Assured clear distance ahead

[8] 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.

[12] 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.

[13][15] Furthermore, modern knowledge of human factors has revealed physiological limitations—such as the subtended angular velocity detection threshold (SAVT)—which may make it difficult, and in some circumstance impossible, for other drivers to always comply with right-of-way statutes by staying clear of roadway.

[25][26][27] By the late 1920s, the term "assured clear distance ahead" came into widespread use as the identity of a standard of care element in choosing safe speed,[28][29] with differing jurisdictions adopting the language to carry its same effects.

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.

[80][81][97][98][99] Electronic access to precise EDR data and rulings with new ideological modeling tools, can now expose judges as consistent political advocates for differing special road user interests.

This distance may be attenuated by specific conditions such as atmospheric opacity,[108] blinding glare,[109] darkness,[1][110] road design,[111][112] and adjacent environmental hazards including civil and recreational activities,[13] horse-drawn vehicle,[37] ridden animal,[37] livestock,[37] deer,[113] crossing traffic,[15] and parked cars.

In such situations, approach speed must be reduced in preparation for entering or crossing a road or intersection or the unmarked pedestrian crosswalks[121][122] and bike paths[123] they create because of potential hazards.

[124][125][54][126][127][128] This jurisprudence arises in-part because of the known difficulty in estimating the distance and velocity of an approaching vehicle,[128][118][119] which is psychophysically explained by its small angular size and belated divergence from an asymptotically null rate of expansion, which is beyond the subtended angular velocity detection threshold (SAVT) limits of visual acuity[129][130][131][16][17] by way of the Stevens' power law[16] and Weber–Fechner law, until the vehicle may be dangerously close; subjective constancy and the visual angle illusion[132] may also play a role.

In this circumstance, it is impossible for the entering driver to have fair notice that his or her contemplated conduct is forbidden by such hazard,[9][119] and any legal expectation to the contrary would implicate violating the vagueness doctrine of the US Constitution.

[124] The distance "di" is the sum of the measured limit line setback distance—which is typically regulated by a Manual on Uniform Traffic Control Devices, at often between 4 and 30 feet in the United States[134][135][136]—and the crosswalk, parking lane, and road shoulder width.

[137] Horizontal clearance is measured from the edge of the traveled way to the bottom of the nearest object, tree trunk or shrub foliage mass face, plant setback, or mature growth.

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.

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 certain urban environments, a straight, traffic-less, through-street may not necessarily be assuredly clear past the entrance of the nearest visually obstructed intersection as law.

[142] While there may be a degree of variance of such skill in seasoned drivers, they generally do not have the discretion in engaging in a behavior such as driving a speed above which no reasonable minds might differ as to whether it is unsafe or that one could come to a stop within the full distance ahead.

[126][141] Drivers and law enforcement alike can apply elementary level arithmetic[143] towards a rule of thumb to estimate minimal stopping distance in terms of how many seconds of travel ahead at their current speed.

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.

Wildlife will frequently transit across a road before a full stop is necessary, however collisions with large game are foreseeably lethal, and a driver generally has a duty not to harm his or her passengers.

[116][117][110][160][3][46] A general principle in liability doctrine is than an accident which would not have occurred except for the action or inaction of some person or entity contrary to a duty such as the exercise of proper care was the result of negligence.

[169] The assumption of risk resulting from the unsafe activity of driving faster than one can stop within one's vision, does not depend ex post facto on what you happened to hit, for which by nature you could not have known; it could have been a moose or a luxury car.

Furthermore, modern times still provide no legal remedies for Darwinian misfortune upon the entire class of unwarnable accidents where drivers and their passengers would not have collided into the likes of a moose,[113] livestock,[37] fallen tree, rock, jetsam, horse-drawn vehicle,[37] stalled vehicle,[170] school bus, garbage truck, mail carrier, snowplow, washout, snow drift, or slid off the road, were it not for their decisions to drive faster than dictated by the assured clear distance ahead.

In this specific regard, jurisdictions which grant drivers the liberty to be fools from their own folly, are also condoning the collateral damage and life loss which is expected to occur.

Moreover, modern life-entrusting consumers of driving services and driverless cars[12] who suffer such caused injury are left without legal remedy for foreseeable outcome of imprudent speed; this in-turn unnecessarily transfers a substantive portion of the ACDA liability space into act of god, government claims, strict liability, or other findings from legal fiction which the justice system generally abhors.

[34][35] The deceleration coefficients and reactions times may change from conveyance by chariot, horse and buggy, internal combustion engine, electric motor, and by driverless car, but the equations governing stopping distances are immutable.

Finally, where it is the policy of the law not to fault well intending diligent citizens for innocent mistakes,[118][119] human life reaps continued benefit from the ACDA duty of which instills the necessary room to survive uninjured from such foreseeable and excusable error while adding redundancy in the responsibility to avoid a collision; mere unilateral duties laid down to assure the safety of others tend to result in hazardous risk compensation by those unfettered parties[172] resulting in a moral hazard.

[114] One of the greatest difficulties created by such an extension of the ACDA is the frequency at which roads reduce their functional classification[114] unbeknownst to drivers who continue unaware they have lost this extended "assurance" or do not understand the difference.

[173] A partial solution to this challenge is to remove driver discretion in determining whether the ACDA is extended beyond line of sight, by explicitly designating this law change to certain marked high functional classification roadways having meet strict engineering criteria.

[12][104][140] As opposed to a strict standard of care,[176][80][81][177] delegation of such standard to a jury[178] assumes the representativeness heuristic[179] for twelve people to determine ordinary care representative of everyone while ignoring its insensitivity to sample size, which of course when applied to multiple cases involving identical situational circumstances results in many verdicts with opposing extreme views,[81] which works against the utility of the law by making it arbitrarily vague.

of 0.8, this equals (.2)2x0.8≈0.03): Now, the total stopping distance is the sum of the braking and perception-reaction distances: Isolating zero as preparation to solve for velocity: Completing the square or invoking the quadratic formula to find the solution: Use small-angle approximation to obtain a more field-able version of the above solution in terms of percent grade/100 "e" instead of an angle θ in radians: Substituting the angle as described produces the form of the formula of case 1 (): The Basic Speed Law constrains the assured clear distance ahead to the total stopping distance, and the small angle value of road grades approximates the superelevation "e." Many roadways are level, in which case the small angle approximations or superelevation may be dropped altogether: This model ignores the effects of air drag, rolling resistance, lift, and relativity as a vehicle's great momentum and weight dominate these factors; they increase the complexity of the formulas while insubstantially changing the outcomes in practically all driving situations except ultra-low-mass bicycles stopping from inherently dangerously high speeds; usability to the layman and conformance with current standard engineering assumptions[106][107] is the objective and a vehicle's lift factor is often inaccessible.