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1. How do traffic signals work?

In Woodbury, traffic signals usually operate in one of two ways: using vehicle detectors, usually a wire loop buried in the pavement, or via video detection, which uses cameras mounted on mast arms or signal poles.

The vehicle detector loops work like a metal detector, sensing the car's magnetism, not weight. Video detection uses a camera that can recognize an approaching vehicle (Radio Drive/Lake Road and Tamarack Road/Interlachen Parkway use video detection).

In both of the systems, the detectors send a signal to a computer, which then changes the light at the appropriate time in the signal sequence. The signals do have some fixed timings entered, such as minimum and maximum green times, yellow time, or pedestrian timing, but otherwise the signal responds to the traffic that is present.

In the middle of the night, the signal might stay green for hours in the same direction if there is no cross-traffic.

2. How much do traffic signals cost?

If there aren't any road improvements needed, such as adding turn lanes, then a complete traffic signal system for a typical four-way intersection will cost about $250,000 to $300,000. When additional costs like control equipment, project inspection, and design are taken into account, the costs generally exceed $300,000. There are also ongoing costs for electricity, painting, re-lamping, and other maintenance.

3. Will a traffic signal make an intersection safer?

A traffic signal should not be expected to reduce the total number of crashes at an intersection. Traffic signals in Minnesota, according to a 2013 study, have a statewide crash rate of about five crashes for every 10 million entering vehicles.

An all-way stop has an average crash rate of four crashes for every 10 million entering vehicles, and a two-way stop has an average of three crashes per 10 million entering vehicles. Of course this can depend on other site conditions as well, but on average, a signal will reduce right-angle crashes while causing an increase in rear-end crashes with an overall increase in crashes. Right angle crashes are often the most severe crashes resulting in injury, while rear end crashes involve property damage but are generally less severe.

Unfortunately, drivers occasionally run red lights or fail to notice stopped traffic in front of them. Many of the city's serious and fatal crashes occur at traffic signals. Therefore, it is very important to study an intersection to ensure a traffic signal is appropriate.

4. How does the City decide where to install signals?

Traffic signals are usually warranted based on minimum traffic volumes during eight hours of a typical weekday. These minimum volumes, or "warrants," vary depending on the speed of the roadway and the number of approach lanes.

For a signal to be considered, it needs to meet the necessary warrants and be justified by an engineering study. Engineering studies review factors such as access and distance to nearby signals, lane geometry, and future road plans. If all the necessary conditions are satisfied and funding is available, a signal installation may be considered.

5. Why is the "Walk" light so short? I can only get halfway across before it starts flashing.

The "Walk" light isn't supposed to stay on for more than a few seconds. The state standard is seven seconds, but can be as little as five seconds. The "Walk" light means that it's okay to start crossing. If you started crossing at the transition from "Walk" to a flashing "Don't Walk" sign, it should flash long enough for most people to make it all the way across the road before the "Don't Walk" stops flashing and the light starts to change. When the signal is flashing "Don't Walk", it is safe to finish crossing the intersection; however, it is not safe to enter the crosswalk at this time. When the "Don't Walk" stops flashing and the light changes, it is unsafe to cross the road.

6. Why can't the signals be timed together to reduce delay?

All traffic signals in Woodbury rely on detectors to maximize efficiency. Traffic signals typically will not turn red for the main roadway until the detectors sense a gap in the traffic and a vehicle is detected on the cross street, or the maximum time for a green light is reached. This is designed to increase efficiency and safety by reducing the number of vehicles confronted with a yellow light.

Unfortunately, gaps in traffic often occur when a large platoon of traffic is approaching but is out of range of the detectors. This causes the entire platoon of vehicles to arrive at a red light, sometimes for the benefit of just a few vehicles entering from the side street.

Traffic signal systems can be coordinated, but doing so will typically lead to longer wait times for side street traffic and for left turns. When traffic signals are coordinated they operate on a fixed timing cycle to remain in step with each other. The cycle must be long enough to accommodate the busiest intersection in the corridor. While this coordination can be a benefit, it is costly to implement and will typically increase the wait time to enter a highway. These coordinated timing cycles also benefit from regular adjustments and maintenance, which can be costly to optimize efficiency.

7. What should drivers do when a traffic signal is flashing or all lights are dark?

When a traffic signal malfunctions but still has power, it will often revert to an all-red flashing mode. When you approach a signal light that is flashing red, you must come to a complete stop (Mn Statute 169.06, Subd. 7). Sometimes the traffic signal will flash red in all directions, making the intersection operate like a four-way stop. At some intersections, the lights may flash yellow for the main roadway, thus making the intersection function like a two-way stop. Always stop at a traffic signal that is flashing red, and be sure it is safe before entering the intersection as the cross traffic may not be required to stop. If you approach a flashing yellow light, you do not need to come to a complete stop. However, be alert for drivers entering the intersection and proceed with caution.

Signals that are in flashing mode can return to normal operation at any time and without warning, even when repair workers are not present. The flashing red may change to a solid red, and drivers often fail to notice the difference, running the red light. Proceeding on a solid red light is always illegal and can lead to a crash. Be alert, and make sure that the light is still flashing and that it is safe to proceed before entering the intersection.

During severe weather season, it is not unusual to have power outages due to high wind or lightning strikes. Traffic signals throughout the city rely on electricity to function. If a traffic signal is in an area of a power outage, it will go dark with no light operation.

If you approach a traffic signal without power, remember that the rules of right-of-way still apply. You should:

• Slow down and prepare to stop.
• Determine who has right-of-way to proceed through the intersection.

If there is a lot of traffic at the intersection, it will naturally begin to function as a four-way stop. Remember that vehicles have right-of-way in the order in which they arrive regardless of which road they arrive from. If two vehicles arrive at the intersection at approximately the same time, the vehicle on the left must yield to the vehicle on the right, and left turns must yield to oncoming traffic.

Always assume that other drivers may react unpredictably or that they may not even slow down when driving through an uncontrolled intersection. Drive defensively, even if it means waiting beyond "your turn" to proceed through the intersection safely.

To report a signal malfunction, call the Washington County Sheriff's Office dispatcher at 651-439-9381. If it is an emergency, call 911.

8. How should drivers respond to a flashing yellow arrow?

When approaching a flashing yellow arrow, drivers are allowed to turn left after yielding to all oncoming traffic and to any pedestrians in the crosswalk.

Oncoming traffic has a green light. Drivers must wait for a safe gap in oncoming traffic before turning. It is illegal to pull into the intersection with a flashing yellow light if the driver is unable to move through the intersection immediately.

To learn more about flashing yellow arrows, visit the Minnesota Department of Transportation website.

9. Which government agency owns which traffic signals?

Within the city, traffic signals are generally owned by the City, Washington County, or the Minnesota Department of Transportation (MnDOT). The following rules are generally true, but there are exceptions:

• Signals at interchange ramps and loops are owned by MnDOT.
• Signals at all intersections involving at least one state road (e.g. Manning Avenue) are owned by MnDOT.
• Signals at intersections involving a county road are owned by the county.
• Signals at intersections that only include city roads are owned by the city.

To report a signal malfunction, contact the highway department responsible for the signals:

• For city signals, contact the City of Woodbury Public Works Department at 651-714-3720.
• For signals on state highways in Woodbury, contact MnDOT at 651-234-7500.
• For signals on Washington County roads (Radio Drive, Woodbury Drive, Valley Creek Road, Century Avenue and Military Road) call Washington County Public Works at 651-430-4300.
• In an emergency, dial 911.