8_intersection_basic-2

8_intersection_basic-2 - Topic: Fundamentals of Signal...

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Topic: Fundamentals of Signal Design and Timing Henry Liu CE 4211/5211 – Traffic Engineering University of Minnesota email: [email protected]
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Four Basic Mechanism 1. Discharge headways at a signalized intersection 2. Critical lane and time budget concepts 3. Effects of left-turning vehicles 4. Delay and other measures of effectiveness
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Discharge Headways, Saturation Flow Rates, Lost Times, and Capacity (1) The discharge headway is based upon expected headway after the point at which headways stabilize. This headway stabilizes at 2.14 seconds for the sixth and subsequent vehicles. The saturation flow rate is computed as: s = 3600/h where s = saturation flow rate (vphgpl), h = discharge headway (sec/veh) The start-up lost time is estimated by the sum of the differences between the observed headway for each of the vehicles before the headway stabilizes.
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Discharge Headways, Saturation Flow Rates, Lost Times, and Capacity (2) ( ) ( ) = = h h i e l i 1
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Discharge Headways, Saturation Flow Rates, Lost Times, and Capacity (3) ( ) ( ) = = h h i e l i 1 Start-up lost time Time req’d to clear N veh. ( ) N h l T + = 1 ( ) N T 1 . 2 7 . 3 + = Greenshields (1947) Updated Greenshields (1978) ( ) N T 1 . 2 1 . 3 + = Saturation Flow s = 3600/h=3600/2.1=1714 vphpl
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Effective Green saturation flow effective green time g rate of discharge time lost time lost time
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Time Budget and Critical Lane The time allocation of 3600 seconds in an hour to conflicting movements and to lost times incurred in the cycle due to the starting and stopping of these movements The amount of time required for each signal phase is determined by the most intensely used lane which is permitted to move during the phase. This lane is referred to as the “ c ritical lane ”. Each signal phase has one and only one critical lane. All other lane movements in the phase require less time than the critical lane.
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Critical Lane Volume Consider the heaviest per-lane volume by phase, the volume that can be handled in the 3600 seconds is: )] 3600 )( )( ( 3600 [ 1 C t N h h T V V L G ci C = = = V c = max sum of critical lane volumes which can be accommodated in an hour (vph) TG = time available for effective green allocation within an hour Also see Figure 16-4
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Adding Consideration of the PHF and v/c Ratio For a given V c , the minimum cycle length is ( ) h V Nt h V Nt C c L c L 3600 1 3600 3600 min = = A desirable cycle length could be found by specifying a desired v/c ratio and PHF ( ) ( ) h c v PHF V Nt C c L des 3600 1 = where v/c ratio is the proportion of available capacity that would be utilized by the given sum of critical lane flows
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Effects of Left-Turn Vehicles A significant factor in signalized intersection analysis Left-turning vehicles look for gaps in the opposing traffic on a permitted movement. This causes delay on themselves as well as vehicles behind. If left turns are moved separately from an opposing
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This note was uploaded on 02/07/2012 for the course CE 4211 taught by Professor Preston,h during the Spring '08 term at Minnesota.

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8_intersection_basic-2 - Topic: Fundamentals of Signal...

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