Public Works & Natural Resources
Traffic engineering services include traffic volume counts, service analysis,
accident and safety studies, traffic control devices, signal timing, and coordinating
the City of Longmont School Safety Program including the School Crossing Guard
Program. This service assists with the identification and prioritization of
capital projects to enhance the safety and efficiency of the City's transportation
FREQUENTLY ASKED QUESTIONS ABOUT TRAFFIC SIGNALS IN THE
CITY OF LONGMONT
The traffic signals in Longmont are installed, timed and maintained by the
City Public Works Division. The following are answers to some of the most
commonly asked questions about the signals.
How do we decide where to install traffic signals?
The primary purpose of traffic signals is simply to assign the right-of-way
at intersections that would otherwise be so congested that excessive delays
to vehicles and pedestrians would occur. Traffic signals can also be used
to address certain types of safety problems, particularly, right angle accident
Unfortunately, traffic signals also often cause an increase in other types
of accidents, especially rear-end accidents. At an intersection that is not
prone to right angle accidents, installing a signal might actually increase
the overall number of accidents. In addition, unwarranted signals can cause
unnecessary stops and idling that lead to excess fuel consumption and increased
Since signals can have significant negative as well as positive impacts,
and because they are very expensive ($75,000 - $200,000), standards are needed
to determine where they should be placed. Instead of using arbitrary standards
that could vary with each jurisdiction a set of standards has been established
by the Federal Highway Administration using the experience and expertise of
traffic professionals nationwide. Vehicle volumes, pedestrian volumes, amount
of delay and number of accidents are all considered in the standards. Intersections
with conditions that meet the standards may benefit from the installation
of a signal. Intersections that don't meet the standards typically would see
more negative impacts than benefits if a signal was installed. Both the State
of Colorado and the City of Longmont have adopted the national standards for
the placement of traffic signals. So, the City only considers signalization
at intersections that meet or are projected to meet (for a new road) these
minimum threshold values.
How is the timing of the traffic signals decided?
Two timing philosophies are used in Longmont:
Along arterial and collector streets that have several traffic signals spaced
at periodic intervals, the traffic signals are synchronized to provide coordinated
movement along the main street. The emphasis of this strategy is to minimize
the amount of fuel consumed and air pollution produced. This is accomplished
by setting the time relationships between adjacent signals and the amount
of green time at each signal in such a way that the number of stops is minimized.
Once the minimum stop solution is determined, green times at individual intersections
are checked and adjusted as needed to minimize the amount of delay experienced
by drivers given the constraints of the minimal stop solution.
The disadvantage to synchronized operation is it leads to longer delays to
side street vehicles and pedestrians waiting to cross or enter onto the arterial.
A fixed cycle length must be used for each intersection in order to keep the
signals synchronized along the arterial (a cycle is the time it takes to give
a green light to all of the different movements at an intersection). To maximize
the efficiency of an arterial, signal cycle lengths of 80 to 115 seconds are
used in Longmont and most of this time must be devoted to giving green lights
to arterial street traffic. Thus, drivers who pull up on a side street just
after the green light ended for their intersection approach, may have to wait
more than a minute before the green light is once again available to the side
street. Although this can seem like a terribly long time when your waiting
on the side street, as mentioned above the overall system efficiency is optimized
by operating in this fashion.
Currently, Hover Road, Ken Pratt Boulevard and Main Street are operated as
synchronized arterials. Also, Francis Street from 9th to 17th is synchronized
for parts of the day when traffic is heaviest. 2nd Avenue, 3rd Avenue, Longs
Peak Avenue and 9th Avenue through the downtown area are also synchronized.
The synchronization obviously works better on some of these roads than on
others. This is because other factors such as signal spacing and the amount
of interference from unsignalized access points play a major role in the performance
of a synchronized signal system.
The other philosophy for traffic signal timing is used at isolated intersections
in Longmont. Isolated intersections are those where vehicle arrivals on each
approach are random. There are typically no other signals nearby. At isolated
intersections, there really isn't any opportunity to minimize stops as there
is no way to predict when vehicles will arrive.
The approach we use at these locations is to time the signals to minimize
the amount of delay experienced by drivers. This is best achieved by using
fully traffic actuated operation. Fully traffic actuated signals have vehicle
detectors on all approaches and cycle only in response to the actual demand
on each approach. Unfortunately, many of our existing isolated intersections
do not have vehicle detection equipment installed on every approach thus,
they cannot work as fully traffic actuated. We have implemented a program
to gradually install new detection equipment at these existing intersections
to make them fully actuated. And, all of our new signals are being designed
with the capability to operate fully actuated.
While delays are typically short at isolated intersections, the downside
to this timing philosophy is that the vast majority of motorists have to stop.
That is the nature of the least delay timing solution. The amount of time
spent waiting is minimal but nearly everyone ends up stopping at least for
a short time. That makes this strategy inappropriate along arterial streets
where signals are closely spaced.
Why are the pedestrian "WALK" lights so short?
The pedestrian signals are timed to allow a person to walk across the street
before conflicting traffic gets a green light. The "WALK" portion
is only intended to get pedestrians started across the street. It is typically
only on long enough for a person to make sure it is clear and then start walking.
Most of the crossing actually occurs during the flashing "DON'T WALK"
that follows the "WALK". Flashing "DON'T WALK" means it
is okay to finish crossing once you've started but you should not start across
if you haven't left the curb area yet.. The steady "DON'T WALK"
is next and it means to wait until the next "WALK" light is displayed
before trying to cross.
Just as vehicles must use the sensors to get a green light, pedestrians must
use the push buttons to let the signal know they are there. If the button
is not pushed, the signal will only stay green as long as vehicles are on
the sensors. If there are only a few cars, the green light won't last long
enough for a pedestrian to get across the street. Pedestrians must use the
push buttons to ensure adequate time to cross. The buttons do not make the
green light come any faster - especially when the signals are coordinated
and have to maintain time relationships with one another. All the buttons
do is ensure enough time for pedestrians to get across the street.
Why do some intersections have left turn arrows and others don't?
The City installs left turn arrows where they can reduce the overall delay
experienced by motorists or at locations that have an identified left turn
accident problem that a left turn arrow can help improve. We do not install
them indiscriminately because, where unwarranted, they can cause more problems
than they solve.
Left turn arrows typically decrease the delay experienced by left turning
drivers. However, what many people don't realize is that they also usually
increase the delay and number of stops for non-turning traffic. This results
because some green time must be taken from other movements in order to give
green time to the arrow. It is not simply a matter of putting in an arrow
to improve things for left turning traffic. There is also a cost to other
drivers that results from a left turn arrow.
At locations where there is a high volume of left turners and left turn delays
are high, the increase in delay to through traffic caused by an arrow can
be offset by the decrease in the delay to the left turners. However, where
the through traffic volume is considerably higher than the left turning traffic
volume, the benefit to left turning traffic usually does not outweigh the
increase in delay and stops for through traffic. If arrows are installed under
these circumstances not only does it lead to more congestion, it also leads
to an increase in fuel consumption, vehicle emissions and, potentially, an
unnecessary increase in rear end accidents (because of the increase in stops
for through traffic).
Most of the left turn arrows in Longmont operate in a protected/permissive
mode. That is, cars can turn left when the arrow is on and then continue to
turn left after yielding to oncoming traffic when the green ball is displayed.
Left turn arrows that operate in a protected/permissive mode are an efficiency
tool much more so than they are a safety tool. Where left turn volumes are
high and delays to left turners are correspondingly high, arrows can be used
to decrease the overall delay at an intersection. Under those conditions arrows
As far as safety goes, we don't see significant differences in left turn
accident rates where we do and do not have left turn arrows operating in a
protected/permissive mode. We suspect the reason for this is that most of
our left turn accidents happen at the end of a green ball interval (after
the arrow has already turned off) when the light changes to yellow. When this
happens some people tend to turn assuming that oncoming traffic can and will
stop. This situation occurs whether the turn arrows are installed or not.
Intersections without arrows are regularly evaluated to determine if changing
traffic volumes and patterns have created the need for left turn arrows. If
it is found that the arrows would improve the overall operation of the intersection
they are installed.
Why are there cameras appearing at some signalized intersections?
The cameras are vehicle detectors -- used in operation of the traffic signal.
They are replacing detectors that in the past were in the pavement. Video
detection is the latest technology available for vehicle detection. It offers
a variety of advantages over the in-pavement detectors.
The cameras give us more flexibility in the placement of vehicle detection
zones than the traditional in-pavement detectors. They do a better job of
detecting bicyclists and motorcyclists. They can be used to count the amount
of traffic lane by lane -- to help us better time the signal. In the event
of a lane closure (due to construction for example) the detection zones can
be modified to still detect cars that may not be in their normal position.
And, lastly, the cameras aren't prone to failure related to construction or
other pavement problems.
The cameras have the capability of sending a signal back to a remote location
for real-time monitoring of an intersection. However, we are not currently
using that feature. Also, the cameras do not show enough detail to identify
individual cars, drivers or passengers. They are set to a wide angle picture
to capture all the traffic approaching the intersection in each lane. They
are not used to identify who or what went through the intersection when. And,
no recording is occurring through the cameras.
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Public Works & Natural Resources