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Aroad junction is a type ofjunction where two or moreroads meet.
Roads are a means of transport, historically linking locations such astowns,forts and geographic features such as riverfords. Where roads met outside of an existing settlement, these junctions often led to a new settlement.Scotch Corner is an example of such a location.
In the United Kingdom and other countries, the practice of giving names to junctions emerged, to help travellers find their way. Junctions often took the name of a prominent nearby business or a point of interest.
As road networks andtraffic flows increased in density, managing the flow of traffic through junctions became of increasing importance, to minimize delays and improve safety. The first innovation was to addtraffic control devices, such asstop signs andtraffic lights that regulated traffic flow. Next came lane controls that limited what each lane of traffic was allowed to do while crossing. Turns across oncoming traffic might be prohibited, or allowed only when oncoming and crossing traffic was stopped.
This was followed by specialized junction designs that incorporated information about traffic volumes, speeds, driver intent and many other factors.
The most basic distinction among junction types is whether or not the roads cross at the same or differentelevations. More expensive, grade-separatedinterchanges generally offer higher throughput at higher cost. Single-gradeintersections are lower cost and lower throughput. Each main type comes in many variants.[1]
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Atinterchanges, roads pass above or below each other, usinggrade separation andslip roads. The termsmotorway junction andhighway interchange typically refer to this layout. They can be further subdivided into those with and without signal controls.
Signalized (traffic-light controlled) interchanges include "diamond" designs such as theregular diamond,diverging diamond,three-level diamond, andtight diamond. Others include thecontraflow left,single-point urban, andpartial cloverleaf interchanges.[2]
Non-signalized designs include bothfree-flow interchanges and non-free-flow ones. Free-flow variants include thecloverleaf,stack,double crossover merging,[3] andwindmill. Non-free-flow variants include thedogbone (featuring tworaindrop roundabouts),dumbbell (featuring two standardroundabouts), androundabout interchange (featuring one largeroundabout).[2]
Autobahnkreuz (literally "autobahn cross"), short formKreuz, and abbreviated as AK, is a four-way interchange on the Germanautobahn network.Autobahndreieck (literally "autobahn triangle"), short formDreieck, and abbreviated as AD, is a three-way interchange on the German autobahn network.
Atintersections, roads crossat-grade. They also can be further subdivided into those with and without signal controls.
Signalized designs include theadvanced stop line,bowtie,box junction,continuous-flow,[4] double-wide,hook turn,jughandle,Michigan left (median U-turn), paired,quadrant,seagull (continuous green T),slip lane,split,staggered,superstreet, andTexas T.[2]
Non-signalized designs include the Maryland T/J,roundabout, andtraffic circle, as well as theTexas U-turn (a type ofturnaround) and unsignalized variations on the continuous-flow (3-leg and 4-leg), median U-turn, and superstreet.[2]
In the EU, it is estimated that around 5,000 of the 26,100 car crash fatalities in 2015 occurred in a junction collision, while it was around 8,000 in 2006.[5] Therefore, during the 2006–2015 decade, around 20% of EU road fatalities occurred at junctions.[5] Among these junction fatalities, 34% were car users, 23% were pedestrians, 21% were motorcyclists, 12% were cyclists, and the remaining were other types of road users.[5]
It has been considered that several causes might lead to fatalities; for instance:[5]
A number of features make thisprotected intersection muchsafer. A corner refuge island, a setback crossing of the pedestrians andcyclists, generally between 1.5–7 metres of setback, a forward stop bar, which allows cyclists to stop for atraffic light well ahead of motor traffic who must stop behind the crosswalk. Separate signal staging or at least an advance green for cyclists and pedestrians is used to give cyclists and pedestrians no conflicts or a head start over traffic. The design makes a right turn on red, and sometimes left on red depending on the geometry of the intersection in question, possible in many cases, often without stopping.[6]
Cyclists ideally have aprotected bike lane on the approach to the intersection, separated by a concrete median with splaykerbs if possible, and have a protected bike lane width of at least 2 metres if possible (one way). In theNetherlands, most one way cycle paths are at least 2.5 metres wide.[7]
Bicycle traffic can be accommodated with the low gradebike lanes in the roadway or higher grade and much saferprotected bicycle paths that are physically separated from the roadway.
In Manchester, UK, traffic engineers have designed a protected junction known as the Cycle-Optimised Signal (CYCLOPS) Junction. This design places a circulatory cycle track around the edge of the junction, with pedestrian crossing on the inside. This design allows for an all-red pedestrian / cyclist phase with reduced conflicts. Traffic signals are timed to allow cyclists to make a right turn (across oncoming traffic) in one turn). It also allows for diagonal crossings (pedestrian scramble) and reduces crossing distances for pedestrians.[8]
Intersections generally must managepedestrian as well as vehicle traffic. Pedestrian aids includecrosswalks, pedestrian-directed traffic signals ("walk light") and over/underpasses. Walk lights may be accompanied by audio signals to aid the visually impaired. Medians can offer pedestrian islands, allowing pedestrians to divide their crossings into a separate segment for each traffic direction, possibly with a separate signal for each.