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SECTION 2. DESCRIPTION AND DEVELOPMENT OF THE DVM
Introduction
This section describes the DVM design and development that has taken place throughout the course of the project. An overview of the five major functional components is also provided.
Purpose and Structure
Figure 1 outlines the information flow within the DVM. The Roadway Geometrics and Driver Geometrics components perform coordinate transformations of the roadway data and compute certain error variables (e.g., vehicle drift rate) that are to be corrected by the driver. The VDM responds to the driver's control input as well as to the gravitational inputs dictated by the roadway geometry to simulate the motion of the vehicle.
The purpose of the driver portion of the DVM is to simulate the driver's perceptual, cognitive, and control processes to generate steering, braking, and acceleration inputs. This module has five major functional components: (1) Perception, (2) Speed Decision, (3) Path Decision, (4) Speed Control, and (5) Path Control. These components are described below.
- Perception: This component translates the physical description of the situation contained in the driver input database into estimates of vehicle states, roadway characteristics, and other relevant variables needed by the decision and control modules. These estimates are degraded as a result of driver information-processing limitations.
- Speed Decision: This component computes the driver's desired speed profiles. The desired speed profile typically varies during the course of a simulation run and, at any instant, the driver attempts to perform one of the following tasks: (1) maintain a constant speed, (2) generate a deceleration or acceleration profile appropriate to entering or exiting a curve, (3) decelerate because of a condition such as a reduction in posted speed or a stop sign that requires reducing speed or coming to a full stop, or (4) accelerate to resume speed after the slow or stop requirement is no longer relevant. Speed decision may be influenced by limited sight distance (SD) as described elsewhere in this document.
- Path Decision: This component computes the desired path profile, which is either the center of the lane for the entire roadway, or-at the option of the user-lane center on tangent sections and a path that optimally "cuts the curve" on horizontal curves.
- Speed and Path Control: These components perform the closed-loop tracking tasks of regulating speed and path about the profiles produced by the high-level decision module. Because lateral deviation generally requires substantially tighter control than speed deviation in the absence of traffic, separate model elements for the two regulation tasks are specified to facilitate different modeling approaches and to allow independent development for speed and path control models in the future.
Figure 1 . Information flow in the Driver Vehicle Model.
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FHWA-HRT-08-019
