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HFG TUTORIALS Version 1.0 Tutorial 1: Real-World Driver Behavior Versus Design Models Much of the information on sight distance presented in Chapter 5 reflects the application of empirically derived models to determine sight distance requirements. Such models, while valu- able for estimating driver behavior across a broad range of drivers, conditions, and situations, have limitations. This tutorial discusses how driver behavior as represented in sight distance models may dif- fer from actual driver behavior. The design models presented in Chapter 5 use simplified con- cepts of how the driver thinks and acts. This simplification should not be viewed as a flaw or error in the sight distance equations. These models are a very effective way of bringing human factors data into design equations in a manner that makes them accessible and usable. After all, the intent of a sight distance equation is not to reflect the complexities of human behavior but to bring what we know about it into highway design in a concise, practical way. However, like any behavioral model, models for deriving sight distance requirements are not precise predictors of every case and there may be some limitations to their generality. Therefore, having an under- standing of certain basic principles of human behavior in driving situations is useful to better interpret these models and to understand how they may differ from the range of real-world driv- ing situations. Sight distance formulas for various maneuvers (presented in Chapter 5) differ from one another, but they share a common simple behavioral model as part of the process. The model assumes that some time is required for drivers to perceive and react to a situation or condition requiring a particular driving maneuver (i.e., PRT), which is followed by some time (i.e., MT) and/or distance required to execute the maneuver. Sight distance equations for some maneuvers may contain additional elements or assumptions; however, all have this basic two-stage model somewhere at their core. The two equations that follow show two versions of the general, two-component model. In both versions, the first term shows the distance traveled during the PRT component and the sec- ond term shows the distance traveled during the MT component. The difference is that the first equation shows a case where the distance traveled while executing the maneuver is based on the time required to make that maneuver (for example, the time to cross an intersection from a Stop), while the second equation shows a case where the distance traveled while executing the maneuver is based directly on the distance required to complete the maneuver (for example, braking distance for an emergency stop). For both forms of this general equation, vehicle speed (V) influences the second (MT) component. The general form of the sight distance equation is: d SD = kVt prt + kVt man , where maneuver time is input or d SD = kVt prt + d manV , where maneuver time is input Where: d = required sight distance V = velocity of the vehicle(s) tprt = PRT tman = MT dmanV = distance required to execute a maneuver at velocity V k = a constant to convert the solution to the desired units (feet, meters) 22-2
