Transcript MICHIGAN DEPARTMENT OF TRANSPORTATION Davison TSC

MICHIGAN DEPARTMENT OF TRANSPORTATION
Davison TSC
Traffic Signals
101
Intersection Safety Myths
The driving public has developed a number of
misconceptions about traffic control solutions over
the years.
Myth : Installing signals always
makes intersections safer.
Reality:
The installation of unwarranted signals, or signals that operate improperly, will create
situations where overall intersection congestion is increased, which in turn can create
aggressive driving behavior.
When signal phasing causes longer waiting times, both drivers and pedestrians tend
to become impatient and violate red lights, or drivers are tempted to cut through
neighborhood streets.
One way of improving waiting times at an intersection with a new signal is to make
sure the minor street waiting times are less than they were before installation of the
signal. This will encourage motorists to use signals on main roads instead of
neighborhood streets.
WHY UNIFORM STANDARDS?
Do you know what the color red signifies? A yellow painted line? An eightsided traffic sign?
A study of driver observance of a standard red octagonal sign with the letters
“SOTP” painted on it was conducted. It was found that practically no one
noticed the misspelled legend but everyone obeyed the sign.
Uniformity is a critical factor relative to signalization because of the serious
consequences which may result from installation of an unwarranted signal.
Properly used, traffic signals reduce certain type of crashes, provide gaps in
the traffic stream benefiting other access points further “downstream”.
Poorly designed, ineffectively placed, or improperly operated signals perform
just the opposite of what is expected of them. Intersections become clogged
with cars, and motorist delay and crash potential is increased.
A set of “warrants” was established, under which signals function properly and provide the
motoring public and pedestrians the most benefit.
The “warrants” are used by traffic engineers nationally to evaluate the need for stop and go
traffic signals.
WARRANTS – GUIDELINES FOR
TRAFFIC SIGNAL INSTALLATION
Signal warrants are a set of guidelines or conditions which allow careful
assessment of the need for signalization.
Warrant 1 – MINIMUM VEHICULAR VOLUME
This warrant requires a minimum number of vehicles on both the major
street and cross street for each of any eight hours during the day.
Warrant 2 – INTERRUPTION OF CONTINUOUS TRAFFIC
Warrant 2 provides for situations where a very high volume of major street
traffic restricts entry of cross street traffic causing excessive delay.
Warrant 3 – MINIMUM PEDESTRIAN VOLUME
Occasionally a situation arises where very high pedestrian activity conflicts
with high traffic volumes.
Warrant 4 – SCHOOL CROSSING
Warrant 4 provides minimum installation guidelines for traffic signals at
established school crossing locations based on the number of children
utilizing the crossing and the availability of adequate gaps in the traffic
stream during crossing periods.
Warrant 5 – PROGRESSIVE MOVEMENT
Between two widely spaced signals in a system, an intermediate traffic signal, properly located,
may serve to regulate the size and speed of the traffic “platoon”. A signal maybe installed under
this warrant if vehicle speeds vary and volumes are high.
Warrant 6 – CRASH EXPERIENCE
Certain conditions at an intersection may result in a high incidence of “right-angle” crashes.
This warrant provides for a reduction in the number of vehicles required under other warrants.
Warrant 7 – SYSTEMS WARRANT
This allows installation of traffic signals at some intersections to encourage concentration and
organization of traffic flow networks.
Warrant 8 – COMBINATION OF WARRANTS
Warrant 8 says when several of the preceding conditions are nearly satisfied, a signal may
provide the best solution at the intersection.
Warrant 9 – FOUR HOUR VOLUMES
This warrant is used when volumes are very high on the major street for a minimum of four
hours a day. The side street volumes can be lower than in Warrant 1.
Warrant 10 – PEAK HOUR DELAY
This warrant would be applied where traffic conditions are such that for one hour of the day, the
minor street traffic suffers excessive delay entering or crossing the main street.
Warrant 11 – PEAK HOUR VOLUME
This warrant is also intended for application where traffic conditions are such that for one hour
of the day, minor street traffic suffers undue delay entering or crossing the main street. The
volume requirement for this warrant is higher than for Warrant 10.
These warrants have one common point; they all require certain minimum vehicular volumes.
EFFECTS OF TRAFFIC SIGNALS
What, then, are the effects of traffic signals? Do they improve operation of the intersection?
Are intersections safer after their installation? When located properly and operated correctly,
signals offer the following advantages:
1. They can provide orderly traffic movement.
2. Where proper physical design and control measures are used, they can increase the traffichandling capacity of the intersection.
3. They reduce the frequency of certain crash types, especially the right-angle type.
4. Under favorable conditions, they can be coordinated to provide continuous movement of
traffic along a given route.
5. They can be used to interrupt heavy traffic at intervals to permit other traffic, vehicular or
pedestrian, to cross.
EFFECTS OF TRAFFIC SIGNALS, cont…
The following factors will result from improper or unwarranted signal installation:
1.
Excessive delay will be caused.
2.
Disobedience of signal indications is encouraged.
3.
The use of less adequate routes may be induced in an attempt to avoid the signal.
4.
Crash frequency (especially rear-end crashes) will be increased.
How a Signal Request is Evaluated
TRAFFIC COUNTS
This survey consists of “machine counts” (the “boxes” and rubber hoses frequently seen along state
highways) taken of all vehicles entering the intersection on a typical weekday. In conjunction with the
machine counts, an eight hour “turning count” is conducted during peak traffic hours in which all incoming
vehicles are tabulated according to the direction they leave the intersection.
CRASH RECORDS
A record of all traffic crashes reported to the Michigan Department of State Police is forwarded to MDOT’s
Traffic and Safety Division. The traffic engineer uses these crash reports to study the problems at the
subject location and determine which crash patterns are susceptible to correction through application of
various measures, including traffic signals.
FIELD INVESTIGATIONS
In addition, the engineer also conducts an on-site investigation to observe the intersection and its operating
characteristics. Some of the points the traffic engineer looks for are type and condition of the road, schools
and “school crossings”; large traffic generators, such as shopping centers; existence of parking and bus
stops; and the proximity of other traffic signals.
Before and after signals
crash study
Right angle
Rear end
AFTER
SIGNALIZATION
BEFORE
SIGNALIZATION
Head on, LT
Other
0
100
200
300
400
500
600
Multiway Stop Applications
Again, the decision to install multiway stop control should be based on an
engineering study. The study will evaluate the following warrants:
A. If a traffic signal is warranted, the multiway stop can be placed as an
interim measure while arrangements for signal installation are being
made.
B. A crash problem as indicated by 5 or more reported crashes in a 12
month period that are correctable with a multiway stop installation. Such
crashes include right- and left-turn collisions as well as right-angle
collisions.
C. Minimum volumes:
Just as in signalization, volumes of vehicles and pedestrians play a large
factor in multiway stop placement.
Major street approaches – 300 vehicles per hour for any 8 hours
Minor street approaches – 200 units per hour for the same 8
hours.
These volumes are reduced when the approach speed of the
major-street traffic exceeds 40mph.
CONCLUSION
We hope the preceding explanation gives you a better understanding of the
complexities involved in signal evaluation from first request to actual installation.
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The decision-making process is lengthy, though justified, since a
professional traffic engineer must make decisions based on
engineering studies, not snap judgments resulting from opinion.
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The necessary data requires time to collect. It may take four to six
months to gather and analyze traffic counts and crash records.
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After approval, signal installation must then be engineered, estimates
prepared, contractual agreements negotiated, material requisitioned,
and installation crews scheduled.
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It is not uncommon for the process to take 12 months.
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Traffic signals are never denied on the basis of cost. Our primary
concern is the public’s well-being.