Vehicles often come into problem with other vehicles and pedestrians because their intended courses of travel intersect, and thereby interfere with each others pathway. The generic principle that establishes who has the right to go first is called "right of way", or "priority". It establishes who has the right to use the conflicting part of the road and who has to wait until the other does so.
Signs, signals, markings and other features are usually used to make priority explicit. Some signs, such as the stop sign, are almost universal. When there are no signs or markings, different rules are followed depending on the situation. These default priority rules differ between countries, and may even vary within countries. Trends toward uniformity are exemplified at an international level by the Vienna Convention on Road Signs and Signals, which advices standardized traffic control devices (signs, signals, and markings) for establishing the right of way where necessary.
Crosswalks (or pedestrian crossings) are common in populated areas, and may show that pedestrians have priority over vehicular traffic. In major modern cities, the traffic signal is used to establish the right of way on the busy roads. Its primary objective is to give each road a duration of time in which its traffic may use the intersection in an organized way. The intervals of time assigned for each road may be altered to take into account factors such as difference in volume of traffic, the needs of pedestrians, or other traffic signals. Pedestrian crossings may be located near other traffic control devices; if they are not also regulated in some way, vehicles must give priority to them when in use. Traffic on a public road usually has priority over other traffic such as traffic emerging from private access; rail crossings and drawbridges are typical exception.