Lec 15: The hierarchy of intersection control

Report
Chapter 18: Hierarchy of 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
1
Conflict points at unsignalized intersections
T intersection
4-leg intersection
Total = 32
Crossing = 3
Merging = 3
Diverging = 3
Total = 9
2
Two factors affecting a driver’s ability
to avoid conflicts
1. A driver must be able to see a potentially conflicting
vehicle or pedestrian in time to implement an avoidance
maneuver, (sight distance related) and
2. The volume levels that exist must present reasonable
opportunities for a safe maneuver to take place (gap
availability related).
3
The Hierarchy of Intersection Control
Intersection Control Options: How much judgment can drivers safely
exercise to avoid collisions? Three levels of control are available.
Level I
Passive control – basic rules of the road apply
 No control
 Guide signs only
 Warning signs with or without guide signs
Level II
Direct assignment of ROW to major street or rotational ROW
 YIELD control (roundabouts are in this category)
 Two-way STOP control
 All-way STOP control
Level III Positive alternate assignment of exclusive ROW
Traffic signals: 2-phase, multiphase
 Traffic control agent/officer
4
18.1 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.
b
From the
similarity of
triangles (1-3-2
and 6-4-5):
dB  a
ab
dA b
ab
How do you
determine dA
and dB?
“Visibility Triangle: Veh A is on minor street”

dA b
dA b
a
 dB  a
 a  dB
ab  ad
A
 ab
dA b
dB 
ad
 dB
A
dA b
5
Rule 1: Both vehicles have at least one safe SSD to the
collision point (AASHTO recommended), p.412
Step 1: Assume that Vehicle A (on minor street) is located one safe SSD
from the collision point. Usually the vehicle is considered to be on the minor
2
road.
SA
d A  1 . 47 S A t 
30 0 . 348  0 . 01 G 
Step 2: Based on the assumed position, determine the
actuallocation of Vehicle B when it first becomes visible.
Call it dB(act)
dB 
ad
A
dA b
Step 3: Rule 1 requires Vehicle B (on major street) to have one safe SSD.
Call it dB(min).
2
d B min  1 . 47 S B t 
SB
30 0 . 348  0 . 01 G 
Step 4: If dBact  dBmin, then adequate SSD for basic rules-of-the-road has been
provided. Otherwise, violated and under Rule 1, not safe.
6
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.412 (Historically, another approach…)
d A  18
dB
1 . 468 S A

d B  12
1 . 468 S B
d B   d A  18 
dA
SB
 12
SA
This dB is used in Step 3 in Rule 1. The
remaining steps are the same as Rule 1.
7
When dBact < 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.
8
Sample problem: Fig. 18.3, p.412
Step 1:
d A  1 . 47 ( 30 )( 2 . 5 ) 
2
 196 . 5 ft
30 ( 0 . 348 )
Step 2:
dB
d Bact 
b
dA
( 30 )
ad
A
dA b

( 20 )( 196 . 5 )
196 . 5  42
 25 . 4 ft
Step 3:
d B min, R 1  1 . 648 ( 40 )( 2 . 5 ) 
a
d B min, R 2  (196 . 5  18 )
40
( 40 )
2
 300 . 3 ft
30 ( 0 . 348 )
 12  298 . 0 ft
30
Step 4:
The sight triangle between Vehicle A and B
fails to meet the criteria for safe operation
under basic rules of the road.
dBact << dBmin,R1 and dBmin,R2
Both rules were violated.
9
Apply the concept you just learned. There are 4 steps to solve this problem.
Remember as long as one side is unsafe, rules of the road cannot be used as
the only form of control.
10
18.2 Level II Control: Yield and STOP Control
YIELD or STOP
control? Remember
the critical approach
speed (CAS) method
we studied in
CE361?, section
8.1.2 of Fricker’s
text book? If CAS is
between 10 to 15
mph, use YIELD. If
less than 10 mph,
use STOP.
http://mutcd.fhwa.dot.gov/HTM/2003r1/part2/part2b1.htm
11
18.2.1 Twoway stop
sign
http://mutcd.fhwa.dot.gov/HTM/2003r1/part2/part2b1.htm
12
Stop Control:
Sample Problem:
Figure 18.4
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
dcl = Distance from the curb line to the center
of the closest travel lane from the direction
under consideration (ft): cl = curb line. (because
LTs must enter the median lane.)
dBmin = 1.47*Smaj*tg
36!
dB 
dB 
ad
A
ad
dA b
dA b
So, what do you have to do with this result?
A
13
18.2.2 Yield control
YIELD
Table 18.3:
Warrants for
YIELD sign
14
18.2.3 Multiway-way
stop control
15
18.3 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.
18.3.1 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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18.3.2 Disadvantages of traffic signal control
1. Excessive delay (when timing is improper)
2. Excessive disobedience of the signal indications
(when red interval is improperly long) – may
encourage red-light running
3. Increased use of less adequate routes as road
users attempt to avoid the traffic control signal
4. Significant increases in the frequency of
collisions (especially rear-end collisions)
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18.3.3 Warrants for traffic signals
Data requirements
1.
2.
3.
4.
5.
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
6.
A condition diagram (ch.11)
7.
A collision diagram (ch.11)
8.
For the two peak AM and two peak
PM hours:
a)
Vehicle-hours of stopped delay
for each approach
b)
Number and distribution of gaps
c)
Speed limits
d)
Pedestrian delays for at least two
30 minute peak pedestrian delay
periods
e)
Queue lengths on STOPcontrolled approaches
(See page 418 and 419.)
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Warrants for Traffic Signals
MUTCD 2009 has 9 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
Signals
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
 Warrant 9: Intersection near a highway-rail crossing
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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 (they do not need to
be consecutive 8 hours).
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).
20
Where do these
values come
from?
My guess is…
Use the random
arrival case and
use the Poisson
distribution.
21
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).
22
Signal warrant 1C
Combination of warrants: Only in
exceptional cases. When none of the
warrants are satisfied but when the first
two warrants of Warrant 1 are satisfied
to the extent of 80% of the stipulated
volumes.
23
Signal warrant 2: 4-hour volume
Check for each of any
4 hr of an average day.
To meet the warrant ,
at least four hours
must plot above the
appropriate decision
curve.
24
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.)
25
Or Signal warrant 3B: delay
This applies to cases in which
STOP control is already in effect
for the minor road. Cannot jump
from no-control or YIELD to
signals.
26
Warrant 4: Pedestrians (4 hours)
(a) Normal criteria
(b) Pop < 10,000, major road
speed exceeds 35 mph
If a signal is warranted only by
this warrant, start with a
semi-actuated signal. A full
actuated signal is possible
at an isolated intersection.
Another one nearby < 300 ft?
Then, this warrant is not
met.
May be reduced as much as by
50% if majority of walkers
walk at less than 3.5 mph.
Fig. 18-7
27
Or, Warrant 4: Pedestrians (peak hour)
Fig. 18.8
28
Warrant 5: School Crossing
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 to cross. The
minimum number is 20 during the highest crossing hour.  If only
this warrant is met, use a pedestrian-actuated signal.
29
Warrant 6: Coordinated System
This should not
result in signal
spacing of less than
1,000 ft.
30
Signal warrant 7: Crash Experience
Warrant 7: Crash Experience.
Less restrictive measures
must be used before this
warrant is used. 5 or more
injury or property-damageonly accidents within a 12month 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 18.5 for details).
31
Signal Warrant 8: Roadway Network
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 18.9
for details.
32
Signal warrant 9:
Intersection Near a
Highway-Rail
Grade Crossing
33
One lane approach
The minor-street volume used in entering either Figure18.9 or
18.10 may be multiplied by up to three adjustment factors: (1) for
train volume, (2) for presence of high-occupancy buses, more than
20 passengers, and (3) for truck presence.
34
Two lane approach
The minor-street volume used in entering either Figure18.9 or
18.10 may be multiplied by up to three adjustment factors: (1) for
train volume, (2) for presence of high-occupancy buses, more than
20 passengers, and (3) for truck presence.
35
The minor-street volume used in entering either Figure18.9 or
18.10 may be multiplied by up to three adjustment factors: (1) for
train volume, (2) for presence of high-occupancy buses, more than
20 passengers, and (3) for truck presence.
36
37
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18.3.5 A sample problem in application of signal warrants
We will walk through 18.3.5 in class
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