The Assured Clear Distance Ahead (ACDA) is the distance ahead of a vehicle or craft which can be seen to be clear of hazards by the driver, within which they should be able to bring the vehicle to a halt. It is one of the most fundamental principles governing ordinary care and the duty of care and is frequently used to determine if a driver is in proper control and is a nearly universally implicit consideration in vehicular accident liability.[1][2][3]

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. It is a spatial component to the common law basic speed rule. 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.

## ACDA as Common Law Rule or Statute

"At common law a motorist is required to regulate his speed so that he can stop within the range of his vision. In numerous jurisdictions, this rule has been incorporated in statutes which typically require that no person shall drive any motor vehicle in and upon any public road or highway at a greater speed than will permit him to bring it to a stop within the assured clear distance ahead."[1] Decisional law usually settles the circumstances by which a portion of the roadway is assuredly clear without it being mentioned in statute.[4] California is a state where the judiciary has established the state's ACDA law.[5][6][7][8][9][Note 1] Most state issued driver handbooks either instruct or mention the ACDA rule as required care or safe practice.[10][11][12][13][14]

Many states have further passed statutes which require their courts to more inflexibly weigh the ACDA in their determination of reasonable speed or behavior. Such statutes do so in part by designating ACDA violations as a citable driving offense, thus burdening an offending driver to rebut a presumption of negligence. States with such explicit ACDA standard of care provisions include: Iowa,[15] Michigan,[16] Ohio,[17] Oklahoma,[18] and Pennsylvania.[19] States which apply the principle by statute to watercraft on navigable waterways include: Montana,[20] Florida,[21] and West Virginia. Explicit ACDA statutes, especially those of which create a citable driving offense, are aimed at preventing harm that could result from potentially negligent behavior—whereas the slightly more obscure common law ACDA doctrine is most easily invoked to remedy actual damages that have already occurred as a result of such negligence. Explicit and implicit ACDA rules govern millions of North American drivers.

## Determining the ACDA

### Static "line-of-sight" distance

The range of visibility of which is the defacto ACDA, is usually that distance before which an ordinary person can see small hazards—such as a traffic cone or buoy—with 20/20 vision. This distance may be attenuated by specific conditions such as atmospheric opacity, blinding glare, and adjacent environmental hazards including civil and recreational activities, deer, livestock, and parked cars. The ACDA may also be somewhat attenuated on roads with lower functional classification.[22] [23] This is because the probability of spontaneous traffic increases proportionally to the density of road access points, and this density reduces the distance a person exercising ordinary care can be assured that a road will be clear; such reduction in the ACDA is readily apparent from the conditions, even when a specific access point or the traffic thereon is not.[24][Note 2] Furthermore, even though a through-driver may typically presume all traffic will stay assuredly clear when required by law, such driver may not take such presumption when circumstances provide actual knowledge under ordinary care that such traffic cannot obey the law.[24]

### Dynamic "following" distance

The ACDA may also be dynamic as to the moving distance past which a motorist can be assured be to able to stay clear of a foreseeable dynamic hazard—such as to maintain a distance as to be able to safely swerve around a bicyclist should he succumb to a fall—without requiring a full stop beforehand, if doing so could be exercised with due care towards surrounding traffic. Quantitatively this distance is a function of the appropriate time gap and the operating speed: dACDA=tgap*v. The assured clear distance ahead rule, rather than being subject to exceptions, is not really intended to apply beyond situations in which a vigilant ordinarily prudent person could or should anticipate.[1]

As such, the Assured Clear Distance Ahead is somewhat subjective to the baseline estimate of a reasonable person. For example, whether one should have reasonably foreseen that a road was not assuredly clear past 75–100 meters because of tractors or livestock which commonly emerge from encroaching blinding vegetation is on occasion dependent on societal experience within the locale. In urban environments, a straight, traffic-less, through-street may not necessarily be assuredly clear past the entrance of the nearest visually obstructed intersection.[22][23][24]

## Relation to the basic speed rule

The ACDA distances are a principal component to be evaluated in the determination of the maximum safe speed (VBSL) under the basic speed law, without which the maximum safe speed cannot be determined. The relation of the ACDA to the basic speed rule may be objectively quantified as follows:

${\displaystyle V_{BSL}={\begin{cases}{\sqrt {\mu ^{2}g^{2}t_{prt}^{2}+2\mu gd_{ACDA(s)}}}-\mu gt_{prt},&{\mbox{if }}V_{ACDA(s)}\leq V_{ACDA(d)}{\mbox{ or }}V_{cl}\\{\frac {d_{ACDA(d)}}{t_{g}}},&{\mbox{if }}V_{ACDA(d)}

The maximum velocity permitted by the Assured Clear Distance Ahead is controlling for only the top and middle cases.

For the top case, the maximum speed is governed by the assured clear "line-of-sight", as when the "following distance" aft of forward traffic and "steering control" are both adequate. Common examples include when there is no vehicle to be viewed, or when there is a haze or fog that would prevent visualizing a close vehicle in front. This maximum velocity is denoted by the case variable ${\displaystyle V_{ACDA(s)}}$, the friction coefficient is symbolized by ${\displaystyle \mu }$—and itself a function of the tire type and road conditions, the distance ${\displaystyle d_{ACDA(s)}}$ is the static ACDA, the constant ${\displaystyle g}$ is the acceleration of gravity, and interval ${\displaystyle t_{prt}}$ is the perception-reaction time—usually between 1.0 and 2.5 seconds.[25]

The pedantic middle case applies when the dynamic ACDA "following distance" (dACDA(d)) is less than the static ACDA "line-of-sight" distance (dACDA(s)). A classic instance of this occurs when, from a visibility perspective, it would be safe to drive much faster were it not for a slower-moving vehicle ahead. As such, the dynamic ACDA is governing the basic speed rule, because in maintaining this distance, one cannot drive at a faster speed than that matching the forward vehicle. The "time gap" tg or "time cushion" is the time required to travel the dynamic ACDA or "following distance" at the operating speed. Circumstances depending, this cushion might be manifested as a two-second rule or three-second rule.

The bottom case is invoked when the maximum velocity for surface control Vcl is otherwise reached. Steering control is independent from any concept of clear distance ahead. If a vehicle cannot be controlled so as to safely remain within its lane above a certain speed and circumstance, then it is irrelevant how assuredly clear the distance is ahead.

## Notes

1. In addition to being old common law principle, ACDA case law jurisprudence buttressed the legislature's implicit intent in its' Basic Speed Law, and further limited transference of liability for ACDA negligence to the state—under the California Tort Claims Act—for insufficient sight distance at the speed of which the driver chose. See CVC § 22350, CVC § 22358.5, Cal Gov. Code § 830.4, Cal Gov. Code § 830.8, and Cal Gov. Code § 831. See CACI Form 1120 for details.
2. For this reason, full corner sight distance is almost never required for individual driveways in urban high-density residential areas, and street parking is commonly permitted within the right-of-way.
3. In most jurisdictions, judicial notice shall be taken of the total stopping distance, and such notice is therefore logically and substantively taken of the maximum speed permitted to brake within the stopping distance as applied to the ACDA. The latter is merely the inverse function of the former. Furthermore, fundamental mathematical relationships are themselves subject to judicial notice. ${\displaystyle V_{ACDA(s)}={\sqrt {\mu ^{2}g^{2}t_{prt}^{2}+2\mu gd_{ACDA(s)}}}-\mu gt_{prt}}$ For example, using the ${\displaystyle \mu =0.7}$ and ${\displaystyle t_{prt}=1.5}$ values that produced Code of Virginia § 46.2-880 Tables of speed and stopping distances, one simply obtains the same velocities that produced the stopping distance in the statute: Metric (SI) – Speed in km/h from distance in meters: ${\displaystyle V_{ACDA}\approx {\sqrt {1372.3+177.8d_{ACDA}}}-37.0}$ US customary – Speed in MPH from distance in feet: ${\displaystyle V_{ACDA}\approx {\sqrt {529.8+20.9d_{ACDA}}}-23.0}$

## References

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5. {{#invoke:Citation/CS1|citation |CitationClass=journal }} See California Official Reports: Online Opinions
6. {{#invoke:Citation/CS1|citation |CitationClass=journal }} See California Official Reports: Online Opinions
7. {{#invoke:Citation/CS1|citation |CitationClass=journal }} See California Official Reports: Online Opinions
8. {{#invoke:Citation/CS1|citation |CitationClass=journal }} Driver traveling at 35 MPH when rain limited visibility to 25 feet held negligent when 65 feet were required to stop car on wet road. See California Official Reports: Online Opinions
9. {{#invoke:Citation/CS1|citation |CitationClass=journal }} See California Official Reports: Online Opinions
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