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

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Passing Lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16-2 Countermeasures for Pavement/Shoulder Drop-offs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16-4 Rumble Strips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16-6 Design Consistency in Rural Driving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16-8 16-1 C H A P T E R 16 Special Considerations for Rural Environments

PASSING LANES Introduction A passing lane is a lane added in one or both directions of travel on a two-lane, two-way highway to improve passing opportunities. This definition includes passing lanes in level or rolling terrain, climbing lanes on grades, and short four- lane sections (1). Passing lanes have been used mostly to allow drivers to bypass vehicles that are unable to maintain normal highway speeds on grades, usually called climbing lanes. Potts and Harwood (2) found that the primary benefit of passing lanes is the improvement of overall traffic operations on two-lane highways. This improved operation has direct implications for driver behavior because a driver stuck behind a slow-moving vehicle may be more likely to experience time delays and frustration, which could lead drivers to increase speeds to unsafe levels to pass a slow-moving vehicle. Design Guidelines RECOMMENDED VALUES OF LENGTH AND SPACING BY AVERAGE DAILY TRAFFIC (ADT) AND TERRAIN (6) ADT (vpd) Level Terrain Rolling Terrain Recommended Passing Lane Length (mi) Recommended Distance between Passing Lanes (mi) 1950 1650 0.8-1.1 9.0-11.0 2800 2350 0.8-1.1 4.0-5.0 3150 2650 1.2-1.5 3.8-4.5 3550 3000 1.5-2.0 3.5-4.0 TYPICAL PASSING LANE SIGNAGE AND MARKINGS (3 ) Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG RURAL ENVIRONMENTS Version 2.0 16-2

Discussion Two-lane highways with passing lanes provide a definite improvement in level of service over those without passing lanes (2). In particular, at medium and high volumes, a roadway with continuous alternating passing lanes can provide improvement by two levels of service over conventional two-lane highway without passing lanes. Similarly, a two-lane highway with less frequent passing lanes typically provides an improvement of one level of service over a conventional two-lane highway (2). Comparable improvements in service provided by passing lanes were found to reduce driver frustration and improve overall quality of service and the benefits of the passing lane extend beyond the confines of the added lane itself (4). Harwood, Hoban, and Warren (3) found that passing lanes improve the percentage of time drivers spend following other cars on those roads by 10% to 31% in comparison to a conventional two-lane highway without passing lanes. Passing lanes are generally well received by drivers; one study conducted in Kansas found that 93% of all respondents were positive about passing lanes and indicated a higher acceptance and satisfaction of the concept. Also, 46% of these drivers thought the passing lanes were just right in length, while 53% thought the passing lanes were too short (5). Mutabazi, Russell, and Stokes (5) also found that 88% of drivers agree more passing lanes are needed. Highway engineers typically provide passing lanes with a primary objective of dispersing platoons and hence reducing travel time, with safety as a secondary objective. However, drivers view safety as the main benefit accrued from passing lanes (5). In the Kansas survey, 93% of drivers thought passing lanes improve safety, while 8% believed it encourages speeding. These driver perceptions are consistent with crash data analyses, which indicate that the installation of a passing lane on a two-lane highway reduces crash rates by approximately 25% (3). Harwood et al. (3) also found that crash frequency per mile per year within passing lanes sections on two-lane highways is 12% to 24% lower than for conventional two-lane highway sections. Signage: Warning signs should be used to give drivers a preview of an upcoming passing lane and to warn drivers that the passing lane is ending. The safety and convenience benefits of passing lanes are reduced if passing lanes are not adequately signed. Clearly defined and well-maintained lane markings provide a similar function that can reduce the likelihood of drivers’ selecting an oncoming lane in an attempt to enter or remain in a passing lane. In a survey of passing-lane signs, Wooldridge et al. (6) found that 61% of motorists prefer the wording “Left Lane for Passing Only” versus 29% who prefer “Keep Right Except to Pass.” When surveyors reviewed the sign “Passing Lane Ahead 2 Miles,” 61% of motorists would wait 2 mi to pass while the rest would pass when ready. This advance signing is useful because it also informs the driver of the repetitive nature of the passing lane design, allowing the driver to understand the purpose and nature of the roadway’s characteristics. The sign should be used if the distance to the next passing lane is less than 12 mi. The sign “Right Lane End” is recommended to be located at a distance that will provide adequate notice that the passing lane is terminating. Design Issues Length: The effective length of the passing lane is defined as the physical length of the passing lane plus the distance downstream to the point where traffic conditions return to a level similar to that immediately upstream of the passing lane (5). Through computer stimulation, Harwood et al. (3) found the effective length to range between 4.8 km (3 mi) and 12.8 km (8 mi), depending on the physical length of the passing lane, traffic flow, traffic composition, and downstream passing opportunities. Width and lane drop: Rinde (7) found the minimum width considered adequate for a two-lane road with a passing lane to be 40 ft. In the opinion of Rinde (7), passing should not be allowed for vehicles traveling in the single lane of three-lane roadways at traffic volumes above 3000 AADT. Also, the use of an appropriate lane-addition transition on the upstream end of a passing lane is needed for effective passing lane operations (2). The recommended length of this transition area is half to two-thirds of the length of the lane-drop taper (2). Cross References Signing Guidelines, 18-1 Marking Guidelines, 20-1 Key References 1. Mutabazi, M.I., Russell, E.R., and Stokes, R.W. (1999). Location and configuration of passing lanes. Transportation Research Record, 1658, 25-33. 2. Potts, I.B., and Harwood, D.W. (2004). Benefits and Design/Location Criteria for Passing Lane. Jefferson: Missouri Department of Transportation. 3. Harwood, D.W., Hoban, C.J., and Warren, D.L. (1988). Effective use of passing lanes on two-lane highways. Transportation Research Record, 1195, 79-91. 4. Hoban, C.J. and Morrall, J.F. (1986). Overtaking Lane Practice in Canada and Australia (ARR 144). Victoria: Australia Road Research Board. 5. Mutabazi, M.I., Russell, E.R., and Stokes, R.W. (1998). Drivers’ attitudes, understanding and acceptance of passing lanes in Kansas. Transportation Research Record, 1628, 25-33. 6. Wooldridge, M.D., Messer, C.J., Heard, B.D., Raghupathy, S., Parham, A.H., Brewer, M.A., and Lee, S. (2001). Design Guidelines for Passing Lanes on Two-Lane Roadways (Super 2) (FHWA/TX-02/4064-1, TTI: 0-4064). College Station: Texas Transportation Institute. 7. Rinde, E.A. (1977). Accident Rates vs. Shoulder Width: Two-Lane Roads, Two-Lane Roads with Passing Lanes (CA-DOT-TR-3147-1-77-01). Sacramento: California Department of Transportation. 16-3 HFG RURAL ENVIRONMENTS Version 2.0

COUNTERMEASURES FOR PAVEMENT/SHOULDER DROP-OFFS Introduction A shoulder is a portion of the roadway contiguous with the traveled way for accommodation of stopped vehicles, for emergency use, and for lateral support of the sub-base, base, and surface courses. The roadway shoulder has been recognized as desirable ever since engineers began paving roadways. However, the width, uniformity, and stability of roadway shoulders have varied greatly from roadway to roadway and along different sections of the same roadway (1). Shoulders on rural roadways serve as structural support for the surfacing and additional width for the traveled way. Shoulder drop-offs occur when there is a difference in height (ranging from a fraction of an inch to several inches) between the pavement surface and the roadside surface (2). This height difference typically arises from tire rutting erosion, excessive wear, or resurfacing. The primary concern related to drop-offs is that if they are too high, then it can pose a crash risk if a vehicle drifts outside the road and has a wheel go over the drop-off. Design Guidelines Vertical or near-vertical shoulder drop-off heights that exceed the indicated table values warrant consideration for drop-off treatment or traffic control (in work zones; adapted from Graham & Glennon (3)). VERTICAL DROP-OFF HEIGHT WARRANTING TRAFFIC CONTROL FOR VARIOUS LANE WIDTHS Drop-off Height Speed (mi/h) 12-ft Lane Width 11-ft Lane Width 10-ft Lane Width 9-ft Lane Width 30 3 in. 3 in. 3 in. 2 in. 35 3 in. 3 in. 2 in. 1 in. 40 3 in. 2 in. 1 in. 1 in. 45 2 in. 1 in. 1 in. 1 in. 50 1 in. 1 in. 1 in. 1 in. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data EXAMPLE OF SAFETY EDGE RECOMMENDED BY THE FHWA (4 ) HFG RURAL ENVIRONMENTS Version 2.0 16-4

Discussion A primary safety concern related to drop-off from the perspective of driver performance occurs when a vehicle leaves the lane and has a tire go over the drop-off (typically the front right tire). This event is often surprising and unfamiliar for most drivers, and a drop-off can interfere with their ability to return the vehicle safely to the lane. In particular, if drivers try to return to the lane at high speed and at low steering angles, their tire can “scrub” against the drop-off edge, impeding their return. A common response is to increase the steering angle towards the lane, which can lead to an abrupt change in heading once the drop-off is overcome. In severe cases, this response can result in a vehicle swerving out of the lane into the opposite-direction travel lane, putting the driver at risk of a collision with an oncoming vehicle. Motorcyclists can also have difficulties traversing drop-offs, although the vehicle control issues are somewhat different. Crash data analyses suggest that the overall frequency of crashes “probably” or “possibly” related to high drop-off is relatively low (less than 3% of rural road crashes for related road types), but that these crashes tend to result in a greater proportion of fatalities or injuries than typical rural road crashes (4). The guideline information is primarily based on an analysis of drop-offs for work zones (3). The original source table also contained drop-off height thresholds that were higher than 3 in., but these were changed in the current guideline to reflect a more conservative assessment of other related driver performance data on driver encounters with drop-offs of various heights (4). Note that the recommendation only represents general guidance related to driver performance; other sources—such as the Roadside Design Guide (5)—recommend that vertical drop-offs with differentials of 2 in. or more should be avoided. What the guideline table is intended to convey is that vertical drop- off heights that exceed the listed values are more likely to be associated with increased difficulty for drivers trying to recover in a controlled manner if one of their tires go over the drop-off edge. Another design aspect related to drop-offs that affects driver performance is the shape of drop-off. In particular, safe return to the lane is significantly more successful if a tire had to overcome a drop-off with a slope of 45° or shallower. The figure accompanying the guideline illustrates the relative “safety” of three drop-off geometries. Lane recovery with a sloped or filleted drop-off is significantly better than a straight vertical or curved drop-off. Moreover, the effectiveness of sloped drop-offs persists at higher speeds and at higher drop-off heights (4). Design Issues There are several accepted approaches for addressing drop-offs that are too high. For example, in work zones MUTCD warning signs for edge drop-off can notify users of present drop-off conditions. The application of a wedge-shaped asphalt material called “Safety Edge” is another possible countermeasure (see figure on previous page). When placed between the roadway and the shoulder, the material can help drivers recover from the shoulder to the driving surface. The asphalt material needs to be compacted to increase strength, otherwise the material will break apart over time due to forces and runoff water. Graham, Richard, and Harwood (6) found the results of empirical Bayes and cross-sectional analysis of sites paved with and without Safety Edge reveal that the material has a net positive effect on the safety of rural highways. Humphreys and Parham (1) found that the shoulder is best resurfaced when the roadway is resurfaced so that shoulder drop-off does not form. They also recommend that the contractor, in areas where road-resurfacing contracts must be bid separately, should be required to provide a 45° angle fillet along the edge of the roadway as part of the scope of work. Cross References Design Consistency in Rural Driving, 16-8 Key References 1. Humphreys, J.B., and Parham, J.A. (1994). The Elimination or Mitigation of Hazards Associated with Pavement Edge Drop-offs During Roadway Resurfacing. Washington, DC.: AAA Foundation for Traffic Safety. 2. Fitzpatrick, K., Parham, A.H., and Brewer, M.A. (2002). Treatments for Crashes on Rural Two-Lane Highways in Texas (FHWA/TX- 02/4048-2). College Station: Texas Transportation Institute. 3. Graham, J.L., and Glennon, J.C. (1984). Work Zone Design Considerations for Truck Operations and Pavement/Shoulder Drop-offs. Washington, DC: FHWA. 4. Hallmark, S.L., Veneziano, D., McDonald, T., Graham, J., Bauer, K.M., Patel, R., and Council, F.M. (2006). Safety Impacts of Pavement Edge Drop-Offs. Washington, D.C.: AAA Foundation for Traffic Safety. 5. AASHTO (2002). Roadside Design Guide. Washington, DC. 6. Graham, J.L., Richard, K.R., and Harwood, D.W. (2009). Safety Evaluation of Safety Edge Treatment—Year 2, Interim Report. Kansas City, MO: Midwest Research Institute. 16-5 HFG RURAL ENVIRONMENTS Version 2.0

R UMBLE S TRIPS Introduction Shoulder rumble strips (SRS) are raised or grooved patterns on the shoulder of a travel lane intended to provide a tactile/haptic and auditory alert to drivers who stray onto the shoulder. When a vehicle’s wheels traverse an SRS, they generate both an increase in sound and haptic (physical) vibrations that drivers feel through their seat, foot pedals, fl oor, and steering wheel . SRSs are best suited for warning inattentive or drowsy drivers who ar e leaving the travelled way. SRSs can potentiall y wa ke drivers who fall asleep; however, this result typically requires a greater level of sound and vibration. In general, SRSs must produce sound and vibration levels that are easily detectable, yet not so loud and jarring that they startle drivers. The design chall enge is balancing the need to provide alerts in a variety of situations (e.g., in heavy trucks or to sleeping drivers) with the need to avoid potentially undesirable startling effects and di fficulties that SRSs can cause bicyclists. Previous safety evaluations of SRSs confirm their overall effectiveness. For example, Griffith ( 1 ) indicates there is a medium- high level of predictive certainty that SRSs reduce all single-vehicle run-off-road (SVROR) crashes by 21% on rural freeways and by 18% on all freeways (i.e., both rural and urban). NCHRP research (2) also indicates that continuous SRSs reduce injury SVROR crashes by 7% on rural freeways and by 13% on all freeways. Rumble strips have been shown to significantly reduce the run-off-road crash rate on some rural highways by up to 80% (3). Design Guidelines C OMMON R OADWAY S OUNDS AND A SSOCIATED D B L EVELS dB Sound dB Sound 60 Freeway driving from inside car 85 Heavy traffic 70 Freeway traffic 90 Truck 75-80 Inside heavy truck cab 95-100 Motorcycle SRS should produce an audible sound betw een 6 and 15 dB louder than background noi se levels. 85 City traffic inside car 110 Car horn E FFECTS OF D IFFERENT SRS D IMENSIONS ON A UDITORY / T ACTILE A LERTS Characteristic Suitable Values Direct Effect on Driver Implications for Effectiveness Lateral placement / offset 6+ in. from lane edge (but depends on other factors) Drivers encounter the alert sooner, the closer it is to the lane edge. The sooner the warning occurs, the more space drivers have to recover before reaching the road edge. Groove Width 16 in. (12 may be acceptable if the shoulder is narrow) Wider SRS will produce sounds/vibrations for a longer duration as the vehicle laterally traverses it. Sounds presented for longer durations are generally easier to detect. Groove Depth 7/16 in. Deeper grooves increase sound and vibration alert levels. Louder sounds and vibrations are easier to detect relative to background noise levels. Groove Separation 11-12 in. Narrower groove separation slightly increases the frequency. Drivers generally perceive higher tones as sounding more urgent. Longitudinal Gaps None if shoulder not shared with bikes 12 ft if shoulder shared with bikes Gaps of 12 ft or less can reduce the chance that a vehicle will miss the SRS completely. Effectiveness will be lower than without gaps because alert duration will be shorter over gap sections. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empir ical Da ta Based Primarily on Empirical Da ta HFG RURAL ENVIRONMENTS Version 2.0 16-6

Discussion Torbic et al. ( 4 ) found that there is no conclusive evidence indicating a clear minimum level of stimulus that a shoulder or centerline rumble strip must generate in order to alert an inattentive, distracted, drowsy, or fatigued driver. However, the applicable research literature generally in dicates that rumble strips that generate a 3 to 15 dBA increase above the ambient in - vehicle sound level can be detect ed by awake drivers. Also some evidence suggests that a sudden change in sound level above 15 dBA could startle a driver. However, a rumble strip generating more than a 15 dBA increase above the ambient sound level should not be automatically assumed to cause negative impacts (e.g., an increase in crashes), but rather to increase the potent ial for startling drivers who encounter the rumble strip. Related guidance for in-vehicle warning tones typically recommends sound intensit y levels for aud itory-only warnings (unaccompanied by vibrations) of between 10 and 30 dB above b ackground noise levels, while not exceeding 90 dB overall. The SRS guidelines differ significantly from the guidance for in-vehicle warning tones because of the presence of haptic vibrations with the SRS. In particular, at least fo r passenger vehicles, background vibration levels are low in small vehicles and even a small change in vibration can be clear ly detected. Laboratory driving simulator studies show that usually drivers easily detect stee ring wheel or brake pedal vibrations of 1.2-1.5 N • m torque presented over half a second. In contrast to passenger vehicles, cab vibrations in heavy trucks are significant and the size and weight of heavy trucks reduce the vibrations generated by SRS; therefore, the vibration component of SRS is viewed to have minimal benefit for alerting heavy truck drivers. It is also worth noting that the effectiveness of SRS for waking sleeping drivers has not been closely examined and that it is likely that SRS are much less effective in this application. The prim ary reasons for this lesser effectivenes s are that greater stim u lus levels are required to wake a sleeping driver rather than to mere ly alert a distracted or drows y driv er and that the increased arousal caused by traversing rum ble strips is brief and insufficient ( 5 ). The rationale for the “suitable values” in the guidelines table is discussed in further detail in FHWA ( 6 ), Spring ( 7 ), and Torbic et al. (4). For the most part, the values also accommodate bicycle traffic on the shoulder. Design Issues An important consideration when installing SRS on non-controlled-access roadways is the impact on bicyclists (and possibly motorcyclists) because several aspects that improve the alerting aspects of rumble strips (e.g., depth) also make SRS more challenging to traverse. A key factor in the suitability of a shoulder for accommodating both SRS and bicycle traffic is the shoulder width (see table below). Bicyclists also need gaps at sufficient frequency to cross the rumble strips in advance of hazards or intersections. Moeur (8) suggests a 12-ft gap in 60-ft cycle, which will result in 80% coverage of the shoulder with rumble strips and exactly 1½ times cycle length for lane line striping. Other options are to use a 40-ft cycle, consisting of a 28-ft long rumble strip with a 12-ft gap, though the 40-ft cycle will provide gaps more frequently for a given speed. Shoulder Width (ft) Is there a Problem? Reasoning 0-1.9 No Shoulder is too narrow for SRS or bicyclists. 2-3.9 Yes Shoulder may be wide enough for SRS or bicyclists. 4-5.9 Yes Shoulder may be wide enough for both SRS and bicyclists. 6+ No Shoulder is wide enough for SRS and bicyclists. Cross References Countermeasures for Pavement/Shoulder Drop-offs, 16-4 Key References 1. Griffith, M.S. (1999). Safety evaluation of rolled-in continuous shoulder rumble strips installed on freeways. Transportation Research Record, 1665, 28-34. 2. Harkey, D.L., Srinivasan, R., Zegeer, C., Persaud, B., Lyon, C., Eccles, K., et al. (2005). NCHRP Research Results Digest 299: Crash Reduction Factors for Traffic Engineering and Intelligent Transportation System (ITS) Improvements: State-of-Knowledge Report. Washington, DC: Transportation Research Board. 3. Harwood, D.W. (1993). NCHRP Synthesis of Highway Practice 191: Use of Rumble Strips to Enhance Safety. Washington, DC: Transportation Research Board. 4. Torbic, D.J., Hutton, J.M., Bokenkroger, C.D., Bauer, K.M., Harwood, D.W., Gilmore, D.K., et al. (2009). NCHRP Report 641: Guidance for the Design and Application of Shoulder and Centerline Rumble Strips. Washington, DC: Transportation Research Board. 5. O’Hanlon, J.F., and Kelley, G.R. (1974). A Psychophysiological Evaluation of Devices for Preventing Lane Drift and Run-Off-Road Accidents. Goleta, CA: Human Factors Research, Inc. 6. FHWA (n.d.). Synthesis of Shoulder Rumble Strip Practices and Policies. Washington DC. Retrieved from http://safety.fhwa.dot.gov/roadway_dept/research/exec_summary.htm. Spring, G.S. (2003). Shoulder Rumble Strips in Missouri. (RDT 03-007, Final Report). Rolla: University of Missouri. 8. Moeur, R. (2000). Analysis of gap patterns in longitudinal rumble strips to accomodate bicycle travel. Transportation Research Record, 1705, 93-98. 7. 16-7 HFG RURAL ENVIRONMENTS Version 2.0

DESIGN CONSISTENCY IN RURAL DRIVING Introduction Design consistency refers to the conformance of a highway’s geometric and operational features with driver expectancy (1). All other factors being equal, drivers will make fewer errors when faced with geometric features that are consistent with their expectations. Note that the guideline information below only provides general information about some of the factors associated with the concept of design consistency. Although research suggests that the factors listed are relevant to design consistency, at this time there is insufficient data to provide detailed quantitative recommendations. Design Guidelines The following driver expectancies (adapted from Lunenfeld & Alexander (2)) and factors (adapted from Fitzpatrick et al. (3)) should be considered in a design consistency review. What is known about driver expectancies? What factors should be considered in a design consistency review? 1. Drivers tend to anticipate upcoming situations and events that are common to the road that they are traveling. 2. The more predictable the roadway, the less likely there will be driver errors. 3. Drivers experience problems when surprised or with inconsistent design or operation. 4. Drivers generally assume they will only have to react to standard situations. 5. The roadway and its environment upstream of a site create expectancy for downstream conditions. 6. Expectancies are associated with all levels of driving performance and all aspects of the driving situation. Cross-section markings Guide signs and route markers Warning and regulatory signs Geometry Sight distance Road type and surface Signals Lighting Consider also: Land use Terrain Typical traffic conditions Typical weather More detailed analyses can be conducted for: Navigation expectancies Guidance expectancies Special geometric and other features Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG RURAL ENVIRONMENTS Version 2.0 16-8

Discussion As noted on the previous page, design consistency refers to the conformance of a highway’s geometric and operational features with driver expectancy (1). In a key study leading to the development of the Interactive Highway Safety Design Model (IHSDM), crash and field data were analyzed and speed prediction models were developed for a variety of different roadway alignments (3). Four design consistency measures were associated with crash frequency: 1. Predicted speed reduction on a horizontal curve relative to the preceding curve or tangent (has the strongest and most sensitive relationship to crash frequency) 2. Ratio of an individual curve radius to the average radius for the roadway section as a whole 3. Average rate of vertical curvature on a roadway section 4. Average radius of curvature on a roadway section Design consistency is an important concept because the driving task requires continuous/frequent: Sampling of visual, auditory, and haptic (touch or feel) cues Processing of these cues and decision making Outputs in the form of steering, brake, and accelerator inputs This requirement to continuously “perceive–think–act” takes considerable effort (even when some activities become more or less automated), especially under challenging circumstances such as poor weather, nighttime conditions, heavy traffic, high speeds, etc. Inconsistent roadway design has the potential for increasing driver uncertainty about—for example—where to look for signs, how much illumination to expect from roadway section to roadway section, and how fast to drive. An inability to anticipate and predict the conditions that shape driving decisions and behaviors can lead to higher workload and, ultimately, decrements in driving performance and safety. Thus, minimizing driver workload through consistent layout and alignment of roadways is an important design goal. Although driver expectancies for a roadway can vary widely with respect to their completeness and correctness, there should ideally be a reasonable match between the geometric and operating characteristics of the rural driving environment and the driver’s expectancies for this environment. The underlying psychological factor supporting the need for design consistency is the notion of mental models or schemas (see Gentner & Stevens (4)), which—broadly defined in the context of system design—is the user’s internal understanding and representation of an external reality. In the driving environment, one type of mental model is the driver’s understanding of the roadway and the surrounding infrastructure, how the roadway system works, and how to operate within it. A key aspect of mental models is that they allow the driver to predict the outcome of his or her driving behaviors. Design Issues The IHSDM is a suite of software analysis tools for evaluating safety and operational effects of geometric design decisions on two-lane rural highways. IHSDM is a decision support tool that checks existing or proposed two-lane rural highway designs against relevant design policy values and provides estimates of a design’s expected safety and operational performance. FHWA’s website for the IHSDM can be found at http://www.tfhrc.gov/safety/ihsdm/ihsdm.htm. Cross References Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems, 17-14 Key References 1. Wooldridge, M.D., Fitzpatrick, K., Harwood, D.W., Potts, I.B., Elefteriadou, L., and Torbic, D.J. (2003). NCHRP Report 502: Geometric Design Consistency on High-Speed Rural Two-Lane Roadways. Washington, DC: Transportation Research Board. 2. Lunenfeld, H., and Alexander, G. J. (1984). Human factors in highway design and operations. Journal of Transportation Engineering, 110(2), 149-158. 3. Fitzpatrick, K., Elefteriadou, L., Harwood, D.W., Collins, J.M., McFadden, J., Anderson, I B., et al. (2000). Speed Prediction for Two-Lane Rural Highways (FHWA-RD-99-171, Final Report). McLean, VA: FHWA. 4. Gentner, D. and Stevens, A.L. (Eds.) (1983). Mental Models. Hillsdale, NJ: Lawrence Erlbaum Associates. 16-9 HFG RURAL ENVIRONMENTS Version 2.0