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HFG RURAL ENVIRONMENTS Version 1.0 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 factors should be considered in a design What is known about driver expectancies? consistency review? 1. Drivers tend to anticipate upcoming Cross-section markings situations and events that are common to the Guide signs and route markers road that they are traveling. Warning and regulatory signs 2. The more predictable the roadway, the less likely there will be driver errors. Geometry 3. Drivers experience problems when surprised Sight distance or with inconsistent design or operation. Road type and surface 4. Drivers generally assume they will only have Signals to react to standard situations. Lighting 5. The roadway and its environment upstream of a site create expectancy for downstream Consider also: conditions. Land use 6. Expectancies are associated with all levels of Terrain driving performance and all aspects of the Typical traffic conditions driving situation. Typical weather More detailed analyses can be conducted for: Navigation expectancies Guidance expectancies Special geometric and other features Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 16-8
HFG RURAL ENVIRONMENTS Version 1.0 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 "perceivethinkact" 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
