LED Traffic Signals

The Pros and Cons of Light Emitting Diodes in Traffic Signals

The advent of the LED signal brings quite a few benefits, but it also brings some problems.

Here are the pros and cons of LED signals:

PROS:

Energy Savings
The LED signal, by emitting energy in only one narrow band, and not emitting much heat, needs much less energy to work.
Long Life
The lifetime is much longer than that of an incandescent bulb. While a standard incandescent bulb lasts about 1000 hours, an LED can last as long as 30000 hours.
Purer Colors
The colors are pure hues, tailored to the specified colors for traffic signals. Colored lenses used with incandescent lamps can not provide such purity (especially for green signals), and can fade over time.
Lower Current Draw
LED signals draw much lower currents, meaning that the signal wires do not need to be as large.
Battery Backup
Because the signal lamps draw much lower currents, traffic signals can operate at low enough currents to be given a battery backup. This keeps the signal operating during power failures.

CONS:

Strange Failure Modes
When an LED signal fails, it can blink, darken parts of the section, blink parts of the section, dim, or go out entirely.
Susceptibility to Lightning Strikes
Lightning is more likely to cause failure of an LED signal lamp than an incandescent lamp. Lamp filaments are more robust than semiconductors.
Signal Changes Don't Trigger the Eye
When an incandescent signal changes, one lamp dims out while the other one brightens. This triggers the rapid change detection nerves in the peripheral vision of the eye, alerting the driver. But when the LED signal changes, one lamp instantly goes out, while at the same instant, the other lamp instantly lights at full brilliance. This does not trigger the rapid change nerves, so the driver is not alerted by his peripheral vision. He may not notice that the light changed.

Look for signal changes while staring at the large red X. Which signal has the more noticeable change? The moving dots simulate traffic and other distractions normally found in real traffic situations. This requires a standard monitor at a normal distance to provide the proper visual angle.
Snow Collects in LED Signal Visors
During a snowstorm, the signal section can become completely obscured by snow. The LED does not provide enough heat to melt snow off of the lens and visor. This was not a problem with incandescent lights, because the lamp heat melted the snow.
Reduced Light and Fading
As an LED signal ages, the output dims. Also, for white LEDs, the yellow component darkens quicker than the blue component, making the signal turn a bluer color. Both necessitate replacing the LED before it goes completely dark.
Lower Current Won't Activate Conflict Monitors
With lower current draws, older conflict monitors may not function properly.

CURES:

Strange Failure Modes, and Susceptibility to Lightning Strikes
Protect the lamp assembly with internal multiple low-pass filters. Use more robust electronics.
Signal Changes Don't Trigger the Eye
If the low-pass filters mentioned above have long enough time constants, they will also make the LEDs dim and brighten as incandescent lights do. So the same device cures three problems.
Snow Collects in LED Signal Visors
There are three known solutions:
1. Place a small heater in the bottom section of each signal face. Connect it to the lamp in the face that is on the most, with a thermostat to prevent it from operating if the temperature is above freezing. But note that the heater can prevent the conflict monitor from detecting a lamp failure unless extra wiring is provided.
2. Special visors are available with air scoops to blow snow off the lenses.
3. Slanted clear plastic inserts that completely fill the visor keep the snow out.
Reduced Light and Fading
Nothing except a relamping schedule can guard against this.
Lower Current Won't Activate Conflict Monitors
Replacing the conflict monitor with a new one cures this.

Unusual Traffic Problems