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chapter7_notes_ - Chapter 7 Traffic Control and Analysis at...

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Chapter 7 Traffic Control and Analysis at Signalized Intersections 7.1 INTRODUCTION Conflicting traffic movements, make roadway intersections are a source of great concern to traffic engineers. Intersections are a major source of crashes and vehicle delay (as vehicles yield to avoid conflicts with other vehicles). Most roadway intersections are not signalized due to low traffic volumes and adequate sight distances. At some point, traffic volumes and crash frequency/severity (and other factors) reach a level that warrant the installation of a traffic signal. Why signalization? Advantages: potential reduction of some types of crashes (particularly angle crashes), provisions for pedestrians to cross the street, provisions for side-street vehicles to enter the traffic stream, provisions for the progressive flow of traffic in a signal-system corridor, possible improvements in capacity, and possible reductions in delay. Disadvantages: poorly-timed signal or one that is not justified can have a negative impact on the operation of the intersection by: increasing vehicle delay, increasing vehicle crashes (particularly rear-end crashes), causing a disruption to traffic progression (adversely impacting the through movement of traffic), and encouraging the use of routes not intended for through traffic (such as routes through residential neighborhoods). Traffic signals considerations: Cost: in excess of $100,000. Warrants: Manual on Uniform Traffic Control Devices (MUTCD) [Millenium Edition, 2000] Eight warrants dealing with vehicle volumes, pedestrian volumes, school crossings, signal coordination, and crash experience. 7.2 INTERSECTION AND SIGNAL CONTROL CHARACTERISTICS Definitions: Intersection : at-grade crossing of two or more roadways.
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2 Approach consists of a lane or group of lanes through which vehicles enter the intersection. Lane group is an individual lane or multiple lanes which are grouped based on the allowed movements (left, through, right) within each lane and the sequencing of allowed movements by the traffic signal. In this figure: Approach 1: a lane for the exclusive use of left turns, a lane for the exclusive use of right turns, and two lanes for the exclusive use of through movements. Approach 3 like approach 1, but: Right turns share the outside lane with the through movements. Note: Turn bays, hold a limited number of queued vehiles. Queuing analysis used to determine the length of bay necessary to prevent: Spillover queued turning vehicles overflowing the bay and blocking the through lanes and/or Spillback queued through vehicles blocking the entrance of the turn bay (known as spillback). Approach 2 consists of a shared through/right-turn lane and an exclusive left turn lane (not a bay in this case because it extends upstream).
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