To: Honorable Mayor and Members of Town Council
From: Ryan Germeroth, P.E., Transportation Planning & Traffic Engineering Manager
Title
Presentation: Town Traffic Signal Operation Practices
Body
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Executive Summary
The purpose of this memorandum is to provide the Town Council with background information on how the Public Works Department manages the operation of the traffic signals within Castle Rock (not including CDOT signals). A signal ownership map is attached for reference (Attachment A). Past Town-wide citizen surveys have shown that traffic operations and traffic signal timing are one of the issues at the forefront of most of our citizens’ minds. The good news is that the survey rating with regard to traffic signal timing improved from 2015 to 2017. This reflects the investment that has been put into the signal system over the course of the past four years as a part of the multi-year traffic signal system upgrade project. It also reflects staff’s continued efforts to optimize and run a system that is as efficient as possible.
The content of the memo will address several key themes or frequently asked questions related to traffic signal operations and efficiencies. Topics of discussion in this memorandum will include:
• How does staff decide when a traffic signal is needed
• The how and why of signal operations and signal timing efficiencies
• And, a brief discussion on why the Town moved away from nighttime flash operations in 2016
History of Past Town Council, Boards & Commissions, or Other Discussions
A similar presentation on this topic was provided to both the Public Works Commission and the Town Council in January 2014 and February 2014 respectively.
Discussion
In general, the most common inquiries that the Public Works Department receives with regard to traffic signals have to do with requests for the installation of new traffic signals and more commonly, with the operation of the existing traffic signals within Castle Rock. These common inquiries were also documented in the most recent Town-wide citizen survey showing that the traffic signals here within Town and their operation are important to the traveling public in Castle Rock. The majority of comments and complaints are about the operation of the CDOT signals on Founders Parkway as most people do not realize that the Town does not control those intersections.
The Town Staff utilizes both nationally accepted documents such as the Manual on Uniform Traffic Control Devices (MUTCD) and internally developed documents such as the Town’s Traffic Signal Management Plan to guide the planning and day to day operation of the signal system.
Glossary
In order to have a better understanding of traffic signal operations, staff has prepared a glossary of some of the more common terms that are used:
• Demand: The number of vehicles that desire to travel past a point during a specified time period (i.e. one hour).
• Capacity: The maximum number of vehicles that can actually travel past a point during a specified time period (i.e. one hour). It is a characteristic of an intersection. It is affected by a variety of features, such as the intersection design, lane width, turning movements, vehicle speeds and the amount of signal “green time” that is available. There is a limit to the capacity of a roadway and an intersection.
• Cycle Length: The time (in seconds) that it takes for every legal vehicular movement to receive a “green” indication over the course of each cycle.
• Signal Coordination: The timing of traffic signals so that traffic can travel along a street without stopping at every light.
Installation of Traffic Signals
Generally, traffic signals are located at the intersection of major streets since this is where the highest traffic volumes of vehicles and pedestrians are found and where most of the crashes occur; because of this, these locations are more likely to benefit from the installation of a traffic signal. However, traffic signals are just one of several types of intersection control; with yield signs, stop signs and roundabouts the other most common options. The MUTCD has a set of criteria that are useful in determining when the operational conditions at an intersection may warrant the installation of a traffic signal. These conditions are evaluated as part of an engineering study which is required to support the installation of a new traffic signal. Traffic signals are most efficient during heavy volume peak periods, which comprises only about 20 to 25 percent of a typical day. This is why it is important for any new traffic signal installation to be justified with an engineering evaluation and why the Town also assesses whether a roundabout would be a better solution in lieu of a traffic signal. Roundabouts are our preferred control due to full day operations efficiencies. At locations where a roundabout is cost prohibitive to construct or where the forecasted operations with a roundabout in place would not be acceptable, a traffic signal is typically the best solution. Examples of some of the criteria from the MUTCD that are used to determine when a traffic signal should be considered include:
• Vehicle volumes and delay
• Pedestrian volumes
• Intersections at school crossings
• Crash history
• Proximity of railroad crossings
Some of the intersections that have been considered and studied for a traffic control change in Town within the last two years include:
• Crystal Valley Pkwy & Lions Paw Street
• Crystal Valley Pkwy & Plum Creek Blvd
• Fifth Street & Valley Drive
• Meadows Blvd & Butterfield Crossing
• Meadows Blvd & Cherokee Drive
• Meadows Blvd & Future Street (Hospital Access)
• Meadows Blvd & Low Meadow Blvd
• Meadows Blvd & Morningview Drive
• Meadows Blvd & Red Hawk Drive
• Plum Creek Pkwy & Emerald Drive
Most people commonly assume that a traffic signal will solve all congestion or safety problems at an intersection. While it is true that a new traffic signal will help to reduce the delay and improve the safety for side street vehicles (fewer right-angle collisions occur), the new signal can increase delay and the number of crashes (typically rear-end collisions) on the main street. This impact to the main street traffic must be carefully considered when considering the installation of a traffic signal and is why all traffic control options are considered before a new signal is installed.
Steps we take to Improve Efficiencies and Ways to Continue to Improve Efficiencies
The Public Works Department’s Strategic Asset Management Plan (SAMP) outlines stakeholder values and provides the framework for rating each of the Town’s transportation assets. The stakeholder values in the SAMP that apply to the traffic signal system include:
• Lowest total life cycle cost
• Reliability
• Low downtime
• High safety value
• Low environmental impacts
• Minimizing delays for vehicular traffic
• Safe and efficient integration of bikes and pedestrians
The Town’s Traffic Signal Management Plan is a supporting document to the SAMP that outlines key requirements and objectives for the operation of the existing traffic signal system. The document also identifies acceptable service levels and how these should be provided. Performance measures are included which help staff identify locations in need of operational improvements as well as proactive maintenance reviews to minimize equipment failures as much as possible. Currently, the Traffic Engineering Division evaluates the operation of the main traffic signal corridors in Town approximately twice per year. Later this year, a newly deployed travel time monitoring system will be in place on all of the signalized corridors in Town. Corridor travel time data will be logged 24 hours a day and automatic alerts will be generated to staff when a corridor exceeds its performance metrics outlined in the Signal Management Plan. This new system will help Town staff better monitor the signalized corridors in Town with the intent of further improving the operational efficiency of the traffic signals in Castle Rock.
Based on the current assessment criteria within the SAMP, the Town’s traffic signal system has an overall Good rating in the 2018 Transportation Infrastructure Report Card. The risks to maintaining this current service level are relatively low given that the Town has recently made significant capital upgrades to the Town’s traffic signal system. The current practices and procedures are ensuring timing plans are as efficient as possible. Physical assets in this asset class are well maintained and are achieving low total life cycle costs.
With regard to the day to day operations, traffic signals within Castle Rock operate either individually (i.e. Meadows & Coachline) or as part of a coordinated group (i.e. Plum Creek Parkway). The determination as to how the signals should operate is based upon the spacing between the intersections. It is more appropriate to operate closely spaced intersections as a coordinated group so that traffic can pass through several signals before possibly having to stop. Coordination between intersections becomes much more difficult the farther apart they are spaced.
Traffic volumes at each intersection are used to prepare a timing plan that balances delay between the intersecting streets. Delay at a traffic signal is unavoidable since to have a green in one direction means having a red in the conflicting direction. Given that traffic volumes and patterns vary throughout the day, staff has developed multiple signal timing plans for an intersection. For example, the morning rush hour, when most motorists are trying to get to I-25, will have a different plan than the afternoon rush hour when most motorists are trying to get home. It is important to find the appropriate balance between the green times provided for each movement at the intersection with the heavier volume movements being allotted more green time. During lower volume periods outside of the peak travel hours, shorter signal cycle lengths are used and the split of time allotted to the main street versus the side street is more balanced. These shorter cycle lengths and the more balanced split times are used to minimize the delay to the side street as much as possible while also still minimizing the number of stops along the corridor for travelers on the main street.
Even with the best efforts given toward balancing delay at each intersection, back-ups still can occur. Only so much traffic can physically travel through an intersection in a given period of time. This is what is referred to as the capacity of an intersection and it is affected by a variety of features, such as the intersection design, lane width, turning movements, and vehicle speeds. When the number of cars exceeds the capacity, back-ups will start to occur. At signalized intersections, the timing plan can also limit capacity since green time must be split between the intersecting streets. Adding green time to one movement in order to alleviate congestion means that green time must be reduced on conflicting movements which in turn increases their congestion.
It is very similar to water trying to enter a funnel or pipe. If each approach to an intersection is a funnel and water enters the system faster than it can be processed at the outlet, water will spill back and overflow the system. It may be possible to modify the outlet to the funnel to provide additional capacity but there will be a trade off in doing so by reducing the size of the funnel outlet on another approach.
Other potential causes for back-ups at signalized intersections include the irregular arrival of traffic at an intersection (i.e. school traffic rush during drop-off or pick-up times), detection equipment failures, pedestrians crossing at the intersections and/or emergency vehicle pre-emptions. Detection equipment failures, whether vehicle or pedestrian detection, is the most common cause of the signal cycling to the side street when no one is there. Public Works monitors detection failures like these via the central traffic signal system software and schedules repairs to get the signal back into normal operation. Prior to the deployment of the new central system software in 2016, the real time monitoring of detection failures was not possible.
Other signal system upgrades completed in Castle Rock since 2015 or that will be completed in the near future that have or are expected to improve signal efficiency include:
• High speed signal communications network via fiber optic cable (complete)
• New central monitoring software and signal controllers (complete)
• Detection malfunction monitoring and remote troubleshooting (complete)
• Battery back-up systems at all signals (complete)
• Real time travel time monitoring on all corridors (end of 2018)
• Closed circuit television cameras for remote traffic management (end of 2018)
In addition to these upgrades, in the recently completed 2017 Traffic Signal Operations Report, Public Works has also identified several next steps to further improve the efficiency and reliability of the Town’s traffic signal system. These initiatives include furthering the development of automated traffic signal performance measures (ATSPMs) by using the available real time date from the upgraded equipment listed above. ATSPMs provide high resolution data to staff to actively manage system performance and improve customer service with the goal of reducing congestion and costs. In addition, staff will continue with the implementation of traffic responsive signal timing plans during off-peaks periods. In a traffic responsive operation, the signal can select from a set of preapproved timing plans that are selected based on the traffic conditions at the intersection. Staff utilized a traffic responsive operation at the main signalized entrance to the Douglas County Fairgrounds during the 2018 Fair. Finally, implement adaptive traffic signal timing plans on appropriate corridors in Town. Town staff is currently in discussions with CDOT on the feasibility of an adaptive deployment on Founders Parkway. Several Town owned corridors could be options as well but would require additional funding for the equipment needed to run a fully adaptive system. In an adaptive operation, the signal system is able to adjust on the fly to changing traffic conditions while also maintaining coordination between the signals along a given corridor.
Pros and Cons of Nighttime Flash Operations
Prior to 2016, most traffic signals in Town ran a nighttime flash operation between midnight and 5:30AM. This type of operation is typically when the main street approaches flash yellow with the side street flashing red. The benefit of having this type of operation is that a driver on the main street can traverse the corridor without having to stop for a red light. A car on the side street will have to come to a complete stop, but if no vehicles are coming on the main street then they can turn onto the main street with minimal delay.
Staff reached out to ten jurisdictions in Colorado during the development of this memo that are good benchmark agencies for Castle Rock. To date five of the ten have replied. Some of these agencies do use nighttime flash while others do not. Those that do utilize it do so at very select locations where traffic conditions or operations are a good candidate for nighttime flash. Those agencies that do not use nighttime flash did at one time but have moved away from the operation for safety reasons. A summary of the feedback gathered will be presented at the Council meeting.
In early 2016, Town staff completed an assessment of the nighttime flash operation that resulted in the removal of nighttime flash operation from the Town. Attached is the staff assessment memorandum that was completed (Attachment B), which includes an assessment from Castle Rock Fire on emergency response times late at night. The Fire Department’s assessment found that the nighttime flash operation was slowing their response time to an increased number of calls for service at night due to the increasing population in the Town. With regard to vehicular and pedestrian safety, right angle crashes tend to escalate during late night flash operations because a driver coming off the side street has a hard time judging the distance of the approaching vehicle. A report done by the Federal Highway Administration (FHWA) titled Removal of Signal Flashing Mode During Late-Night/ Early Morning Operation looked at eight locations in Winston-Salem, North Carolina. In the assessment for the report, it was found that the total crash, right angle crash and injury crash rates were lower when the signals were run in full operations at night as compared to a flashing operation. Due to these safety and emergency response reasons, staff implemented the removal of the nighttime flash operations when the signal controllers were upgraded in 2016.
In order for Town staff to remove nighttime flash while minimizing the impact to the traveling public, the goal was to keep both the main street and side street delays manageable. In order to accomplish this, shortened timing plans were implemented in the traffic signal controllers that were adjusted to the lower volume demand late at night in order to minimize the delay experienced by a driver waiting at a red light late at night. During this operation, progression is maintained by having the main streets remain green (green rest), and only cycle to the side street when a vehicle approaches and comes to a complete stop. To date, the delay to the traveling public with the signals operating in green rest late at night has not been substantially different from the nighttime flashing operation. Generally, the time spent waiting for the signal to change late at night should be ten seconds or less. If drivers notice a longer delay time, there could be a malfunction that needs addressed. Staff strives to asses these malfunctions and repair them within three days of notification. Due to this, staff is not planning to change the late night operation back to nighttime flash at this time.
Budget Impact
This is an informational presentation to get feedback from Town Council and has no budget impacts.
Attachments
Attachment A: Signal Ownership Map
Attachment B: Recommendation to Remove Late Night Flash - April 2016 Staff Memo