Traffic Signals

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Frequently Asked Questions 



The City of Naperville has more than 3,400 street intersections. Nearly half are controlled by signs (stop or yield) or by traffic signals. Requests for new traffic controls or changes to existing traffic controls are handled by the city's Transportation, Engineering and Development Business Group. 

Decisions about establishment of traffic controls are made only after a detailed investigation of each intersection involving consideration of the character of the streets involved, the accident history of the intersection, the physical characteristics of the intersection, and the relationship of the intersection to surrounding community facilities, such as parks or schools.

City of Naperville has developed signal system timing plans for arterial progression on individual signal systems and most significantly for coordination between systems and crossing coordination for signals in Naperville under the jurisdiction of the City of Naperville, DuPage County Highway Department and the Illinois Department of Transportation.  Individual traffic signals are coordinated into signal systems, which are then coordinated with adjacent signal systems.

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For questions or concerns related to traffic signals, please use our Citizen Support Center and select the topic "Traffic Signals" to reach City staff.


Frequently Asked Questions

What is the Purpose of a Traffic Signal?
Traffic signals are designed to:

  • Provide for the orderly and efficient movement of people.
  • Reduce the frequency and severity of certain types of crashes.
  • Provide appropriate levels of accessibility for pedestrians and side street traffic.

Are Traffic Signals the Answer to Solving Traffic Problems? 
Advantages of Traffic Signals
Signals offer maximum control at intersections. They relay messages of both what to do and what not to do. The primary function of any traffic signal is to assign right-of-way to conflicting movements of traffic at an intersection. This is done by permitting conflicting streams of traffic to share the same intersection by means of time separation.

By alternately assigning right of way to various traffic movements, signals provide for the orderly movement of conflicting flows. They may interrupt extremely heavy flows to permit the crossing of minor movements that could not otherwise move safely through an intersection. When properly timed, a traffic signal increases the traffic handling capacity of an intersection, and when installed under conditions that justify its use, a signal is a valuable device for improving the safety and efficiency of both pedestrian and vehicular traffic. In particular, signals may reduce certain types of accidents, most notably right-angle (broadside) collisions.

Disadvantages of Traffic Signals 
While many people realize that traffic signals can reduce the number of right-angle collisions at an intersection, few realize that signals can also cause a significant increase in rear-end collisions. Traffic signals are not a cure-all for traffic problems. The primary goal of the traffic engineer is to attain the safest and most efficient overall traffic flow possible. In addition to an increase in accident frequency, unjustified traffic signals can also cause excessive delay, disobedience of signals, and diversion of traffic to residential streets.

Uniformity of Traffic Signals
The purpose of traffic control devices (signs, signals, and pavement markings) is to help provide safe, orderly, and predictable movement of traffic. Improper or overuse of devices may reduce safety and efficiency of traffic flow. State and national standards and uniform state vehicle codes have been developed to provide uniformity of appearance and proper application of devices. For example, imagine the confusion of drivers if each city or county used traffic signals with a different color, shape, and meaning.

What are the Criteria for Considering the Installation of a Traffic Signal?
Before installing a traffic signal at an intersection, established minimum criteria (per the Federal Highway Administration’s Manual on Uniform Traffic Control Devices) must be satisfied in order to consider the installation of a traffic signal. A review primarily includes:

  • The amount of vehicular and pedestrian traffic
  • The need to provide interruption to the major flow for side street vehicles and pedestrians
  • The accident history of the intersection. 

It is important to know that even if a particular intersection meets one or more of these criteria it does not mean that installation of a traffic signal is automatically justified at a particular location.  The criteria are one of the factors that must be considered when completing a comprehensive engineering study of an intersection. 

How Many Traffic Signals are in Naperville?
There are approximately 155 traffic signals within the City of Naperville municipal boundaries.  Of these 155 locations, 91 are maintained by the City of Naperville.  The remaining signals are maintained by either DuPage County (75thStreet, Chicago Avenue, Hobson Road, Naperville Road/I-88 Interchange) or the Illinois Department of Transportation (IL Route 59).

How Much do Signals Cost?
If there are not any road improvements needed, such as adding turn lanes, then a traffic signal contract will usually cost approximately $200,000 to $250,000 (2008). The City also spends approximately $2,000 per year per traffic signal for ongoing electricity and maintenance costs. 

How are Traffic Signals Timed?

Traffic engineers must first collect an extensive amount of data before developing settings for an intersection. Traffic counts, the number of lanes, the visibility, the speed limit, the proximity to other signals, and the grade are only a few examples of the type of information engineers look at before they can start timing an intersection.

Once the data has been collected, signal optimization software is then used to analyze an intersection and to determine the best settings.   These settings are then refined in the field through observation of actual traffic patterns.  Usually, three different timing scenarios are developed to address morning peak traffic, afternoon peak traffic and off¬peak traffic patterns that occur throughout the day.

A typical cycle (the total time it takes a signal to serve every movement at an intersection) can range from 80 seconds to 160 seconds (120 seconds is the most common cycle length).  Traffic actuated signals use detectors located in the pavement on the approaches to traffic signals to monitor and assign the right of way on the basis of changing traffic demand. These signals attempt to assign most of the available green time to the heaviest traffic movements.

Are the Traffic Signals in Naperville Coordinated?
The greatest benefits to the public for each dollar spent on traffic operations improvements come from the coordination of adjacent traffic signals to provide smooth movement of the traffic through groups of signals on an arterial street. The coordination of traffic signals to facilitate smooth traffic flow (progressed movement) along a street is a proven technique. The quality of flow along a street is basically a function of the spacing of the signals along the street, the prevailing speed of traffic on the street, number of intersections with other major streets, and the traffic signal cycle length. The amount of traffic and the proportion of the green time given to the progressed movements are also important.

Nearly all of the traffic signals within the City of Naperville are interconnected by dedicated wires to form a series of coordinated signal system corridors.   These corridors are typically on arterial roadways such as Washington Street, Naper Blvd, etc.  These systems are often multi-jurisdictional that include City, IDOT, and DuPage County traffic signals.  

In general, traffic signals within the City of Naperville are coordinated using the following guidelines:

  • During the morning rush hours, traffic signal systems are generally programmed to facilitate northbound or eastbound traffic (direction of heaviest traffic volumes). 
  • During the mid-day, a balanced approach is typically taken because there is often no predominant direction of traffic.
  • During the evening rush hours, the traffic signal systems are typically coordinated to increase southbound or westbound traffic.
  • In the late evening and early morning, traffic signals are typically allowed to run independently because the arrival of traffic becomes random.  Side streets are served more quickly when the signal is operating in this mode.

Traffic Signal Coordination Goals 
Many drivers ask why they have to wait so long for a signal to change. Many of these drivers are waiting to enter a major arterial street from a side street. This is even more frustrating when no traffic can be seen on the arterial. To allow the coordination of the arterial, the side street must wait until the main traffic movement on the arterial has gone through the intersection. It is possible that the arterial traffic can’t be seen immediately, but will soon be passing through the intersection.

The goal of coordination is to get the greatest number of vehicles through the system with the fewest stops and/or shortest travel time. It would be ideal if every vehicle entering the system could proceed through the system without stopping. This is not possible, even in well-spaced, well designed systems. Therefore, in traffic coordination, the majority rules and the busiest traffic movements are given precedence over the smaller traffic movements.  This means that side street traffic will typically experience a longer wait time. However, once on the main street, motorists should generally experience better free flowing traffic conditions.
An associated benefit of traffic signal coordination is a reduction in pollution.  Improved traffic flow reduces vehicular emissions through reduced idle time and more efficient operation.

What are the Results of Napervilles Signal Coordination and Timing (SCAT) Studies?

Signal Coordination and Timing (SCAT) Studies are comprehensive reviews of the programming and coordination of the signals within a particular system.  Based upon the results of this work, new timing plans are implemented and adjusted in the field. 

From 2005 to 2008, the City of Naperville conducted SCAT studies on numerous signal systems.  These corridors include:

  • Mid-Washington Street System (Fifth Avenue to Gartner Road) – 2005
  • Washington Street - Bailey Rd to Royce Rd - 2006
  • 95th Street - Cedar Glade Dr to Book Rd - 2006
  • Ogden Avenue - Fort Hill Dr to Columbia St - 2007
  • Plainfield Naperville Road - 87th St to 95th St – 2008
  • North Aurora Rd - Frontenac to Fairway – 2008
  • Washington Street - Bauer Rd to Warrenville Rd – 2008
  • Other systems were also adjusted as new traffic signals have been installed.

As part of the SCAT work, a before and after comparison of travels time and pollutant emission analysis is perform.  A summary of the results of these studies can be found by clicking on the links below.  It is important to remember that these results are representative.  Your travels time may vary significantly depending upon the conditions encountered on any particular day. Following are the results for Naperville's SCAT Study.

DuPage County and IDOT have also completed SCAT studies on systems in our area, including:

  • Illinois Route 59 – Glacier Park to Beebe Drive – 2006
  • Illinois Route 59 – Ferry Road to North Aurora Road – 2000
  • 75thStreet – Fort Hill Drive to Wherli Road – 2003

As the city has recently completed SCAT studies on all of the systems it maintains, no additional SCAT studies are currently planned.  If major changes in traffic patterns are observed, the city may decide to perform a revised SCAT study in the future. City staff does routinely make minor adjustments to signal timings at individual intersections if local traffic conditions change.

Does This Mean I Will Never Have to Stop at a Red Light?
Unfortunately, the answer to this question is "No". There are many reasons why, even when traffic signals are coordinated, you will still have to stop at red lights.  In order to operate traffic signals safely, several things must be considered.  Due to the fixed amount of time for the "coordinated" traffic signal to provide a green light for all of the traffic movements, each of the following has a direct relationship to the amount of green time available for an intersection approach and/or the ability to remain in synchronization within a coordinated system along a roadway.

  • Emergency Vehicles Pre-emption: When an emergency vehicle pre-empts the normal operation of the signal, the signal is thrown out of coordination with the rest of the signal system.  It can take several cycles, before an individual traffic signal can “catch up” and return to coordination with the rest of the system.
  • Pedestrian Crossing: For safety, enough time must be allowed for a pedestrian to cross the street from curb-to-curb walking at a pace of four (4) feet per second (typical). This is called the pedestrian clearance interval and is represented by the flashing "DON'T WALK" or upraised hand symbol. The wider the street, the more time needed to cross and the less time available for the green light in the opposite direction.  Similar to emergency vehicle pre-emption, long pedestrian times needed at very wide intersections can throw a signal out of coordination with the rest of the system.
  • Cross Traffic: Like pedestrian crossings, enough time should be allocated to clear the waiting traffic on the cross street. The heavier the cross traffic, such as experienced near schools, businesses, and other heavy traffic generators, the more time needed to clear them through the intersection and the less time available for the green light in the "coordinated" direction.  Further, a coordinated system can intersect other coordinated systems that run in different directions (for instance, where a major north/south street intersects with a major east/west street).
  • Left-Turn Signals: Where left-turning traffic is especially heavy and/or the amount of opposing traffic is so heavy that there are not enough gaps in the traffic to safely complete a left-turn, then left-turn signals are usually installed. The amount of time for left-turning traffic also limits the time permitted for the "through" traffic flow in the opposite direction.
  • Construction:  If there is construction in your area, traffic signal coordination is often disrupted.  When a street is repaved or curb is replaced, the detector loops for the traffic signal are often damaged.  This requires that the signal be placed in an automatic mode to make sure that all movements are served.  Unfortunately, this sometimes requires traffic to stop when no vehicles are present or approaching from the conflicting direction.
  • Two-Way Traffic Flow: Another thing that limits the amount of time for the green light in one direction is the need for "coordination" in the other direction as well. The distance between traffic signals and the speed of the traffic determine the way in which the green lights at the next traffic signal "line up". If the spacing is not equal between traffic signals, the green lights may only "line up" well in one direction. When this happens, the green lights will normally "line up" better in the direction with the most traffic. The traffic in the other direction may have to stop.
  • Off-Peak Traffic Periods: Another reason that you may have to stop is that the traffic signals are not coordinated. During times when traffic is light, traffic signals often are allowed to run independently. Traffic signals are most often coordinated during the "peak" travel times when traffic is heaviest. These times are usually between 7-9 a.m. and 4-6 p.m.

Why do I have to Wait so Long on a Side Street?
In order to have "coordinated" traffic signals, each traffic signal in the group must be able to allow the green light for all movements during a common fixed time period. The time period chosen is usually determined by the largest intersection with the most different movements. This will most often be an intersection that has left-turn arrows for all directions and wide cross streets. For that reason, the time period that is fixed for each traffic signal may be rather long. So, if you are waiting for a green light to cross the "coordinated" street where there are no left-turns arrows and very light traffic on the side street, chances are very good that you will feel like you are waiting for a very long time. Actually, you should rarely have to wait any longer than about two minutes. This can sometimes seem like a very long time.

How Does a Traffic Signal Know How Long to Stay Green for Each Movement?
Various devices are frequently used to detect the presence of vehicles at intersections. These detections are used by a controller (computer) at each intersection to adjust the timing of the traffic signal based on demand. The most common type of detection is provided by a series of wire loops in the pavement which detect when vehicles disturb the small magnetic field around the loop. Often the loops can be seen as lines making circles or rectangles on the pavement. (See illustration below)  The thin lines are the sealant used to cover the wire. Newer forms of detection include small overhead cameras which give an image of the approaches to the intersections, and the zones of detection maybe drawn
on a computer screen rather than in the pavement.


Motorcycles can usually be detected with inductive loops.  However, it is important that the motorcycle be directly over the sensors.  Due to their relatively low amount of conductive materials, bicycles cannot typically be detected with the in-pavement loops.  Bicyclist should use the pushbuttons near the sidewalk to activate the signal when approaching from a side street.

What do the Pedestrian Signals Mean and Why don't they Allow Pedestrians to Cross the Street Immediately after the Button is Pushed?
Pedestrian indications consist of the illuminated words WALK and DONT WALK or the illuminated symbols of a walking person (symbolizing WALK) and an upraised hand (symbolizing DONT WALK). The steadily illuminated DONT WALK indication means that a person should not enter the roadway in the direction of the signal. The flashing DONT WALK means that the pedestrian should not enter the roadway, but that any pedestrian that has started to cross may proceed to cross the street or to a safety island. The WALK indication means that a pedestrian may proceed toward the signal, but caution should still be used in watching for potential turning vehicles. Pushing the pedestrian crossing button is similar to the vehicle detectors mentioned above. The signal controller will provide a WALK indication during the normal sequence of the signal
lights when vehicle movement conflicts are minimized.

Pushing the pedestrian button will not instantaneously bring up the “WALK” symbol.

The following brochure provides additional information regarding the operation of pedestrian signals.

Why do Signals Sometimes Flash Red and What does it Mean?
Signals which normally operate with a sequence of green, yellow, and red lights may revert to a flashing operation in certain situations. For example, signals contain a fail-safe program to automatically begin flashing if an event occurs which would interfere with normal safe operations such as a brief power interruption. Signals may also be flashed for special events or response to unique situations. Drivers approaching a flashing red light should treat it as they would a stop sign.

What should Drivers do When a Signal is not on (ie. No Power)?

When a power failure or other malfunction occurs and the lights for a traffic signal are not illuminated, the driver of any vehicle approaching the intersection is required to stop at the intersection and may proceed with caution when it is safe to do so.

What is Emergency Vehicle Pre-Emption?
Emergency vehicle pre-emption can be used for any authorized emergency vehicle, including fire engines, ambulances, and police vehicles. The purpose is to obtain a green light for the emergency vehicle as soon as possible or to hold an existing green light. To obtain a green light, existing green lights, including pedestrian walk intervals, are abbreviated. After the yellow change interval, a green light for through and left turn traffic is given to the approach to be used by the emergency vehicle.

What are the Floodlights on the Signal Arms For?
These white lights are activated by emergency vehicles, when the system receives a signal to override its normal timing program. The flashing white light is displayed for the direction of the approaching emergency vehicle.  A solid white light is displayed for the other two or three intersection approaches.  Emergency Vehicle Pre-emption (EVP) is activated by a special transmitter within the emergency vehicle which forces the system to give a green light to that approach, thereby reducing emergency response times and the risk of a crash. It’s illegal to possess one of these devices unless you’re driving an emergency vehicle.

What are Some of the Common Myths About Traffic Signals?

MYTH # 1 – If all the traffic signals were just retimed/synchronized properly, it would eliminate congestion on our streets.

FALSE – Periodic retiming of traffic signal systems can often result in modest increases in the efficiency of traffic flow.  However, timing adjustments can rarely make significant reductions in congestion at major intersections.  When traffic volumes become extremely high, the only method to significantly reduce congestion is to add travel lanes.  The expansion of the intersection of 75th Street and Washington Street is an example of the type of capacity improvement needed to meet the traffic demand.

In some respects, traffic flow is similar to water flow.  Trying to get an extremely large number of cars through a small intersection simultaneously could be compared to trying to extinguish a major building fire with a garden hose.   

MYTH #2- Traffic signals use weight or pressure to detect vehicles.

FALSE – Modern traffic signals use inductive loops in the pavement or video sensors to detect vehicles.  In addition, some people think that if they back up and drive forward again, they will make the signal change quicker. This does not work.  The mechanism does not count the number of vehicles waiting.

MYTH #3 – Pushing the pedestrian button more than once will make the signal change faster.

FALSE – When a pedestrian button is pushed, a call for that movement is placed into the signal controller.  The walk symbol will appear at the appropriate time within the normal traffic signal cycle/sequence.  Pushing the pedestrian button addition times does not influence the operation of the traffic signal or make the walk symbol appear sooner.

What are Some Tips for Improving My Travel Time through Naperville?

Tips for Motorists
Drive the speed limit. Signals are timed to work best when traffic goes the speed limit. Driving faster will simply get you to the next signal too early, causing you to stop more often. The best way to avoid stopping is to slow down when you see a red light ahead and give it time to change to green.
Stop behind stop bars at red lights. Many intersections have vehicle detectors (wire loops) embedded in the pavement. These signals can detect the presence of vehicles and let the controller know that vehicles are waiting. Stopping behind the bar insures that the controller "senses" your car and keeps the crosswalk clear for pedestrians.
Be aware that the City of Naperville has red light photo enforcement cameras on some intersection approaches to discourage red light violations.

Tips for Bicyclists
Use the push buttons to activate the pedestrian signal on side streets if no vehicles are present.  Due to the relatively low content of conductive materials, bicycles cannot typically be detected with the inductive loops in the road pavement.

Tips for Pedestrians
When crossing dedicated right turn lanes, pedestrians should pay extra attention to right turning vehicles.  Inconsiderate or distracted drivers often fail to yield to pedestrians as they are required to do when making right turn movements.

How Do I Report a Malfuntioning Traffic Signal?
Call the City of Naperville’s Dispatch Center at (630) 420-6187 for operational problems such as lamp outages, pole knockdowns, flashing red conditions, etc. The city’s contractor will investigate and make the necessary repairs. When you call, please give us specific information about the location of the signal so we can make the repair as quickly as possible.

For traffic signal timing concerns, please contact the Transportation, Engineering, and Development Business Group at (630) 420-6100 or email at   Please be prepared to provide specific information about the intersection, day, time of day, and any unusual conditions that you observed before calling.