lec 15 ch 8 pp 291 304 signal timing objective
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Lec 15, Ch.8, pp.291-304: Signal Timing (Objective). Be able to state the objectives of signal timing Learn by heart signal-timing related terms (very important) Understand how the yellow interval is determined

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lec 15 ch 8 pp 291 304 signal timing objective
Lec 15, Ch.8, pp.291-304: Signal Timing (Objective)
  • Be able to state the objectives of signal timing
  • Learn by heart signal-timing related terms (very important)
  • Understand how the yellow interval is determined
  • Know how to design signal timing for a fixed (pretimed) signal at an isolated intersection using the Webster’s model
what we cover in class today
What we cover in class today…
  • Terms relating to signal timing
  • Objectives of signal timing
  • Concept behind the determination of yellow interval & dilemma zone
  • Determining cycle length for a pretimed, isolated signal using the Webster method
  • Allocating green time to each signal phase
terms relating to signal timing
Terms relating to signal timing

Cycle length

Phase

Interval

Change interval

All-red interval (clearance interval)

Controller

Phase B

terms relating to signal timing cont
Terms relating to signal timing (cont)

Offset

All signal analyses are based on through passenger cars. This way the analyses became portable to any sites. Then you need conversion of other-than-passenger cars and other-than-through vehicles. Hence we need:

Split phase

Phase 1A

Phase 1B

Passenger car equivalent (PCE)

Turning movement factors

terms relating to signal timing cont5
Terms relating to signal timing (cont)

Peak-hour factor (PHF):A measure of the variability of demand during the peak hour. The smallest time period used for traffic analyses is 15 min.

If PHF is known, Design hourly volume (DHV) is computed by

DHV = (peak-hour volume)/PHF

412

390

380

375

Volume during peak hour

PHF =

4 x Vol. During peak 15 min within peak hour

375 + 380 + 412 + 390

0.945

=

=

4 x 412

terms relating to signal timing cont6
Terms relating to signal timing (cont)

Lane Group: consisting of one or more lanes on an intersection approach and having the same green phase

Critical Lane Group: the lane group that requires the longest green time in a phase. The critical lane group determines the green time that is allocated to that phase.

Saturation flow rate: the flow rate in veh/hr that the lane group can carry if it has the green indication continuously (see eq. 8.3 for the formula used by the Highway Capacity Manual 2000.)

terms relating to signal timing cont7
Terms relating to signal timing (cont)

Once you have adjusted for truck and turn movement factors, you are ready to design phasing. One hint: if the adjusted left turn passenger car equivalent volumes is greater than 120 vph, we usually need a left-turn phase, which means that we need a left-turn bay. Up to that value, the yellow interval can provide enough time for LTs. There are a few methods to deal with left-turns. In this class you need to know only the methods shown below (i.e. protected left-turn phases) and in Example 8-5. All others will be discussed in CE562. It is a complex issue.

130

600/2=300

140

576/2=288

A

B

C

D

Critical movement: The maximum volume (vph) in a phase.

objectives of signal timing
Objectives of signal timing
  • Reduce the average delay of all vehicles
  • Reduce the probability of accidents

Minimize the possible conflict points by assigning the right of way to different traffic movements

  • Two conflicting objectives:
  • More phases, less conflict
  • More phases, more lost time
  • So, if at all possible, use:
  • 2 phases
  • Short cycle length
yellow interval dilemma zone
Yellow interval & Dilemma zone

X0: the min. distance from the intersection for which a vehicle traveling at the speed limit u0 during the yellow interval cannot go through the intersection without accelerating

Distance traveled during yellow interval

Where τmin: yellow interval

Xc: the distance within which a vehicle traveling at the speed limit (u0) during the yellow interval cannot stop before encroaching on the intersection.

yellow interval dilemma zone cont
Yellow interval & Dilemma zone (cont)

For the dilemma zones to be eliminated, X0 = Xc:

Comfortable deceleration rate:

a = 0.27g

Solve for the yellow interval:

If the effect of grade is added:

G = grade in decimals

cycle lengths of pretimed signals by the webster method
Cycle lengths of pretimed signals by the Webster Method

Optimal cycle length C0 by the Webster method:

L = Total lost time per cycle (sec), usually you lose 3 seconds per distinct phases.

Yi = Max value of the ratios of approach flows (called “critical movement”) to saturation flows for all lane groups using phase i, qij/Sj

 = Number of phases

Vij = Flow on lane groups having the right of way during phase i

Sj= Saturation flow on lane group j; for a through lane, about 1900 pcphgpl

cycle lengths of pretimed signals by the webster method cont
Cycle lengths of pretimed signals by the Webster Method (cont)

Lost time for phase i:

Total lost time:

R = Total all-red during the cycle

green time allocation
Usually li is about 3 seconds.Green time allocation

Available total effective green time, Gte = C - L

Distribute effective green to each phase i by:

And the actual green time for each phase i is:

minimum green time
Minimum green time

At an intersection where a significant number of pedestrians cross, it is necessary to provide a minimum green time that will allow the pedestrians to safely cross the intersection. After allocating green time, you must check with the minimum green time for each phase.

For WE > 10 ft

For WE≤ 10 ft

At an intersection where only a few pedestrians cross, this will be the length of green time for pedestrian actuated green.

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