Module 6 : Traffic Intersection Control

Lecture 30 : Uncontrolled Intersection

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	Categories of Intersection An intersection is a road junction where two or more roads either meet or cross at grade. This intersection includes the areas needed for all modes of travel: pedestrian, bicycle, motor vehicle, and transit. Thus, the intersection includes not only the pavement area, but typically the adjacent sidewalks and pedestrian curb cut ramps. All the road junctions designated for the vehicles to turn to different directions to reach their desired destinations. Traffic intersections are complex locations on any highway. This is because vehicles moving in different direction want to occupy same space at the same time. In addition, the pedestrians also seek same space for crossing. Drivers have to make split second decision at an intersection by considering his route, intersection geometry, speed and direction of other vehicles etc. A small error in judgment can cause severe accidents. It causes delay and it depends on type, geometry, and type of control. Overall traffic flow depends on the performance of the intersections. It also affects the capacity of the road. Therefore, both from the accident perspective and the capacity perspective, the study of intersections are very important by the traffic engineers. Intersection design can vary widely in terms of size, shape, number of travel lanes, and number of turn lanes. Basically, there are four types of intersections, determined by the number of road segments and priority usage. Priority Intersection: Occur where one of the intersecting roads is given definite priority over the other. The minor road will usually be controlled by some form of sing marking, such as stop or yield sign; thus ensuring that priority vehicles travailing on the main street will incur virtually no delay. Space sharing intersection: Are intended to permit fully equally priority and to permit continuous movement for all intersecting vehicle flows; example would be rotaries and other weaving areas. Time Sharing Intersection: Are those at which alternative flows are given the right of way at different point in time. This type of intersection is controlled by traffic signal or by police officer. Uncontrolled intersection: are the most common type of intersection usually occurs where the intersecting roads are relatively equal importance and found in areas where there is not much traffic shown in Fig. 1. At uncontrolled intersection the arrival rate and individuals drivers generally determine the manner of operation, while the resulting performance characteristics are derived from joint consideration of flow conditions and driver judgment and behavior patterns. In simplest terms, an intersection, one flow of traffic seeks gaps in the opposing flow of traffic. At priority intersections, since one flow is given priority over the right of way it is clear that the secondary or minor flow is usually the one seeking gaps. By contrast at uncontrolled intersection, each flow must seek gaps in the other opposing flow. When flows are very light, which is the case on most urban and rural roads large gaps exist in the flows and thus few situation arise when vehicles arrive at uncontrolled intersection less than 10 second apart or at interval close enough to cause conflicts. However when vehicles arrive at uncontrolled intersection only a few second apart potential conflicts exist and driver must judge their relative time relationships and adjusts accordingly. Generally one or both vehicles most adjust their speeds i.e. delayed somewhat with the closer vehicle most often taking the right of way; in a sense, of course, the earlier arriving vehicle has priority and in this instance when two vehicles arrive simultaneous, the rule of the road usually indicate priority for the driver on the right. The possibility of judgmental in these, informal priority situation for uncontrolled intersection is obvious. At an Uncontrolled intersection: Service discipline is typically controlled by signs (stop or yield signs) using two rules two way stop controlled intersection (TWSC) and all way stop controlled intersection (AWSC). Figure 1: Example showing uncontrolled intersection