CTC-340 Signals - Basics

Terms & Definitions (review) Cycle - Cycle Length - Interval -. change interval - clearance interval- change + clearance = Yi green interval - Gi red interval - Ri

Terms & Definitions (review) Phases - Green + change + clearance Pretimed Operation - Full Actuation - Computer Controlled -

Left turns Permitted Left turns - Protected Left turn - Protected/ Permitted Left turn -

Basic mechanisms Discharge headways - – Figure 20-1 –it is assumed that the first 4 vehicles in the stopped queue will have longer headways than the remainder of the queue. Saturation headways are not reached until the 4th vehicle has passed the stop bar

Saturation flow rate s= 3600/h vehicles per hour of green time T 20-2 Start up lost time l 1 h(i) = actual headway for vehicle i Clearance lost time l 2 = Y i -e e is the extension of green time Use s for flow rate

Saturation flow rate Use l i to decrease the green time per phase Can now find time required to get N vehicles through a green signal T n = l 1 + nh effective green time g i =G i +Y i – (l 1 + l 2 ) knowing the effective green, it is easy to calculate the capacity of a given movement

Critical Lane & Time budget time budget - time is constant critical movement - the movement that needs the most time to work properly during each phase only one critical movement per phase maximum sum of critical movement volumes F 20-2

Critical Lane & Time budget except for lost times one of the critical movements is always moving can find total lost time per cycle L=Nt L the total lost time per hour is L H = L(3600/C) the total remaining time is allocated to effective green time T G = L H

Critical Lane & Time budget 2 important points 1. As the cycle length increases the capacity also increases because the lost time per cycle is constant and the number of cycles decreases Figure 20-4, Capacity increases as the number of phases is reduced - but left turn movements greatly affect capacity

PHF & v/c To find C we use the equation C min = Nt L /(1-V c /3600/h) –gives an answer where the v/c is 1.00 (all effective green is used) does not account for fluctuations in volumes due to peaking –This leads to a better equation for the cycle length which allow for input of the PHF and a desired v/c

PHF & v/c C des = Nt L /(1- [10000/(3600/h*PHF*v/c)] –gives an answer where the desired v/c is known and it accounts for fluctuations in volumes due to peaking

Effect of Left turning vehicles most difficult process to model through car equivalence –Number of through cars which could have crossed the stop line during the time the lane was blocked by 1 left turning vehicle –F 20.7, 20.8

Delay & other MOEs at signalized intersections LOS is defined in terms of delay Delay is a surrogate for driver discomfort, frustration, fuel consumption, and lost travel time Delay is made up of several components: control delay, geometric delay, traffic, and incidents When added together the components make up total delay

Delay & other MOEs at signalized intersections For signalized, unsignalized, and arterials only the control delay is analyzed Delay is complex and is dependent upon several variables –quality of progression, cycle length, green ratio, and v/c computed as seconds per vehicle F 20.9

Delay & other MOEs at signalized intersections length of queue at a given time (Usually a critical time such as at beginning of green) –F 20-10

Webster’s Delay Assumes 1. Arrival function is uniform 2. Aggregate delay is the area bounded by the arrival rate, departure rate, and a horizontal line intersecting the arrival rate at the beginning of the red interval

Delay model in the Highway Capacity Manual The average control delay d is a combination of the uniform delay, incremental delay (overflow queues) and residual demand (Overflow queues existing before the study period) delay

Delay model in the Highway Capacity Manual Progression Factor - when does majority of traffic flow arrive at signal F the better the progression - the lower the delay d 3 is found using Chapter 24 in the HCM All of the delay calculation only work for uniform or Poisson arrival rates.