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Public Works & Natural Resources

Engineering Services

Traffic Engineering

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 system.


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 problems.

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 air pollution.

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 are installed.

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

Updated 2/8/12