Abstract
It is well known that road accidents tend to be more frequent in locations where a sudden change in road characteristics violates the driver's expectations. Most methods used to assess the design consistency are based on simplified speed profiles that give a coarse description of the vehicle dynamics along the road. This paper presents a new approach to quantify the road design consistency, based on continuous operating speed profiles. These profiles are based on the Gipps’ car-following equations, adapted to simulate the driver behaviour in the vicinity of horizontal curves under free-flow conditions. A methodology to calibrate and validate the Gipps’ behavioural parameters from field data is presented and applied to predict the speed profiles of three drivers for a set of rural road segments. The calibration is based on trajectory data collected with an instrumented vehicle and it follows an automated procedure that aims to minimize the differences between the predicted and observed speed profiles. The new consistency index is based on the deceleration distances and it allows to overcome some limitations of the existing methods.
Highlights
The concept of design speed is used in most manuals to define the minimum road geometric design parameters [1]
The concept of design consistency was defined as a way to prevent unexpected changes to geometrical characteristics of contiguous road elements and combinations of elements that do not comply with drivers’ expectations
Some of the methods that exist today to evaluate the design consistency on two-lane rural highways are based on the estimate of operating speed profiles that do not replicate the actual driver behaviour
Summary
The concept of design speed is used in most manuals to define the minimum road geometric design parameters [1]. The concept of geometric design consistency was defined as a way to prevent sudden changes on geometrical features of contiguous road elements and the use of combinations of elements that do not comply with the driver’s expectations. The operating speed is the most frequently used criterion to evaluate road design consistency and it is frequently estimated by the 85th percentile of the speed distribution observed in real driving conditions [4]. The Highway Capacity Manual (HCM) [5] denotes speed as the most adequate parameter to ensure an economical and environmental analysis regarding two-lane rural highways
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