In urban areas, the capacity of two-lane streets can be increased by adding a continuous third lane which can be used for left turns from both of the opposing lanes of traffic flow. On lightly travelled rural roads; on heavily travelled urban streets, left-turn movements can cause notable delays in traffic flow. If the stop sign is at an intersection which has considerable traffic on a cross street, the traffic flow on the major street will be diminished substantially more. The "Highway Capacity Manual" is generally considered to be the basic source of information on the analysis of intersections to determine if traffic signals are warranted, and on the timing of the cycles of traffic signals. Pedestrians crossing at unsignalized intersections of lightly travelled rural roads rarely cause a significant interruption to vehicular traffic. The impact on traffic flow by pedestrian crossings can be substantial. The volume of pedestrian crossings is often considered when estimating the capacity of urban streets.