Academic journal article Human Factors

# Speed Choice and Steering Behavior in Curve Driving

Academic journal article Human Factors

# Speed Choice and Steering Behavior in Curve Driving

## Article excerpt

INTRODUCTION

Car-driving behavior in curves may be regarded as an interesting case in which steering, as an example of operational performance, is intimately related to behavior on the tactical level - in this case, the choice of speed as a function of curve radius. The distinction between the operational and the tactical level of car-driving behavior has been made by several authors (see Michon, 1985) and might form a fruitful basis for the development of modern driver behavior theories (see Ranney, 1994). Until now, studies of car-driving behavior in curves have focused exclusively on either speed choice or steering behavior, and no attempt has been made to integrate these two lines of research.

A consistent finding in studies of speed choice in curves is that speed has a curvilinear relation to curve radius (see Kanellaidis, Golias, & Efstathiadis, 1990) and an inverse relation to lateral acceleration. This means that with smaller radii, speed is lower but lateral acceleration is higher than with larger radii (see McLean, 1981). Sometimes an inverse linear relation is reported (Ritchie, McCoy, & Welde, 1968), whereas other studies have found an inverse nonlinear relation between speed and lateral acceleration (Herrin & Neuhardt, 1974; Macura, 1984). These results have encouraged the idea that drivers use lateral acceleration as a cue in speed choice, in that they accept a smaller lateral acceleration as a safety margin at higher speeds (and thus larger radii).

In studies of steering behavior during curve negotiation, speed is usually held constant. Donges (1978) presented a two-level steering control model that incorporated negotiation of curves. Anticipatory open-loop control begins with a steering action some time before the curve is entered; this is followed by a steering wheel angle maximum, [[Delta].sub.sa], in the curve. Then a period of stationary curve driving begins, during which the driver generates correcting steering actions in a compensatory closed-loop mode.

In a survey of models of steering behavior, Reid (1983) argued that driver models should incorporate both lane tracking and speed control. In Donges's model the parameters estimated to fit the model on experimental data were influenced by vehicle speed and confounded with road curvature. Curve radius and speed during curve negotiation affect required operational performance because both factors affect the required steering wheel angle.

Godthelp (1986) described this phenomenon as follows: The required steering wheel angle for a particular curve can roughly be characterized as [[Delta].sub.sr] = GL(1 + [Ku.sup.2])/[R.sub.r]. In this, [[Delta].sub.sr] represents the required steering wheel angle, [R.sub.r] the road radius in meters, G the steering system gear ratio, L the wheelbase, K a vehicle-related stability factor, and u the longitudinal speed in m/s. For any given speed, required steering wheel angle then increases with smaller radii, but for a given radius, it increases with higher speed, if K is larger than zero, which is the case for a normal understeered car.

If the steering wheel angle during curve negotiation matches the required steering wheel angle perfectly, speed is restricted only by an upper limit at which the vehicle begins to skid. The speed at which this occurs is generally much higher than actual speed in curves. The hypothesis of the present study is that steering errors play an important role in speed choice, such that speed is adapted to operational performance. There is some evidence that steering errors increase linearly with required steering wheel angle (see Godthelp, 1985, 1986). Because negotiating curves with a smaller radius requires a larger steering wheel angle, the implication is that steering error is larger in curves with smaller radii than with wider curves. If steering error is a linear function of required steering wheel angle, the fraction defined as steering error divided by required steering wheel angle should be constant over radii. …

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