Chapter 21: Fundamentals of Signal Timing and
Design: Pretimed Signals
Chapter objectives: By the end of this chapter the
student will be able to:
Explain the basics of signal timing
Know how to handle left-turn vehicles by various phase
Define terms related to phasing
Explain how change and clearance intervals are determined
Explain how pedestrians are dealt with in signal timing
State a few more ways to deal with left turns
Get familiar with typical steps for simple signal timing
Chapter 21 1
Components of signal timing development, p.489
2. Determination of vehicular
1. Development of a signal needs:
phase plan and sequence (a) Timing of change interval (y)
and clearance interval (ar)
3. Determination of (b) Determination of critical lane
pedestrian signal volume (Vc)
(c) Determination of lost time per
(a) Determine min. phase (tL) and per cycle (L)
(d) Determination of cycle length
(b) Check if vehicular (C)
greens meet min
(e) Allocation of effective green
(c) Check the need of
peds actuators or of The process is not exact, nor is there often a single
adjusting timing “right” design and timing for a traffic control signal.
Chapter 21 2
21.1 Development of signal phase plans
The most critical aspect of signal design and timing is
the development of an appropriate phase plan.
21.1.1 Treatment of left turns (the single most important
feature that drives the development of a phase plan)
Two general guidelines (not absolute criteria):
• vLT ≥ 200 veh/h
• vLT*(vo/No) ≥ 50,000 (Cross product rule)
2 left turn vehicles/cycle may be able to turn
left as “sneakers” during the yellow interval.
Chapter 21 3
LT treatment (continue), p.490
Permitted LT phasing should be provided when the following
1. The LT demand flow within the peak hour falls within the “permitted”
portion of the figure below.
2. The sight distance for LT vehicles not restricted.
3. Fewer than 8 LT accidents have occurred within the last 3 years at any one
approach with permitted-only phasing. (Permitted LT phase must exist to
apply this criterion.)
Figure 21.1 Chapter 21 4
LT treatment (continue), p.491
Fully protected phasing is recommended when any TWO of
the following criteria are met:
1. LT flow rate is greater than 320 veh/h
2. Opposing flow rate is greater than 1,100 veh/h
3. Opposing speed limit is greater than or equal to 45 mph
4. There are two or more LT lanes (in this case, only protected LT
phase is used.
Chapter 21 5
LT treatment (continue), p.491
Fully protected phasing is also recommended when any ONE of
the following criteria are met:
1. There are 3 opposing lanes, and the opposing speed is 45 mph or greater
2. LT flow rate is greater than 320 veh/h, and the percent of heavy vehicles
3. The opposing flow rate exceeds 1,100 veh/h, and the percent of LT
4. Seven or more LT accidents have occurred within 3 years under
compound phasing (must have a compound phasing now)
5. The average stopped delay to LT traffic is acceptable for fully protected
phasing, and the engineer judges that additional LT accidents would
occur under the compound phasing option. (must have a compound
Compound phasing (protected-permitted) may be considered when LT
protection is needed but none of these criteria are met. – Use compound
Chapter 21 6
phasing at less critical areas because it is a confusing phasing.
21.1.2 General considerations in signal phasing
Phasing can be used to A phase plan must be implemented in
minimize conflicting movements accordance with the standards and
and associated hazards. But the criteria of the MUTCD, and must be
higher number of phases means accompanied by the necessary signs,
decreased efficiency and markings, and signal hardware needed to
increased delay. Each phase identify appropriate lane usage. See
add about 3 to 4 seconds of MUTCD about signal heads, associated
effective red (lost time). signs, etc.
The more phases you have The phase plan must be consistent
more delay you create. But, with the intersection geometry, lane use
saturation flow rate increases assignments, volumes and speeds, and
because of less conflicts. Hence, pedestrian crossing requirements.
look for balance between them. e.g. If there is no LT-bay or exclusive
The LT saturation flow rate LT-lane, do not provide a protected LT
is at the mercy of on-coming phase. It’s useless!
vehicles for permitted left turns.
Chapter 21 7
21.1.3 Phase and ring diagrams
Chapter 21 8
21.1.3 Phase and ring diagrams (continued)
Phase diagram: Shows
all movements being
made in a given phase
within a single block of
Ring diagram: Shows
which movements are
controlled by which
“ring” on a signal
A “ring” of a controller generally controls one set of signal faces. Thus, while a
phase involving two opposing through movements would be shown in one block of
a phase diagram, each movement would be separately shown in a ring diagram. 9
21.1.4 Common phase plans and their use
Basic two-phase signalization
This works for either case of having a left-turn bay or not having a left-turn bay.
If a left-turn bay is available, performance and safety increases.
Chapter 21 10
This LT phase may be
With a inefficient if only one
direction has a lot of LTs.
General guidelines to deal with left-turn vehicles:
LT protection is rarely used for LT volumes of less than 100 vph. But even
under 100 vph, it may be used if sight distance is a problem.
LT protection is almost always used for LT volumes of more than 250-300 vph
Between these bounds, the provision of LT protection must consider opposing
volumes and number of lanes, accident experience, system signal constraints, etc.
Chapter 21 11
Leading and lagging green phases:
splitting the exclusive left-turn phase
Count the number of How many
phases in one ring to phases are there?
find how many lost
Leading green for EB LT
Lagging green for WB LT
A2 is a compound
phase if LTs are
We do not have to have both leading and lagging permitted.
phases. We may have only a leading or lagging
phase depending on the prevailing conditions.
Chapter 21 12
Exclusive LT phase with leading green phase
Note how LTs are
direction (WB in
this case) has a
short LT phase.
Chapter 21 13
Eight-phase actuated control (NEMA)
The term “phase” is loosely used
sometimes. “Eight-phase” here is
that it is possible to have 8
phases, but usually 4 phases as NEMA no longer includes lead-lag
Chapteroption and this 8-phase scheme
you see in the ring diagram. replaced it.
Lead vs. Lag
“No significant Difference.”
Chapter 21 15
Safety Lead vs. Lag
Chapter 21 16
Protected-permitted left-turns (1)
This order may have a problem.
This one, too.
LTs may be trapped in the intersection
because there is no clearance time for
LTs beyond the yellow interval. Possible rear-end collisions for LTs because
(sometimes there is AR – then possibility the LT driver might hesitate for an instant.
for rear-end collision) Chapter 21 17
Chapter 21 18
(Source, pages 61-65, “Manual of Traffic Signal Design” by Kell & Fullerton, ITE)
Chapter 21 19
Lag-left turn with no opposing left turn
Chapter 21 20
Lag-left turns moving simultaneously
Chapter 21 21
The exclusive pedestrian phase
An exclusive pedestrian phase is added. This was
started in New York City by then Traffic
commissioner Henry Barnes, hence called “Barnes
Dance.” Not any more in NYC but you see right next
to Clyde Building. Visit 900N & Campus Dr.
Chapter 21 22
T-intersection & 5-leg intersection
If this street is a one-way street heading
south, then it is not that bad (3 phases).
But disallow “U-turn from the south
approach into the diagonal link.
Chapter 21 23
This was created
by a data set
collected in Salem,
Oregon. It may not
apply to other
cities, but it is
useful to make
This figure shows a
case of one-lane
Note that this
eliminated in the
3rd and 4th edition
of the text. But, I
thought it is useful.
Chapter 21 24
21.2 Determining vehicular signal requirements
21.2.1 Change and clearance intervals
All red = clearance interval
The MUTCD carries no
requirement for an all red or
Yellow = change interval
But, ITE recommends use of
Yellow change intervals should have both a yellow change interval
a normal range of approximately 3 to 6 and an all-red clearance interval.
seconds. Generally the longer intervals
are appropriate to higher approach
p.441 2nd edition: If there is no all-read interval, it is the driver’s responsibility
to check if the intersection was cleared of traffic. “…over 60% is not aware of
this legal responsibility. Also, 60% indicated that they did not bother to look
for traffic from the conflicting street when given the GREEN indication.” --
Note that this statement was eliminated in the 3rd edition. So, this is not in the
4th edition, either. Chapter 21 25
Safety stop at or before the stop bar or clear
the intersection & dilemma zone (using SSD
To safely stop: 2
X c D vot r
2 g ( f G)
To safely clear: X o D votY (W L) Cannot stop or cannot
finish crossing (Xc>Xo)
Chapter 21 26
Eliminating the dilemma zone
When Xc = Xo, there is no dilemma zone - at least theoretically.
votY (W L) vot r
2 g ( f G)
votY vot r (W L)
2 g ( f G)
vo (W L)
tY t r
ITE took this 2 g ( f G) vo
vo (W L) This part as the
part as the tY t r
vo length of the all-
length of the 2 g ( G) red interval.
interval. vo (W L)
2 2 gG vo
Chapter 21 27
ITE-recommended practice on change (yellow)
interval, eq. 21-2, p.503
This part shows the
1.47 S85 1.47 S85
y t t effect of gravity on
2a 2 gG 2a gG deceleration vector.
Note that where
approach speeds are
not measured and the
speed limit is used,
a = deceleration rate (e.g., 10 ft/s2)
both the y and ar
g = gravity (32.3 ft/s2) intervals will be
determined using the
G = grade of approach (in decimals) same value of speed.
(p.504 Left col.) Not
t = perception-reaction time (1.0 sec)
S85 = 85th percentile speed or the speed limit, mph
Chapter 21 28
21.2.1ITE-recommended practice on clearance (AR)
intervals: (3 cases to evaluate and use the longest)
Case 1: Practically no pedestrian (low
ar (S15 in mph)
Case 3: Some pedestrian traffic:
W L P
ar max ,
Case 2: Significant number of
pedestrians OR where the crosswalk is
protected by pedestrian signals
PL S15 = S – 5
1.47 S15 S85 = S + 5 If LTs are
S = average approach speed Chapter 21 more critical29
21.2.2 Determining lost times
G y ar R
B l1 e l2 R
C tL g R
r g r
A. Actual signal indications
B. Actual use of green and yellow; e is extended green, i.e. part
of the yellow used as green
C. Lost times l1 and l2 are added and placed at the beginning of
the green for modeling purposes l2 Y e
D. Effective green and effective red Y y ar
l1 = 2 sec/phase t L l1 l2
e = 2 sec/phase n
L t Li
i = Number of
(by HCM 2000) phases
Chapter 21 30
21.2.3 Determining the sum of critical-lane volume
Two factors require special attention:
Simple volumes cannot be simply compared: heavy
vehicles, left turns and right turns affect traffic flow
Where phase plans involve overlapping elements, the ring
diagram must be carefully examined to determine which
flows constitute critical-lane volumes.
Convert all demand volumes to equivalent through vehicle
units (TVUs) first.
Chapter 21 31
The effect of turning vehicles are included in Vc by multiplying ELT and
ERT as shown in Table 21.1 and 21.2 (Note these are different from the
ones we use for computing fHV in capacity analysis.)
VLTE VLT * ELT
VRTE VRT * ERT
VEQ VLTE VTH VRTE
Chapter 21 N 32
Figure 21.11 Determining critical lane volumes
Chapter 21 33
21.2.4 Determining the desired cycle length
This simple method gives you the direction for detailed signal timing.
This method using the formula for the “time budgeting” method as the
S = 1900 pcphgpl, but use 85% of it. Hence, s = 0.85x1900 = 1615
pcphgpl (h = 3600/1615 = 2.23 sec/veh)
12-ft lane width, no parking or local buses, 5% heavy vehicles, +1% grade,
a CBD location, and a lost-time/phase of 3 seconds
Nt L 3N
PHF (v / c)(3600 / h) 1615PHF (v / c)
21.2.5 Splitting the green
Once the cycle length is determined, the available effective
green time in the cycle must be divided (split) among the
various signal phases in proportion to Vci/Vc.
gTOT C L
g i gTOT *
Finding actual green interval values (Gi):
Gi = gi – Yi + tLI
Do the sample timing in page 507.
Chapter 21 35
21.3 Determining pedestrian signal requirements
HCM 2000 requirements:
GP 3.2 (2.7 * ) For WE > 10 ft
p (width of crosswalk in ft)
GP 3.2 (0.27 * N ped ) For WE ≤ 10 ft
The sum total of 1st and 2nd term is WALKmin. The third term is
FLASHING DON’T WALK.
If Gp > G + Y, (a) change the signal timing to satisfy this requirement, or (b) install
pedestrian detectors (buttons) at the intersection. When the button is pressed, the
controller will provide a G + Y equal to Gp during the next available green phase.
When Gp controls, make changes in green times to maintain the original ratio of
vehicular green time. See pages 509 and 510.
Chapter 21 36
Relationship between vehicular signal
indications and pedestrian signal indications,
Chapter 21 37
21.4 Compound signal timing
• Treat the protected and permitted portions of the phase as if
they were separate phases.
• In converting volumes to tvu’s, use different equivalents
(Table 21.1) as appropriate for each portion of the phase.
• Estimate the cycle length (C) and green splits (g) treating
the protected and permitted portions of the phase separately.
•Remember that there will be “yellow” between the green
arrow and the green ball as the phase transitions from
protected to permitted (or vice versa). This yellow counts as
green time for left turns.
Chapter 21 38
21.5 Simple signal timing applications
1. Develop a reasonable signal phase plan. Decide how to deal
with left-turning vehicles.
2. Convert all left-turning and right-turning volumes to through
car equivalents (tvu’s) using Tabs 21-1 and 21-2.
3. Establish a reasonable phase plan and draw a ring diagram.
4. Determine yellow and all-red intervals for each signal phase.
5. Determine lost times per cycle using Eq. 21-5 through 21-7.
for 6. Determine the actual sum of critical lane volumes, Vc, using
this plan. Check the sum of critical lane volumes in tvu’s for
reasonableness. Using Equation 21-11, determine the desirable
cycle length based on a desired v/c ratio (0.85-0.90), the PHF
7. Allocate the available effective green time within the cycle in
proportion to the critical lane volumes (in tvu’s) for each
8. Check pedestrian requirements and adjust signal timing as
Chapter 21 39