Assured Clear Distance Ahead

The Assured Clear Distance Ahead (ACDA) is the distance ahead of any terrestrial locomotive device such as a land vehicle,watercraft,skates, or skis, although commonly an automobile, which can be seen to be clear of hazards by the driver, within which they should be able to bring the device to a halt; drivers generally may not pose an "immediate hazard" upon where or when they cannot assure such distance ahead is clear. It is one of the most fundamental principles governing ordinary care and the duty of care for all methods of conveyance, and is frequently used to determine if a driver is in proper control and is a nearly universally implicit consideration in vehicular accident liability. The rule is a precautionary trivial burden required to avert the great probable gravity of precious life loss and momentous damage—the former of which has a priceless intrinsic value and a pricey to invaluable instrumental value. Satisfying the ACDA rule is necessary but not sufficient to comply with the more generalized basic speed law, and accordingly, it may be used as both a layman's criterion and judicial test for courts to use in determining if a particular speed is negligent, but not to prove it is safe. As a spatial standard of care, it also serves as required explicit and fair notice of prohibited conduct so unsafe speed laws are not void for vagueness.

This distance is typically both determined and constrained by the proximate edge of clear visibility, but it may be attenuated to a margin of which beyond hazards may reasonably be expected to spontaneously appear. The rule is the specific spatial case of the common law basic speed rule, and an application of volenti non fit injuria. The two-second rule may be the limiting factor governing the ACDA, when the speed of forward traffic is what limits the basic safe speed, and a primary hazard of collision could result from following any closer.

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