Lexington Center Streetscape Project

Modern Traffic Signals

The traffic signals that are proposed for the Woburn St / Mass Ave intersection are different from the ones that most of us are familiar with. The technology has advanced significantly since signals ran on fixed timers, resulting in the elimination of unnecessary waits and the handling of traffic in a more efficient, safer manner. The two central features of modern traffic signals are detection and programming.

What is meant by “Detection”?

The new signals include a camera with a single low-resolution fish-eye lens that continuously monitors all of the approaches and identifies vehicles - even bicycles - as they near the intersection. Note that these cameras are object detectors, not traffic cameras. These cameras are not capable of producing an identifiable image.

The signal immediately processes the information from the camera (the “programming” part) and uses it to increase the efficiency of the traffic flow through the intersection.


  • When a vehicle is detected, the signal can change to provide a green light to the approaching vehicle without the delay that was seen years ago when signals were strictly on timers.
  • If no vehicles or bicycles are waiting for a green signal, other traffic proceeds without interruption.
  • Similarly, left-turn signals are activated only when there is a car actually waiting in a left-turn lane.
Camera at Spring St / Hayden Ave Intersection

The white bell-shaped object is the fish-eye camera at the intersection of Spring St / Hayden Ave. Click to enlarge.

What is meant by “Programming”?

Old-style traffic signals ran on set timers so that each part of the cycle lasted the same amount of time, every time.

Modern traffic signals are programmable. This eliminates the stopping of traffic when there is no reason to wait and allows for adjustments based on time or traffic pattern changes. When traffic is stopped, signals return to green seconds after vehicles have cleared the intersection.

Among the advantages:

  • As mentioned above, detectors will identify vehicles and bicycles that are waiting at an intersection and the signal can react appropriately.
  • Crosswalks are activated by buttons pushed by pedestrians. All directions of vehicle traffic stop when the crosswalk ”walk” is illuminated. If no pedestrians need to cross, that part of the cycle is skipped.
  • The programmed cycle can be different for different times of the day so that it works better with the traffic at those times. There can be a morning rush program, an afternoon rush program, a middle of a weekday program, a nighttime program, etc.
  • As traffic patterns change, the programming can be easily modified to make adjustments as needed.
  • Radio receivers detect when emergency vehicles are approaching in order to give a green signal to the emergency vehicle and a red in all other directions.

What about privacy issues regarding the cameras?

Camera images are neither monitored nor saved. They are simply processed to detect traffic. The high angle of the camera and the low resolution of the fisheye images make it impossible to read license plates or recognize faces. The use of the cameras is not tied to law enforcement.

Will the new traffic signals mean more unsightly wires over the streets?

No. The lights are mounted on solid masts. All electrical connections to the signal run underground.

Mass Ave / Waltham St Traffic Signal Mast

One of the signal masts at Mass Ave / Waltham St. Note the lack of wires. Click to enlarge.

Are there examples of signals in Lexington that use the same technology as the signal proposed for Woburn St / Mass Ave?

Yes. The following intersections use the same signal technology:

  • Mass Ave / Waltham St
  • Waltham St / Worthen Rd
  • Spring St / Hayden Ave
  • Spring St / Concord Ave
  • Waltham St / Concord Ave

The cameras are white, bell-shaped, and are mounted on the mast arm.

Additionally, the following intersections use smart technology, but not the single fish-eye type camera that would be installed at Woburn St / Mass Ave:

  • Marrett Rd / Waltham St and Bedford St / Revere St. These intersections use multiple cameras for detection (one camera in each direction). The single, fish-eye type camera proposed for the Woburn St / Mass Ave intersection is less expensive, requires less maintenance, and offers enhanced technology.
  • Marrett Rd / Spring St. This intersection uses detectors under the pavement rather than cameras. While this is better than no detection system, it is inferior to the camera systems, especially the single fish-eye lens, and is more expensive.

Next: Beautification

Friends of the Lexington Center Streetscape Project | Lexington, MA