In legal terminology, the assured clear distance ahead (ACDA) is the distance ahead of any terrestrial
locomotive device such as a
land vehicle, typically an
automobile
A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods.
The year 1886 is regarde ...
, or
watercraft
Any vehicle used in or on water as well as underwater, including boats, ships, hovercraft and submarines, is a watercraft, also known as a water vessel or waterborne vessel. A watercraft usually has a propulsive capability (whether by sail, ...
, within which they should be able to bring the device to a halt.
It is one of the most fundamental principles governing
ordinary care
In tort law, a duty of care is a legal obligation that is imposed on an individual, requiring adherence to a standard of reasonable care while performing any acts that could foreseeably harm others. It is the first element that must be establis ...
and the
duty of care
In tort law, a duty of care is a legal obligation that is imposed on an individual, requiring adherence to a standard of reasonable care while performing any acts that could foreseeably harm others. It is the first element that must be establi ...
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.
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
In tort law, the standard of care is the only degree of prudence and caution required of an individual who is under a duty of care.
The requirements of the standard are closely dependent on circumstances. Whether the standard of care has been b ...
, it also serves as required explicit and fair notice of prohibited conduct so unsafe speed laws are not
void for vagueness
In American constitutional law, a statute is void for vagueness and unenforceable if it is too vague for the average citizen to understand, and a constitutionally-protected interest cannot tolerate permissible activity to be chilled within the ran ...
.
The concept has transcended into
accident reconstruction
Traffic collision reconstruction is the process of investigating, analyzing, and drawing conclusions about the causes and events during a vehicle collision. Reconstructionists conduct collision analysis and reconstruction to identify the cause ...
and engineering.
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
''Volenti non fit iniuria'' (or ''injuria'') (Latin: "to a willing person, injury is not done") is a common law doctrine which states that if someone willingly places themselves in a position where harm might result, knowing that some degree of h ...
''. The
two-second rule
The two-second rule is a rule of thumb by which a driver may maintain a safe trailing distance at any speed. The rule is that a driver should ideally stay at least two seconds behind any vehicle that is directly in front of his or her vehicle. ...
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.
As the original common law driving rule preceding statutized traffic law,
it is an ever important foundational rule in today's complex driving environment. Because there are now protected classes of roadway users–such as a
school bus,
mail carrier
A mail carrier, mailman, mailwoman, postal carrier, postman, postwoman, or letter carrier (in American English), sometimes colloquially known as a postie (in Australia, Canada, New Zealand, and the United Kingdom), is an employee of a post ...
,
emergency vehicle
An emergency vehicle is a vehicle used by emergency services. Emergency vehicles typically have specialized emergency lighting and vehicle equipment that allow emergency services to reach calls for service in a timely manner, transport equipment ...
,
horse-drawn vehicle,
agricultural machinery
Agricultural machinery relates to the mechanical structures and devices used in farming or other agriculture. There are many types of such equipment, from hand tools and power tools to tractors and the countless kinds of farm implements that ...
,
street sweeper
A street sweeper or street cleaner may refer to a person's occupation or to a machine that cleans streets.
Street sweepers have been employed in cities as "sanitation workers" since sanitation and waste removal became a priority. A stre ...
,
disabled vehicle,
cyclist
Cycling, also, when on a two-wheeled bicycle, called bicycling or biking, is the use of cycles for transport, recreation, exercise or sport. People engaged in cycling are referred to as "cyclists", "bicyclists", or "bikers". Apart from two ...
, and
pedestrian–as well as
natural hazards
A natural hazard is a natural phenomenon that might have a negative effect on humans and other animals, or the environment. Natural hazard events can be classified into two broad categories: geophysical and biological.
An example of the distincti ...
which may occupy or obstruct the roadway beyond the edge of visibility,
negligence may not depend ''
ex post facto'' on what a driver happened to hit, could not have known, but had a concurrent duty to avoid.
Furthermore, modern knowledge of
human factors
Human factors and ergonomics (commonly referred to as human factors) is the application of psychological and physiological principles to the engineering and design of products, processes, and systems. Four primary goals of human factors learnin ...
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.
As common law rule or statute
Origins
As with the genesis of most legal doctrine governing problems which precede a legislative solution, the ACDA principle generally originates to decisional
precedent
A precedent is a principle or rule established in a previous legal case that is either binding on or persuasive for a court or other tribunal when deciding subsequent cases with similar issues or facts. Common-law legal systems place great valu ...
by high courts which
reasoned
Reason is the capacity of consciously applying logic by drawing conclusions from new or existing information, with the aim of seeking the truth. It is closely associated with such characteristically human activities as philosophy, science, lan ...
general common sense rules of conduct of which naturally follow from the repetitive process of determining specific culpability.
Legislation often subsequently followed which either superfluously codified and endorsed or revised these principles,
of which courts would in turn continue to flesh out the details.
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, with differing jurisdictions adopting the language to carry its same effects. Much of the earliest published record naturally pertains to high stakes wrecks
among vessels or vehicles
as defined in those times, though the obvious principle applies to
chariots and might in fact be
time immemorial
Time immemorial ( la, Ab immemorabili) is a phrase meaning time extending beyond the reach of memory, record, or tradition, indefinitely ancient, "ancient beyond memory or record". The phrase is used in legally significant contexts as well as ...
.
Present
Horses may still be expected to use the roadways, as well bicycles and automobiles.
The former are a regular appearance in both
urban areas
An urban area, built-up area or urban agglomeration is a human settlement with a high population density and infrastructure of built environment. Urban areas are created through urbanization and are categorized by urban morphology as cities, ...
and in the
country
A country is a distinct part of the world, such as a state, nation, or other political entity. It may be a sovereign state or make up one part of a larger state. For example, the country of Japan is an independent, sovereign state, while ...
, and are commonly exercised by
commuters
Commuting is periodically recurring travel between one's place of residence and place of work or study, where the traveler, referred to as a commuter, leaves the boundary of their home community. By extension, it can sometimes be any regu ...
and
Amish
The Amish (; pdc, Amisch; german: link=no, Amische), formally the Old Order Amish, are a group of traditionalist Anabaptist Christian church fellowships with Swiss German and Alsatian origins. They are closely related to Mennonite churc ...
. Many roads are unchanged since the 1800s while
controlled-access highways
A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway and expressway. Other similar terms i ...
have been invented specifically for the automobile. Ships now have
marine radar
Marine radars are X band or S band radars on ships, used to detect other ships and land obstacles, to provide bearing and distance for collision avoidance and navigation at sea. They are electronic navigation instruments that use a rotating ...
that allows one to view tens of miles beyond the eye. "At
common law
In law, common law (also known as judicial precedent, judge-made law, or case law) is the body of law created by judges and similar quasi-judicial tribunals by virtue of being stated in written opinions."The common law is not a brooding omnipresen ...
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
A statute is a formal written enactment of a legislative authority that governs the legal entities of a city, state, or country by way of consent. Typically, statutes command or prohibit something, or declare policy. Statutes are rules made by le ...
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."
Decisional law usually settles the circumstances by which a portion of the roadway is assuredly clear without it being mentioned in statute.
States where the judiciary has explicitly established the state's ACDA law include Indiana, Iowa,
Kansas, Louisiana, Michigan,
New York,
North Carolina,
Ohio,
Tennessee,
Vermont, Wisconsin,
and California.
[ Se]
California Official Reports: Online Opinions
/ref>
Many states have further passed statutes
A statute is a formal written enactment of a legislative authority that governs the legal entities of a city, state, or country by way of consent. Typically, statutes command or prohibit something, or declare policy. Statutes are rules made by le ...
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
In tort law, the standard of care is the only degree of prudence and caution required of an individual who is under a duty of care.
The requirements of the standard are closely dependent on circumstances. Whether the standard of care has been b ...
provisions include: Iowa, Michigan, Ohio, Oklahoma, Pennsylvania, and Texas.
States which apply the principle by statute to watercraft on navigable waterways include all 174 member states of the International Maritime Organization
The International Maritime Organization (IMO, French: ''Organisation maritime internationale'') is a specialised agency of the United Nations responsible for regulating shipping. The IMO was established following agreement at a UN conference ...
, notwithstanding membership: Great Britain and its common law inheriting Commonwealth of Nations
The Commonwealth of Nations, simply referred to as the Commonwealth, is a political association of 56 member states, the vast majority of which are former territories of the British Empire. The chief institutions of the organisation are the Co ...
, The United States, Florida, Hawaii, Illinois, Louisiana, Michigan, Montana, Oregon, Texas, and West Virginia.
Most state-issued and some Canadian driver handbooks instruct or mention the ACDA rule as required care or safe practice.
Explicit ACDA statutes and regulations, especially those of which create a citable driving or maritime 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. Unsafe speed statutes are immune from being void for vagueness when they contain explicit ACDA clauses. Explicit and implicit ACDA rules govern millions of North American drivers.
Universal standard of care
Not all jurisdictions have applied the rule uniformly, most often differing over exceptions for specific "sudden emergencies." There has been an increased interest in the ACDA codified as a universal standard of care
In tort law, the standard of care is the only degree of prudence and caution required of an individual who is under a duty of care.
The requirements of the standard are closely dependent on circumstances. Whether the standard of care has been b ...
that has been brought about by recent technological and social changes such as event data recorder
An event data recorder (EDR), more specifically motor vehicle event data recorder (MVEDR), similar to an accident data recorder (ADR) sometimes referred to informally as an automotive black box (by analogy with the common nickname for flight re ...
s, dashcams
A dashboard camera or simply dashcam, also known as car digital video recorder (car DVR), driving recorder, or event data recorder (EDR), is an onboard camera that continuously records the view through a vehicle's front windscreen and sometime ...
, self-driving cars
A self-driving car, also known as an autonomous car, driver-less car, or robotic car (robo-car), is a car that is capable of traveling without human input.Xie, S.; Hu, J.; Bhowmick, P.; Ding, Z.; Arvin, F.,Distributed Motion Planning for S ...
, safe cities and multi-use movements, and a movement to reduce claims by speeders against governments for "dangerous conditions" when operating speeds exceed a road's inferred design speed.
Collision liability has historically benefited the law profession by being cloaked as a mixture of fact and law, but with EDR's precisely preserving "a state of facts" often repeated with differing trial outcomes, collisions are less a question of fact, but of law. 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. Furthermore, the law needs to be clear, precise, and uniform at a national level for the panoply of automobile manufacturers with the strict liability
In criminal and civil law, strict liability is a standard of liability under which a person is legally responsible for the consequences flowing from an activity even in the absence of fault or criminal intent on the part of the defendant.
...
for their programming of law-abiding self-driving vehicles. It is foreseeable that two self-driving car makes can collide because their algorithm of the law letter is different; a resolvable issue that has been troubling human drivers for decades. The ACDA is a standard with descriptive mathematics, much of which are used in reverse by road engineers when designing or re-engineering roads to a speed criteria—for which its users were expected to follow.
Determining the ACDA
Static ACDA
Forward "line-of-sight" distance
The range of visibility of which is the ''de facto'' ACDA, is usually that distance before which an ordinary person can see small hazards—such as a traffic cone
Traffic cones, also called pylons, witches' hats, road cones, highway cones, safety cones, channelizing devices, construction cones, or just cones, are usually cone-shaped markers that are placed on roads or footpaths to temporarily redirect tra ...
or buoy
A buoy () is a floating device that can have many purposes. It can be anchored (stationary) or allowed to drift with ocean currents.
Types
Navigational buoys
* Race course marker buoys are used for buoy racing, the most prevalent form of yac ...
—with 20/20 vision. This distance may be attenuated by specific conditions such as atmospheric opacity,[ 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. Se]
California Official Reports: Online Opinions
/ref> blinding glare, darkness,[ Se]
California Official Reports: Online Opinions
/ref> road design
Highway engineering is an engineering discipline branching from civil engineering that involves the planning, design, construction, operation, and maintenance of roads, bridges, and tunnels to ensure safe and effective transportation of people and ...
,[ Se]
California Official Reports: Online Opinions
/ref> and adjacent environmental hazards including civil and recreational activities, horse-drawn vehicle, ridden animal, livestock, deer, crossing traffic, and parked cars. The ACDA may also be somewhat attenuated on roads with lower functional classification
The functional classification of a road is the class or group of roads to which the road belongs. There are three main functional classes as defined by the United States Federal Highway Administration
The Federal Highway Administration (FHWA) ...
.[
Se]
Official Reports Opinions Online
/ref>[
Se]
Official Reports Opinions Online
/ref>
This is because the probability
Probability is the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...
of spontaneous traffic increases proportionally to the density of road access points, and this density reduces the distance a person exercising ordinary care
In tort law, a duty of care is a legal obligation that is imposed on an individual, requiring adherence to a standard of reasonable care while performing any acts that could foreseeably harm others. It is the first element that must be establis ...
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.[
Se]
Huetter v. Andrews, 91 Cal. App. 2d 142
Berlin v. Violett, 129 Cal.App. 337
Reaugh v. Cudahy Packing Co., 189 Cal. 335
an
/ref> 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. During times of darkness, commercial vehicles can see ahead about 250 feet with low beams, and about 350–500 feet with high beams. This clear distance corresponds to a maximum safe speed of 52 mph and 65-81 mph respectively on dry pavement with good tires, which is attenuated further by convex
Convex or convexity may refer to:
Science and technology
* Convex lens, in optics
Mathematics
* Convex set, containing the whole line segment that joins points
** Convex polygon, a polygon which encloses a convex set of points
** Convex polytope ...
and lateral road curvature; safe speed is always dynamic. Non-commercial vehicles have even shorter lighting distances. Drivers commonly drive the maximum posted speed limit at night, often in violation of the ACDA rule and this shows up in accident data.
=Intersections
=
As a corollary to the rule that drivers generally must not pose an "immediate hazard" upon where or when they cannot assure such distance ahead is clear, it follows that others may presume that no vehicle is posing an "immediate hazard" from beyond where they can see with proper lookout. Where there are cross roads or side roads with view obstructions, the assured clear distance terminates at the closest path of potential users of the roadway until there is such a view which assures the intersection will remain clear. In such situations, approach speed must be reduced in preparation for entering or crossing a road or intersection or the unmarked pedestrian crosswalks and bike paths they create because of potential hazards. This jurisprudence
Jurisprudence, or legal theory, is the theoretical study of the propriety of law. Scholars of jurisprudence seek to explain the nature of law in its most general form and they also seek to achieve a deeper understanding of legal reasoning a ...
arises in-part because of the known difficulty in estimating the distance and velocity of an approaching vehicle, which is psychophysically explained by its small angular size
The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...
and belated divergence from an asymptotically null rate of expansion, which is beyond the subtended angular velocity detection threshold (SAVT) limits of visual acuity
Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal ...
by way of the Stevens' power law
Stevens' power law is an empirical relationship in psychophysics between an increased intensity or strength in a physical stimulus and the perceived magnitude increase in the sensation created by the stimulus. It is often considered to supersede ...
and Weber–Fechner law
The Weber–Fechner laws are two related hypotheses in the field of psychophysics, known as Weber's law and Fechner's law. Both laws relate to human perception, more specifically the relation between the actual change in a physical stimulus and ...
, until the vehicle may be dangerously close; subjective constancy and the visual angle illusion In human visual perception, the visual angle, denoted ''θ'', subtended by a viewed object sometimes looks larger or smaller than its actual value. One approach to this phenomenon posits a subjective correlate to the visual angle: the perceived vis ...
may also play a role. Vehicles that are approaching an intersection from beyond the SAVT limit cannot be reliably distinguished between moving or parked, though they may be traveling at such an imprudent speed as to pose an immediate hazard. 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, and any legal expectation to the contrary would implicate violating the vagueness doctrine
In American constitutional law, a statute is void for vagueness and unenforceable if it is too vague for the average citizen to understand, and a constitutionally-protected interest cannot tolerate permissible activity to be chilled within the ran ...
of the US Constitution
The Constitution of the United States is the supreme law of the United States of America. It superseded the Articles of Confederation, the nation's first constitution, in 1789. Originally comprising seven articles, it delineates the nation ...
. It is the duty of the through-driver to decelerate and apply the ACDA principle specifically to the intersection.
See Table of detection thresholds.
When approaching an un-signalized intersection controlled by a stop sign, the assured clear distance ahead is:
:
Normal acceleration "" for a passenger vehicle from a stop up to 20 mph is about 0.15g, with more than 0.3g being difficult to exceed. The distance "" is the sum of the measured limit line setback distance—which is typically regulated by a Manual on Uniform Traffic Control Devices
The ''Manual on Uniform Traffic Control Devices for Streets and Highways'' (usually referred to as the ''Manual on Uniform Traffic Control Devices'', abbreviated MUTCD) is a document issued by the Federal Highway Administration (FHWA) of the Unit ...
, at often between 4 and 30 feet in the United States—and the crosswalk
A pedestrian crossing (or crosswalk in American English) is a place designated for pedestrians to cross a road, street or avenue. The term "pedestrian crossing" is also used in the Vienna and Geneva Conventions, both of which pertain to road ...
, parking lane, and road shoulder
A shoulder, hard shoulder (British) or breakdown lane, is an emergency stopping lane by the verge of a road or motorway, on the right side in countries which drive on the right, and on the left side in countries which drive on the left. Many wid ...
width. A vehicle accelerating from a stop travels this distance in time while through traffic travels a distance equal to their speed multiplied by that time. The time , for the stopped motorist, is the sum of perception time and the time required to actuate an automatic transmission or shift to first gear which is usually between to one second.
ACDA as a function of horizontal sight distance
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. Horizontal sight distance is not to be confused with the clear recovery zone which provides hazardous vegetation set-back to allow errant vehicles to regain control, and is exclusive to a mowed and limbed-up forest which can allow adequate sight distance, but unsafe recovery. The height and lateral distance of plants restrict the horizontal sight distance, at times obscuring wildlife which may be spooked by an approaching vehicle and run across the road to escape with their herd
A herd is a social group of certain animals of the same species, either wild or domestic. The form of collective animal behavior associated with this is called ''herding''. These animals are known as gregarious animals.
The term ''herd'' is ...
. This principle also applies to approaching vehicles and pedestrians at uncontrolled intersections and to a lesser degree by un-signalized intersections controlled by a yield sign
In road transport, a yield or give way sign indicates that merging drivers must prepare to stop if necessary to let a driver on another approach proceed. A driver who stops or slows down to let another vehicle through has yielded the right of w ...
. Horizontal sight distance "" affects the ACDA because the time it takes for an intercepting object, animal, pedestrian, or vehicle with speed "" to transverse this distance after emerging from the proximate edge of lateral visibility affords a vehicle with speed "" a clear distance of "." Thus, the assured clear intercept distance "" is:
:
The faster one drives, the farther down-road an interceptor must be in order to be able to transverse the horizontal sight distance in time to collide, however this says nothing of whether the vehicle can stop by the end of this type of assured clear distance. Equating this distance to the total stopping distance and solving for speed yields one's maximum safe speed
Safe Speed was a United Kingdom-based pressure group that campaigned against speed cameras, claiming that it did so on the grounds of road safety.
History
The organisation was started in 2001 by Paul Smith, a former computer electronics engineer ...
as purely dictated by the horizontal sight distance.
Dynamic "following" distance
The ACDA may also be dynamic as to the moving distance past which a motorist can be assured to be 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: . 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. A common way to violate the dynamic ACDA is by tailgating
Tailgating is the action of a driver driving behind another vehicle while not leaving sufficient distance to stop without causing a collision if the vehicle in front stops suddenly.
The safe distance for following another vehicle varies depend ...
.
Measurement
The most accurate way to determine the ACDA is to directly measure it. Whereas this is impractical, sight distance formulas can be used with less direct measurements as rough baseline estimates. The empirical
Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...
assured clear distance ahead calculated with computer vision
Computer vision is an interdisciplinary scientific field that deals with how computers can gain high-level understanding from digital images or videos. From the perspective of engineering, it seeks to understand and automate tasks that the hum ...
, range finding
Length measurement, distance measurement, or range measurement (ranging) refers to the many ways in which length, distance, or slant range, range can be measurement, measured. The most commonly used approaches are the rulers, followed by transit ...
, traction control
A traction control system (TCS), also known as ASR (from german: Antriebsschlupfregelung, lit=drive slippage regulation), is typically (but not necessarily) a secondary function of the electronic stability control (ESC) on production motor vehicle ...
, and GIS
A geographic information system (GIS) is a type of database containing Geographic data and information, geographic data (that is, descriptions of phenomena for which location is relevant), combined with Geographic information system software, sof ...
, such as by properly programming computer hardware used in autonomous cars
A self-driving car, also known as an autonomous car, driver-less car, or robotic car (robo-car), is a car that is capable of traveling without human input.Xie, S.; Hu, J.; Bhowmick, P.; Ding, Z.; Arvin, F.,Distributed Motion Planning for S ...
, can be recorded to later produce or color baseline ACDA and safe speed maps for accident investigation, traffic engineering, and show disparities between safe speed and 85th percentile "operating" speed. Self-driving cars may have a higher safe speed than human driven vehicles for a given ACDA where computer perception-reaction times are nearly instantaneous.
Discretion
The Assured Clear Distance Ahead can be subjective to the baseline estimate of a reasonable person
In law, a reasonable person, reasonable man, or the man on the Clapham omnibus, is a hypothetical person of legal fiction crafted by the courts and communicated through case law and jury instructions.
Strictly according to the fiction, it is ...
or be predetermined by law. 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. Within the assured clear distance ahead, there is certainty that travel will be free from obstruction which is exclusive of a failure to appreciate a hazard. Collisions generally only occur within one's assured clear distance ahead which are "unavoidable" to them such that they have zero comparative negligence including legal acts of god and abrupt unforeseeably wanton negligence by another party. Hazards which penetrate one's proximate edge of clear visibility and compromise their ACDA generally require evasive action.
Drivers need not and are not required to precisely determine the maximum safe speed from real-time mathematical calculations of sight distances and stopping distances for their particular vehicle. Motor vehicle operators of average intelligence are constantly required to utilize their kinesthetic memory in all sorts of driving tasks including every time they brake to a full stop at a stop line in a panoply of conditions. Like throwing a softball, one does not have to mathematically calculate a trajectory or firing solution in order to hit a target with repeated accuracy. During the earliest stages of learning how to drive, one develops a memory of when to start braking (how long it takes) from various speeds in order to stop at the limit line. 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.
Seconds of distance to stop rule
Drivers and law enforcement alike can apply elementary level arithmetic towards a rule of thumb
In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
to estimate minimal stopping distance in terms of how many seconds of travel ahead at their current speed. For speed "" in miles per hour, this rule of thumb is as follows:
:::
If this distance is greater than the ACDA, they need to decelerate. While most experienced drivers develop a broad intuition
Intuition is the ability to acquire knowledge without recourse to conscious reasoning. Different fields use the word "intuition" in very different ways, including but not limited to: direct access to unconscious knowledge; unconscious cognition; ...
required by everyday braking, this rule of thumb can still benefit some to recalibrate expectations for rare hard braking, particularly from high speeds. Additional simple corrections can be made to compensate for the environment and driving ability. Read more about the Seconds of Distance to Stop Rule.
ACDA rule-specific case generalized to the Basic Speed Law
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. As mathematical statements are more precise than verbal statements alone, the relation of the ACDA as a subset
In mathematics, Set (mathematics), set ''A'' is a subset of a set ''B'' if all Element (mathematics), elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are ...
of the basic speed rule for land based vehicles may be objectively quantified as follows:
: