Thursday, May 27, 2010

Perpendicular intersections

Perpendicular intersections Also known as a "four-way" intersection, this intersection is the most common arrangement for roads that cross each other, and the most basic type.

If traffic signals do not control a 4-way intersection, signs or other features are typically used to control movements and make clear priorities. The most common placement is to indicate that one road has priority over the other, but there are complex cases where all traffic approaching an intersection must yield and may be required to stop.

In the United States, South Africa, and Canada, there are four-way intersections with a stop sign at every entrance, called four-way stops. A failed signal or a flashing red light is the same as a four-way stop, or an all-way stop. Significant rules for four-way stops may include:

1. In the countries that use four-way stops, pedestrians always have priority at crosswalks – even at unmarked ones, which exist as the logical continuations of the sidewalks at every intersection with approximately right angles – unless signed or painted otherwise.
2. Whichever vehicle first stops at the stop line – or before the crosswalk, if there is no stop line – has priority.
3. If two vehicles stop at the same time, priority is given to the vehicle on the right.
4. If three vehicles stop at the same time, priority is given to the two vehicles going in opposite directions, if possible.
5. If four vehicles stop, drivers usually use gestures and other communication to establish right-of-way.

In Europe and other places, there are similar intersections. These may be marked by special signs (according to the Vienna Convention on Road Signs and Signals), a danger sign with a black X representing a crossroads. This sign informs drivers that the intersection cannot be controlled and that default(custom rules) apply. In Europe and in many areas of North America the default rules that apply at uncontrolled four-way intersections are almost the same:

1. Rules for pedestrians differ by country, in the United States and Canada pedestrians generally have priority at such an intersection.
2. All vehicles must give priority to any traffic approaching from their right,
3. Then, if the vehicle is turning right or continuing on the same road it may continue.
4. Vehicles turning left must also give priority to traffic approaching from the opposite direction, unless that traffic is also turning left.
5. If the intersection is congested, vehicles must alternate directions and/or circulate priority to the right one vehicle at a time.

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