Chapter 16: Introduction to intersection control

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Chapter 16 Introduction to intersection control
Chapter objectives By the end of these chapters
the student will be able to

• Explain why some sort of control is essential to
  allocate ROW at intersections
• Explain there is hierarchy of intersection
  control (Traffic signal is NOT a cure-all)
• Check sight distances available at an
  intersection
• Use the warrants for stop and yield controls
• Evaluate traffic signal warrants

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Conflict points at unsignalized intersections
T intersection
4-leg intersection
Total 32
Crossing 3
Merging 3
Diverging 3
Total 9
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16.1 The Hierarchy of Intersection Control
Intersection Control Options How much judgment
can drivers safely exercise to avoid collisions?
Three levels of control are available.
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16.2 Level I Control Basic rules of the Road
Primary prerequisite for safety under basic
rules-of-the-road Sight distance must be
adequate for the driver before he is accountable
for full responsibility for his action. ? The
first thing you want to check is whether adequate
SSD is available at the intersection. This must
be satisfied before traffic volume concerns come
into consideration.
From the similarity of triangles (1-3-2 and
6-4-5)
How do you determine dA and dB?
Visibility Triangle Veh A is on minor street
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Rule 1 Both vehicles have at least one safe SSD
to the collision point (AASHTO recommended), p.447
Step 1 Assume that Vehicle A is located one safe
SSD from the collision point. Usually the vehicle
is considered to be on the minor road.
Step 2 Based on the assumed position, determine
the actual location of Vehicle B when it first
becomes visible. Call it dB(act)
Step 3 Rule 1 requires Vehicle B to have one
safe SSD. Call it dB(min).
Step 4 If dB(act) ? dB(min), then adequate SSD
for basic rules-of-the-road has been provided.
Otherwise, violated and under Rule 1, not safe.
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Rule 2 Vehicle A must travel 18 feet past the
collision point in the same time that Vehicle B
travels to a point 12 feet before the collision
point, p.448 (Historically, another approach)
dB
dA
This dB is used in Step 3 in Approach 1. The
remaining steps are the same as Approach 1.
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When dBact lt Dbmin then operation with no
control cannot be permitted.

• Implement intersection control, using STOP- or
  YIELD-control, or traffic signals.
• Lower the speed limit on the major street to a
  point where sight distances are adequate.
• Remove or reduce sight obstructions to provide
  adequate sight distances.

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Sample problem Fig. 16.3
Step 1
Step 2
dB
dA
Step 3
Step 4
dB(act) ltlt dB(min,R1) and dB(min,R2) Both rules
were violated.
The sight triangle between Vehicle A and B fails
to meet the criteria for safe operation under
basic rules of the road.
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Table 16.1 Warrants for two-way STOP sign
16.3 Level II Control Yield and STOP Control
16.3.1 Two-way stop control
(Check the example in page 450)

• Because the STOP sign causes a substantial
  inconvenience to motorists, it should be used
  only where warranted. A STOP sign may be
  warranted where one or more of the following
  conditions exist
• Intersection of a less important road with a main
  road where application of the normal ROW rule
  would not be expected to provide reasonably safe
  operation.
• Street entering a through highway or street.
• Unsignalized intersection in a signalized area.
• High speeds, restricted view, or crash records
  indicate a need for control by the STOP sign.

http//mutcd.fhwa.dot.gov/HTM/2003r1/part2/part2b1
.htm
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Sample Problem Figure 16.4
18ft (10 ft from the edge of curb to the stop
line) (8 ft form the front bumper to the
driver)
dA-STOP 18 dcl
dBmin 1.47Smajtg
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Table 16.2 Warrants for YIELD sign
16.3.2 Yield control

• The YIELD sign may be warranted
• When the ability to see all potentially
  conflicting traffic is sufficient to allow a road
  user traveling at the posted speed, 85th
  percentile speed, or the statutory speed to pass
  through the intersection or stop in a safe
  manner.
• If controlling a merge-type movement on the
  entering roadway where acceleration geometry or
  sight distance is not adequate for merging
  traffic operations.
• At a second crossroad of a divided highway, where
  the median width is 30ft or greater. A STOP sign
  may be installed at the entrance to the first
  roadway of a divided highway, and a YIELD sign
  may be installed at the entrance to the second
  roadway.
• At an intersection where a special problem exists
  and where engineering judgment indicates that the
  problem is susceptible to correction by use of a
  YIELD sign.

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Table 16.3 Warrants for multiway STOP sign (more
specific)
16.3.3 Multiway-way stop control

• Multiway stop signs Requires that all vehicles
  approaching the intersection stop before entering
  it. Used when the traffic volumes on all the
  approaches are about equal. Warrants are
• Where traffic signals are justified, the multiway
  STOP sign is used as an interim measure.
• Crash problems susceptible to correction by a
  multiway STOP sign exist, like right- and
  left-turn collisions as well as right-angle
  collisions.
• Minimum volumes (1) at least 300 veh/h for any 8
  hours of an average day on the major street, and
  (2) at least 200 units (peds, bikes, vehs
  combined/h for the same 8 hours in the minor
  street with an average vehicle delay 30 sec/h
  during the highest hour , but (3) if the 85th
  percentile approach speed of the major highway
  exceeds 40 mph, the min vehicular volume warrants
  are 70 of the above values.
• Where no single criterion is satisfied, but where
  criteria B, C1, and C2 are all satisfied to 80
  of the minimum values. Criterion C3 is excluded
  from this condition

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16.4 Level III Control Traffic Control Signals

• While warrants and other criteria for STOP and
  YIELD signs are somewhat general in the MUTCD,
  warrants for signals are quite detailed.

16.4.1 Advantages of traffic signal control

• Provide for the orderly movement of traffic
• Increase the traffic-handling capacity of the
  intersection if applied properly and physical
  layout is conducive
• Reduce the frequency and severity of certain
  types of crashes, esp. right-angle collisions
• Provide for continuous or nearly continuous
  movement at a definite speed when coordinated (by
  coordination)
• Used to interrupt heavy traffic at intervals to
  permit other traffic, vehicular or pedestrian, to
  cross

http//mutcd.fhwa.dot.gov/HTM/2003r1/part4/part4c.
htm
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16.4.2 Disadvantages of traffic signal control

• Excessive delay (when timing is improper)
• Excessive disobedience of the signal indications
  (when red interval is improperly long) may
  encourage red-light running
• Increased use of less adequate routes as road
  users attempt to avoid the traffic control signal
• Significant increases in the frequency of
  collisions (especially rear-end collisions)

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Data requirements
16.4.3 Warrants for traffic signals

• Traffic volumes in each approach, at least 12
  consecutive hours (24 hrs count preferred)
  containing the highest of 24-hour traffic
  (include two peak periods)
• 15-min counts by approach and movement classified
  by vehicle type during the 2 hours in the AM and
  PM peak periods (4 hours total)
• Pedestrian counts in each crosswalk during the
  same 4 hours in item 2
• Information on nearby facilities and centers
  serving the movement disadvantaged (young,
  elderly, or disabled)
• Posted or statutory speed limit and/or the 85the
  percentile speed

• A condition diagram
• A collision diagram
• For the two peak AM and two peak PM hours
• Vehicle-hours of stopped delay for each approach
• Number and distribution of gaps
• Speed limits
• Pedestrian delays for at least two 30 minute peak
  pedestrian delay periods
• Queue lengths on STOP-controlled approaches

(See page 454 and 455.)
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Warrants
MUTCD 2000 has 8 warrants for a traffic signal.
They are guides, not specs. Use professional
judgments.

• Warrant 1 Eight-hour vehicular volume (1a
  minimum traffic volume 1b interruption of
  traffic 1c combination of warrants)
• Warrant 2 Four-hour vehicular volume
• Warrant 3 Peak hour
• Warrant 4 Pedestrian volume
• Warrant 5 School crossing
• Warrant 6 Coordinated signal system
• Warrant 7 Crash experience
• Warrant 8 Roadway network

Signals
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Signal warrant 1A 8-hour volume
Min. vehicle volume Principal factor is the
intersection traffic volume. Must satisfy for
each of any 8 hour of an average day.

• May reduce the values by 30 if the 85th
  percentile speed on the major approach is greater
  than 40 mph or population is less than 10,000
  (built-up area of isolated community).

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Signal warrant 1B
Interruption of continuous traffic The volume
requirements must be met for each of any 8 hours
of an average day.

• May reduce the values by 30 if the 85th
  percentile speed on the major approach is greater
  than 40 mph or population is less than 10,000
  (built-up area of isolated community).

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Signal warrant 1C

• Combination of warrants Only in exceptional
  cases. When none of them are satisfied but when
  the first two warrants of Warrant 1 are satisfied
  to the extent of 80 of the stipulated volumes.

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Signal warrant 2 4-hour volume
Check for each of any 4 hr of an average day
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Signal warrant 3 Peak-hour volume
Justified where traffic condition during 1 hr of
the day result in undue delay to traffic on the
minor street. (factories, office park etc.)
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Signal warrant 3B delay
This applies to cases in which STOP control is
already in effect for the minor road. Cannot jump
from no-contro or YIELD to signals.

• This warrant is satisfied when the delay during
  any four consecutive 15-minute periods on one of
  the minor-street approaches (one direction only)
  controlled by a STOP sign is equal to or greater
  than specified levels and the same minor-street
  approach (one direction only) volume and the
  total intersection entering volume are equal to
  or greater than the specified levels
• Delay experienced by minor approaches? 4
  vehicle-hours for a one-lane approach and 5
  vehicle-hours for a two-lane approach.
• Volume on the same minor approaches ? 100 vph
  for one moving lane of traffic, 150 vph for two
  moving lanes of traffic.
• The total entering volume serviced during the
  hour equals or exceeds 800 vph entering volume
  for intersections with four or more approaches or
  650 vph for intersections with 3 approaches.

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Signal warrants (4,5,6)
Warrant 4 Pedestrians. The pedestrian volume
crossing the major street ? At least 100 for each
of any 4 hr or 190 during any 1 hr and there are
fewer than 60 gaps per hr that are acceptable by
pedestrians for crossing. AND, the nearest signal
along the major street should be at least 300 ft
away. See Table 16.6 for details.
Warrant 5 School Crossing. The frequency of
occurrence of gaps and the lengths of gaps are
inadequate for safe crossing of the street. The
number of acceptable gaps is less than the number
of minutes in that period, e.g., if the crossing
lasts 30 minutes, there must be at least 30
occurrence of critical gaps for children the
cross. The minimum number is 20 during the
highest crossing hour. ? If met only this
warrant, use a pedestrian-actuated signal.
Warrant 6 Coordinated System. Used when lights
would not otherwise have been installed.
Justifies a signal if it will help maintain a
proper grouping (platooning) of vehicles and
effectively regulate group speed. See Table 16.7
for details.
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Signal warrants (7,8)
Warrant 7 Crash Experience. Less restrictive
measures must be used before this warrant is
used. 5 or more injury or property-damage-only
accidents within a 12-month period, AND signal
control is a suitable countermeasure. AND, the
traffic and pedestrian volumes should not be less
than 80 of the requirements of warrants 1A and
1B. See Table 16.8 for details.
Warrant 8 Roadway Network. When lights help to
encourage concentration and organization of
traffic networks. (1) The total existing or
immediately projected volume is at least 1000
during the peak hour AND the 5-year projected
traffic volumes satisfy the requirements of one
or more of the Warrants 1, 2, 3, on average
weekday, OR (2) The total existing or projected
entering volume is at least 1000 vehicles for
each of any 5 hr of a Saturday and/or a Sunday.
See Table 16.9 for details.
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16.4.5 A sample problem in application of signal
warrants

• We will walk through 16.4.5 in class