civil engineering Civil engineering is a regulation and licensure in engineering, professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads ...

, clearance refers to the difference between the

loading gauge A loading gauge is a diagram or physical structure that defines the maximum height and width dimensions in railway vehicles and their loads. Their purpose is to ensure that rail vehicles can pass safely through tunnels and under bridges, and k ...

and the

structure gauge A structure gauge, also called the minimum structure outline, is a diagram or physical structure that sets limits to the extent that bridges, tunnels and other infrastructure can encroach on rail vehicles. It specifies the height and width of s ...

in the case of

railroad car A railroad car, railcar (American English, American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and International Union of Railways, UIC), also called a tra ...

s or

tram A tram (also known as a streetcar or trolley in Canada and the United States) is an urban rail transit in which Rolling stock, vehicles, whether individual railcars or multiple-unit trains, run on tramway tracks on urban public streets; some ...

s, or the difference between the size of any

vehicle A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered land vehicle, human-powered vehicles (e.g. bicycles, tricycles, velo ...

and the width/height of doors, the width/height of an

overpass An overpass, called an overbridge or flyover (for a road only) in the United Kingdom and some other Commonwealth countries, is a bridge, road, railway or similar structure that is over another road or railway. An ''overpass'' and '' underpa ...

or the

diameter In geometry, a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest Chord (geometry), chord of the circle. Both definitions a ...

of a

tunnel A tunnel is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two portals common at each end, though there may be access and ve ...

as well as the

air draft Air draft (or air draught) is the vertical distance from the surface of the water to the highest point on a vessel. This is similar to the '' deep draft'' of a vessel which is measured from the surface of the water to the deepest part of the h ...

under a

bridge A bridge is a structure built to Span (engineering), span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, whi ...

, the width of a

lock Lock(s) or Locked may refer to: Common meanings *Lock and key, a mechanical device used to secure items of importance *Lock (water navigation), a device for boats to transit between different levels of water, as in a canal Arts and entertainme ...

or diameter of a tunnel in the case of

watercraft A watercraft or waterborne vessel is any vehicle designed for travel across or through water bodies, such as a boat, ship, hovercraft, submersible or submarine. Types Historically, watercraft have been divided into two main categories. *Raf ...

. In addition, there is the difference between the deep draft and the

stream bed A streambed or stream bed is the bottom of a stream or river and is confined within a Stream channel, channel or the Bank (geography), banks of the waterway. Usually, the bed does not contain terrestrial (land) vegetation and instead supports d ...

sea bed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

of a

waterway A waterway is any Navigability, navigable body of water. Broad distinctions are useful to avoid ambiguity, and disambiguation will be of varying importance depending on the nuance of the equivalent word in other ways. A first distinction is ...

. For

roadway A carriageway (British English) or roadway (North American English) is a width of road on which a vehicle is not restricted by any physical barriers or separation to move laterally. A carriageway generally consists of a number of traffic lane ...

s and waterways, the clearance is typically specified as the width/height of a structure that the vehicle needs to pass instead of the difference between the vehicle and the structure.

Railways

In railways, clearance is the difference between the

and the

. A clearance standard is established using static

rolling stock The term rolling stock in the rail transport industry refers to railway vehicles, including both powered and unpowered vehicles: for example, locomotives, Railroad car#Freight cars, freight and Passenger railroad car, passenger cars (or coaches) ...

outline (static gauge) as the starting point. This is a cross-sectional outline of a maximum size rolling stock when it is not running. The standard then defines maximum kinematic rolling stock outline for when rolling stocks are running to account for suspension and lateral motion on the track. This is also known as "kinematic

envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter (message), letter or Greeting card, card. Traditional envelopes are made from sheets of paper cut to one o ...

". The standard also defines base operating standard for clearance which is larger than the kinematic envelope. This should be maximum outline of the normal rail operation and can only be infringed in special circumstances. The standard then adds another outline called maintenance intervention standard outline that larger than the base operating standard by defining a safety margin (contingency gap) from the kinematic envelope. When there is an infringement of this outline, a maintenance work is required to bring to clearance standard. This establishes the loading gauge. Finally, the standard includes structure outline or structure gauge, leaving a space between the loading gauge and structure gauge as clearance.

Roadways

Vertical clearance

In roadways, vertical clearance is the measurement from the ground or the road pavement to the bottom of overpasses or bridges.

American Association of State Highway Officials The American Association of State Highway and Transportation Officials (AASHTO) is a standards setting body which publishes specifications, test protocols, and guidelines that are used in highway design and construction throughout the United St ...

(AASHO) established

Interstate Highway standards Standards for Interstate Highways in the United States are defined by the American Association of State Highway and Transportation Officials (AASHTO) in the publication ''A Policy on Design Standards: Interstate System''. For a certain highway t ...

which included minimum vertical clearance of . The

Department of Defense The United States Department of Defense (DoD, USDOD, or DOD) is an executive department of the U.S. federal government charged with coordinating and supervising the six U.S. armed services: the Army, Navy, Marines, Air Force, Space Force, ...

later informed that the clearance was not sufficient for national defense purposes and wanted the vertical clearance to be raised to . Eventually, the new standards were approved in 1960 to have the minimum vertical clearance of new structures to be . There were up to 2,650 existing overpasses in 1967 that were not in compliance with the new standards. The decision was made to only raise those 350 overpasses that served up to 95% of major military installations. Other overpasses were left to be reconstructed to the new minimum vertical clearance at later times. Australia defines minimum vertical clearance based on types of roads. The minimum vertical clearance is for main roads and highways, and for other local roads with road authority approval. For high and very high clearance roads, the values are between and . Eurocode 1: Actions on structures has a definition of "physical clearance" between roadway surface and the underside of bridge element. The code also defines the clearance that is shorter than the physical clearance to account for sag curves, bridge

deflection Deflection or deflexion may refer to: Board games * Deflection (chess), a tactic that forces an opposing chess piece to leave a square * Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers Mechanics * Deflection ...

and expected settlements) with a recommendation of minimum clearance of . In Singapore, the minimum vertical clearance is . The clearance for overhead signs is and the clearance for the soffit or underside of overpasses is . In South Africa and the southern region of Africa, the minimum vertical clearance of modern bridges is , although the legal height limit of road vehicles is still at . United Kingdoms has a standard on minimum clearance of a public highway at . Any bridges that do not meet the clearance requirement are considered to be "low bridges" and they require to have signage to indicate the clearance.

Bridge and tunnel strikes

Bridge or tunnel strikes are collisions of vehicles with bridge or tunnel structures. These may involve over-height vehicles, or low vertical clearance bridges or tunnels. These accidents occur frequently and are a major issue worldwide. In United Kingdom, railway bridge strikes happen on an average of once every four and half hours with total of 1789 times in 2019. Several bridges being hit over 20 times in a single year. The total cost borne by the state was around £23 million. In Beijing, China, 20% of all bridge damages are caused by bridge strikes.

Texas Department of Transportation The Texas Department of Transportation (TxDOT ) is a Texas state government agency responsible for construction and maintenance of the state's immense Texas state highway system, state highway system and the support of the state's maritime trans ...

estimated in 2013 that an average cost to repair a bridge strike is US$180,000. A high overpass bridge near St Petersburg, Russia, is known as the "Bridge of Stupidity" because it is often struck by vehicles despite many warning signs. In May 2018, after it was struck for the 150th time by a

GAZelle A gazelle is one of many antelope species in the genus ''Gazella'' . There are also seven species included in two further genera; '' Eudorcas'' and '' Nanger'', which were formerly considered subgenera of ''Gazella''. A third former subgenus, ' ...

truck, a birthday cake was presented to the bridge. This made national news. Similarly, an

in Durham, North Carolina, US, was frequently struck by vehicles, and made the news a number of times until it was raised in 2019. Infrared sensors, which trigger warning signs when a high vehicle approaches, were added to an

underpass A subway, also known as an underpass, is a grade-separated pedestrian crossing running underneath a road or railway in order to entirely separate pedestrians and cyclists from motor or train traffic. Terminology In the United States, as ...

Frauenfeld Frauenfeld (Alemannic German, Alemannic: ''Frauefäld'') is the capital of the Cantons of Switzerland, canton of Canton of Thurgau, Thurgau in Switzerland. The official language of Frauenfeld is (the Swiss variety of Standard) Swiss Standard Ge ...

, Switzerland, only after several incidents. A similar situation exists at an

Guy Street Guy Street (officially in ) is a north-south street located in downtown Montreal, Quebec, Canada. Concordia University's Integrated Engineering, Computer Science and Visual Arts Complex is located on this street, as is the John Molson School of B ...

Montreal Montreal is the List of towns in Quebec, largest city in the Provinces and territories of Canada, province of Quebec, the List of the largest municipalities in Canada by population, second-largest in Canada, and the List of North American cit ...

, which has a clearance of .

Horizontal clearance

In United States, the term "horizontal clearance" is used interchangeably with "lateral offset". This is the space from the edge of the roadway that is clear from vertical obstructions such as sign posts, utility poles, and fire hydrants. The horizontal clearance is used in urban environments where these objects are expected to be near roadways. The horizontal clearance are to prevent overhung elements such as side mirrors of large vehicles driven at the extreme edge of the roads to hit such objects. It also allows opening curbside doors of parked vehicles. Minimum horizontal clearance in US standard is . Horizontal clearance is not the same concept as clear zone which is used in non-urban highways. Some countries have specific horizontal clearances from the edge of the roads for specific types of objects next to the roads. For example, India has horizontal clearance of for electrical and telecommunication poles, and for street light poles of roads with curbs. For roads without curbs, the clearance for that is given that the minimum clearance from the center line of the roads is . For roadways that require passing under some structures such as tunnels, there are standards on the entire width of the roads known as horizontal curb-to-curb and wall-to-wall clearances.

American Association of State Highway and Transportation Officials The American Association of State Highway and Transportation Officials (AASHTO) is a standards setting body which publishes specifications, test protocols, and guidelines that are used in highway A highway is any public or private road ...

(AASHTO) recommends having minimum curb-to-curb clearance for two-lane highways of and wall-to-wall clearance of , while desired curb-to-curb clearance should be , and wall-to-wall clearance should be . For

bike path A bike path or a cycle path is a bikeway separated from motorized traffic and dedicated to cycling or shared with pedestrians or other non-motorized users. In the US a bike path sometimes encompasses '' shared use paths'', "multi-use path", or ...

s, there are two types of horizontal clearances. The first type is the horizontal clearance of any obstacles on the paths. An example is the use of

bollard A bollard is a sturdy, short, vertical post. The term originally referred to a post on a ship or quay used principally for mooring boats. In modern usage, it also refers to posts installed to control road traffic and posts designed to pre ...

s to prevent cars from entering bike paths. The horizontal clearance on the paths defines a minimum clearance of adjacent obstacles such as those bollards to allow clear flowing of bike traffic. Some European countries have that specification between and . The second type is horizontal clearance next to the paths. This clearance is a distance from the edge of the biking paths to any vertical obstacles such as poles, fences, and tree branches to prevent pedal or handlebar from hitting such obstacles. The clearance values may depend on the heights and types of the obstacles. For example, in

Flanders Flanders ( or ; ) is the Dutch language, Dutch-speaking northern portion of Belgium and one of the communities, regions and language areas of Belgium. However, there are several overlapping definitions, including ones related to culture, la ...

, the horizontal clearance next to the paths for poles, lampposts and trees is , and for walls and fences is . The horizontal clearance from the edge of the path to the

curb A curb (American English) or kerb (British English) is the edge where a raised sidewalk/pavement or road median/central reservation meets a street/other roadway. History Although curbs have been used throughout modern history, and indeed ...

is based on curb heights. For curbs of heights up to , the horizontal clearance is , and for higher curbs, the clearance is .

Waterways

In waterways, "bridge span clearance" is a measurement from water surface to the underside of bridge span. The most conservative clearance uses the water level at the mean highest high water (MHHW), the average value of the highest high tide of a measurement period. This is known as "clearance below", "vertical clearance" and "charted height" On other hand, the "overhead clearance" is a measurement from the top most part of a given vessel to the underside of the bridge. The "underkeel clearance" is the distance between the lowest part of a given vessel to the waterway bed. The other type of clearances for the bridges is "clearance above" the bridge floor. This is the vertical clearances for road traffic on the bridge.

References

{{Reflist Engineering concepts Approximations