civil engineering Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewage ...

, 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 ke ...

and the structure gauge in the case of

railroad car A railroad car, railcar (American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and UIC), also called a train car, train wagon, train carriage or train truck, is a ...

s or

tram A tram (called a streetcar or trolley in North America) is a rail vehicle that travels on tramway tracks on public urban streets; some include segments on segregated right-of-way. The tramlines or networks operated as public transport are ...

s, or the difference between the size of any

vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), wa ...

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

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

or the

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

of a tunnel as well as the

air draft Air draft (or air draught) is the 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 hull below th ...

under a

bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...

, the width of a lock or diameter of a tunnel in the case of

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, ...

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

stream bed A stream bed or streambed is the bottom of a stream or river (bathymetry) or the physical confine of the normal water flow (Channel (geography), channel). The lateral confines or channel margins are known as the stream Bank (geography), banks ...

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 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 languages. A first distinction is necessary b ...

. For roadways 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 structure gauge. 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, freight and passenger cars (or coaches), and non-revenue cars. Passenger vehicles can b ...

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 or card. Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a shor ...

". 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 establish 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 to ...

which included minimum vertical clearance of . The

Department of Defense Department of Defence or Department of Defense may refer to: Current departments of defence * Department of Defence (Australia) * Department of National Defence (Canada) * Department of Defence (Ireland) * Department of National Defense (Philippin ...

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 government agency in the American state of Texas. Though the public face of the agency is generally associated with the construction and maintenance of the state's immense state highway system ...

estimated in 2013 that an average cost to repair a bridge strike is $180,000 USD. A high

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

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 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 in Frauenfeld, Switzerland, only after several incidents. A similar situation exists at an underpass on Guy Street in

Montreal Montreal ( ; officially Montréal, ) is the List of the largest municipalities in Canada by population, second-most populous city in Canada and List of towns in Quebec, most populous city in the Provinces and territories of Canada, Canadian ...

, 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 . It is to be noted that 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 quality control, protocols, and guidelines that are used in highway design and construction through ...

(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 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 "Class III bikewa ...

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. It now also refers to posts installed to control road traffic and posts designed to prevent automotive v ...

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 (, ; Dutch: ''Vlaanderen'' ) is the Flemish-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, ...

, 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 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 Span is the distance between two intermediate supports for a structure, e.g. a beam or a bridge. A span can be closed by a solid beam or by a rope. The first kind is used for bridges, the second one for power lines, overhead telecommunication lines ...

. 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