnell, A. Brown, and T. Varunjikar. Superelevation Criteria for Sharp Horizontal Curves on Steep Grades. Final Report for NCHRP Project 15-39, MRIGlobal, 2013. Text on pg. 74 of 2009 MUTCD Section 2B.33 STAY IN LANE Sign (R4-9) Option: A STAY IN LANE (R4-9) sign (see Figure 2B-10) may be used on multi-lane highways to direct road users to stay in their lane until conditions permit shifting to another lane. Guidance: If a STAY IN LANE sign is used, it should be accompanied by a double solid white lane line(s) to prohibit lane changing. Where the STAY IN LANE sign is intended to discourage lane changing on sharp horizontal curves on steep downgrades on multi-lane highways, consideration may be given to using a single solid white lane line marking to supplement the R4-9 sign. Text on pg. 362 of 2009 MUTCD Where crossing the lane line markings is discouraged, the lane line markings shall consist of a normal or wide solid white line. Option: Where it is intended to discourage lane changing on the approach to an exit ramp, a wide solid white lane line may extend upstream from the theoretical gore or, for multi-lane exits, as shown in Drawing B of Figure 3B-10, for a distance that is determined by engineering judgment. Where lane changes might cause conflicts, a wide or normal solid white lane line may extend upstream from an intersection. In the case of a lane drop at an exit ramp or intersection, such a solid white line may replace a portion, but not all of the length of the wide dotted white lane line. Where a solid white lane line marking is intended to discourage lane changing by motorists on sharp horizontal curves on steep downgrades on multi-lane highways, a single solid white lane line may extend upstream, on, and downstream of the horizontal curve for a distance that is determined by engineering judgment. Metric U.S. Customary Design speed (km/h) Limiting superelevation rate (%) Design speed (mph) Limiting superelevation rate (%) 20 8 15 8 30 8 20 8 40 10 25 10 50 11 30 11 60 11 35 11 70 12 40 11 45 12 Table 3-19. Limiting superelevation rates.

Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S.DOT United States Department of Transportation