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A collision avoidance system (CAS) operates, generally, through a sensor installed at the front end of a vehicle, which constantly scans the road ahead for vehicles or obstacles. When found, the system determines whether the vehicle is in imminent danger of crashing, and if so, a collision avoidance maneuver is undertaken. Most CASs are non-cooperative, that is, detection is independent of whether other vehicles on the road are equipped with collision avoidance devices.
An alternative technology relies on vehicle-to-vehicle communications to exchange information on vehicles' presence, location, lane of travel, and speed among other factors. In addition to the front-end sensor, vehicles require a rear-end transponder as well, since communication, and therefore detection, only occurs among equipped vehicles.
The criteria to activate the collision avoidance system are:
- The time-to-collision criterion: the system decides whether a collision is likely to occur at prevailing speeds and distances, within a certain time interval. In a car-following scenario, the time-to-collision is the time taken for the two vehicles to collide if they maintain their present speed and heading.
- The worst-case criterion: the system infers that the vehicle preceding the CAS-equipped vehicle could brake at full braking power at any time. Basically, it operates on a "critical headway distance," that is, the minimum distance necessary for the CAS-equipped vehicle to come to a stop in the event the leading car abruptly brakes.
Collision avoidance maneuvers:
- Headway distance control: the system alerts the drivers when their cars are following the leading car too closely. Some systems include automatic speed control, in such a way, the car could automatically reduce its speed in order to maintain a safe headway with the leading vehicle.
- Hazard warning: the system alerts the driver of an object (moving or stationary) within its projected path, so that the driver has enough time to avoid a collision.
- Automatic vehicle control: the system controls the vehicle's brakes and steering wheel, and applies them automatically when it determines it necessary.
- Visual head-up displays: warnings are displayed on the windshield in the driver's field of view, so that their content can be assimilated in conjunction with the driving scene ahead. These displays are intended to minimize distraction from driving tasks, in addition to ensuring that the warning does not go undetected.
- Audio/voice signals: in comparison to visual signals, auditory signals appear to be less intrusive on driving tasks. They are also insensitive to external conditions such as poor light, bad weather, or a dirty windshield. Two different auditory warnings have been developed: speech (synthesized voice) or non-speech (buzzer) displays.
- Haptic devices: They provide redundant information via alternative sensory modalities, given that the primary visual or auditory channel may be degraded or overburdened. Research suggests that one possibility is to increase the force needed to push the gas pedal.
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