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Intersection at Tverskaya Zastava Square in Moscow, Russia
Intersection at Tverskaya Zastava Square in Moscow, Russia
The intersection between Ayala Avenue and Makati Avenue in Makati, Philippines
The intersection between Ayala Avenue and Makati Avenue in Makati, Philippines
An intersection in rural Grande Champagne, France
An intersection in rural Grande Champagne, France

An intersection or an at-grade junction is an junction where two or more roads converge, diverge, meet or cross at the same height, as opposed to an interchange, which uses bridges or tunnels to separate different roads. Major intersections are often delineated by gores and may be classified by road segments, traffic controls and lane design.

Types

Road segments

One way to classify intersections is by the number of road segments (arms) that are involved.

Intersection along the Veterans Memorial Parkway, an at-grade limited-access road in London, Ontario.
Intersection along the Veterans Memorial Parkway, an at-grade limited-access road in London, Ontario.

Traffic controls

Another way of classifying intersections is by traffic control technology:

Lane design

Turns

At intersections, turns are usually allowed, but are often regulated to avoid interference with other traffic. Certain turns may be not allowed or may be limited by regulatory signs or signals, particularly those that cross oncoming traffic. Alternative designs often attempt to reduce or eliminate such potential conflicts.

Turn lanes

At intersections with large proportions of turning traffic, turn lanes (also known as turn bays)[2] may be provided. For example, in the intersection shown in the diagram,[clarification needed] left turn lanes are present in the right-left street.

Turn lanes allow vehicles to cross oncoming traffic (i.e., a left turn in right-side driving countries, or a right turn in left-side driving countries), or to exit a road without crossing traffic (i.e., a right turn in right-side driving countries, or a left turn in left-side driving countries). Absence of a turn lane does not normally indicate a prohibition of turns in that direction. Instead, traffic control signs are used to prohibit specific turns.[3]

Turn lanes can increase the capacity of an intersection or improve safety. Turn lanes can have a dramatic effect on the safety of a junction. In rural areas, crash frequency can be reduced by up to 48% if left turn lanes are provided on both main-road approaches at stop-controlled intersections. At signalized intersections, crashes can be reduced by 33%. Results are slightly lower in urban areas.[4]

Turn lanes are marked with an arrow bending into the direction of the turn which is to be made from that lane. Multi-headed arrows indicate that vehicle drivers may travel in any one of the directions pointed to by an arrow.

Turn signals

Traffic signals facing vehicles in turn lanes often have arrow-shaped indications. North America uses various indication patterns. Green arrows indicate protected turn phases, when vehicles may turn unhindered by oncoming traffic. Red arrows may be displayed to prohibit turns in that direction. Red arrows may be displayed along with a circular green indication to show that turns in the direction of the arrow are prohibited, but other movements are allowed. In some jurisdictions, a red arrow prohibits a turn on red.[5] In Europe, if different lanes have differing phases, red, yellow and green traffic lights corresponding to each lane have blacked-out areas in the middle in the shape of arrows indicating the direction(s) drivers in that lane may travel in. This makes it easier for drivers to be aware which traffic light they need to pay attention to. A green arrow may also be provided; when it is on, drivers heading in the direction of the arrow may proceed, but must yield to all other vehicles. This is similar to the right turn on red in the US.[6]

Disadvantages to turn lanes include increased pavement area, with associated increases in construction and maintenance costs, as well as increased amounts of stormwater runoff. They also increase the distance over which pedestrians crossing the street are exposed to vehicle traffic. If a turn lane has a separate signal phase, it often increases the delay experienced by oncoming through traffic. Without a separate phase, left crossing traffic does not get the full safety benefit of the turn lane.

Lane management

Alternative intersection configurations, formerly called unconventional intersections, can manage turning traffic to increase safety and intersection throughput.[7] These include the Michigan left/Superstreet (RCUT/MUT) and continuous flow intersection (CFI/DLT), to improve traffic flow, and also interchange types like Diverging diamond interchange (DDI/DCD) design as part of the Federal Highway Administration's Every Day Counts initiative which started in 2012.[8]

Diagram of an example intersection of two-way streets as seen from above (traffic flows on the right side of the road). The east-west street has left turn lanes from both directions, but the north-south street does not have left turn lanes at this intersection. The east-west street traffic lights also have green left turn arrows to show when unhindered left turns can be made. Some possible markings for crosswalks are shown as examples.
Diagram of an example intersection of two-way streets as seen from above (traffic flows on the right side of the road). The east-west street has left turn lanes from both directions, but the north-south street does not have left turn lanes at this intersection. The east-west street traffic lights also have green left turn arrows to show when unhindered left turns can be made. Some possible markings for crosswalks are shown as examples.

Vulnerable road users

Pedestrians

Times Square is the hub of the Broadway theater district and a major cultural venue in Midtown Manhattan, New York City. The pedestrian intersection also has one of the highest annual attendance rates of any tourist attraction in the world, estimated at 60 million.[9]
Times Square is the hub of the Broadway theater district and a major cultural venue in Midtown Manhattan, New York City. The pedestrian intersection also has one of the highest annual attendance rates of any tourist attraction in the world, estimated at 60 million.[9]

Intersections generally must manage pedestrian as well as vehicle traffic. Pedestrian aids include crosswalks, pedestrian-directed traffic signals ("walk light") and over/underpasses. Traffic signals can be time consuming to navigate, especially if programmed to prioritise vehicle flow over pedestrians, while over and underpasses which rely on stairs are inaccessible to those who can't climb them. 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.

Famous intersection of Highways 61 and 49 in Clarksdale, Mississippi sung about by Robert Johnson
Famous intersection of Highways 61 and 49 in Clarksdale, Mississippi sung about by Robert Johnson

Some intersections display red lights in all directions for a period of time. Known as a pedestrian scramble, this type of vehicle all-way stop allows pedestrians to cross safely in any direction, including diagonally. All green for non motorists is known from the crossing at Shibuya Station, Tokyo.[10]

Cyclists and motorcyclists

Poor visibility at junctions can lead to drivers colliding with cyclists and motorcyclists. Some junctions use advanced stop lines which allow cyclists to filter to the front of a traffic queue which makes them more visible to drivers.

Safety

A European study found that in Germany and Denmark, the most important crash scenario involving vulnerable road users was:

At grade railways

In the case of railways or rail tracks the term at grade applies to a rail line that is not on an embankment nor in an open cut. As such, it crosses streets and roads without going under or over them. This requires level crossings. At-grade railways may run along the median of a highway. The opposite is grade-separated. There may be overpasses or underpasses.

See also

References

  1. ^ Steyn, Hermanus. (2014). Displaced Left-turn Intersection Informational Guide. Washington, D.C.: U.S. Department of Transportation, Federal Highway Administration.
  2. ^ "Manual on Uniform Traffic Control Devices (MUTCD), Part 1" (PDF). U.S. DOT, Federal Highway Administration. December 11, 2009. Retrieved November 28, 2011.
  3. ^ "Traffic Signals Brochure - How is it Determined if a Traffic Signal is Needed?". Maryland State Highway Administration.
  4. ^ D.W. Harwood, et al., Safety Effectiveness of Intersection Left- and Right-Turn Lanes, Federal Highway Administration Office of Safety Research and Development, 2002, [1]
  5. ^ New York State Driver's Manual, Chapter 4.
  6. ^ Vienna Convention on Road Signs and Signals
  7. ^ Badger, Emily. "Could These Crazy Intersections Make Us Safer?". The Atlantic Cities. Atlantic Media Company. Retrieved 29 January 2013.
  8. ^ https://rosap.ntl.bts.gov/view/dot/29474/dot_29474_DS1.pdf?download-document-submit=Download
  9. ^ Rachel Sugar (December 19, 2016). "More tourists visited NYC in 2016 than ever before". Vox Media. Retrieved April 24, 2018.
  10. ^ "Saturday afternoon at Shibuya Crossing, Tokyo". April 21, 2007. Archived from the original on 2021-11-14.
  11. ^ "Indev Project -" (PDF).