INCREASING SIGNAL PUNCH
Improve the Visibility of Traffic Signals
THE NEW DEVICES
Traffic engineers have long looked for ways to increase the "punch" of
signal indications.
In the past, higher wattages, special lens designs, and multiplying signal faces
were used. But now some new technologies have become available:
- LED signal lamps are much brighter and use less power. They made red arrows
practical.
- Black signal housings are now permitted.
- Black signal backplates are now mounted even with the edges of the signal housings,
instead of behind the housings.
- The MUTCD (Manual on Uniform Traffic Control Devices) now allows a retroreflective
yellow border at the outside of the backplate. This has several advantages.
- Battery backup systems for traffic signals are now easier with LED lamps.
COMPARE THE NEW METHODS
Compare how well the signals can be seen and how well the colors can be identified
in the large image.
Then try it again in the small image below that as though seen from a distance.
To see what it looks like from even farther away, walk away from the monitor and
look at the small image.
Also see what the signal looks like at night when the signal is malfunctioning or
has no power.
An unlit signal legally becomes a stop sign. You must stop when this happens. Can
you see it?
SIGNAL
DESIGN |
Signal Housing Color |
|
Highway Yellow |
Flat Black |
Flat Black |
Flat Black |
|
| Backplate Color |
|
No Backplate |
No Backplate |
Flat Black |
Flat Black |
|
| Backplate Border Color |
|
No Backplate |
No Backplate |
Flat Black |
Highway Yellow |
|
SIGNAL
ENVIRONMENT |
|
|
| Daytime Sky Background |
| Daytime Overcast Background |
| Nighttime Black Sky |
| Nighttime Power Failure |
| |
| DESIGN |
Signal Colors |
|
Yellow
Housing |
Black
Housing |
Back-
plate |
Yellow
Border |
|
SIGNAL SEEN
AT A DISTANCE |
|
|
| Daytime Sky Background |
| Daytime Overcast Background |
| Nighttime Black Sky |
| Nighttime Power Failure |
| |
OBSERVED VISIBILITY DISTANCES
The page author tested daytime visibility distances for several different signal
displays. The data collected are represented in the following chart.
One outlier was removed from the Y-- data set because the sun reflecting from a sign
bridge hampered visibility.
Where longer distance data are not available, extrapolations (dashed
lines) are made from the slope at the greatest distance available. Greater distance
data do not exist because hilly terrain prevents viewing any signals of those types in
the area at greater distances.
Right click the chart and select 'view image' to see a larger version:
|
Ratings Key |
| 10 | As at own Signal |
| 9 | Easy |
| 8 | Good |
| 7 | Good to Fair |
| 6 | Fair |
| 5 | Medium |
| 4 | Poor |
| 3 | Harder |
| 2 | Difficult |
| 1 | Barely Seen |
| 0 | Invisible |
|
| Chart Code |
Signal Configuration |
Maximum Indication Visibility Distance (Rating 2 on Chart) |
| Y-- | Yellow Housing No Backplate | 0.43 mi |
| Y--F | Yellow Housing Foliage Background | 0.75 mi (extrapolated) |
| K-- | Black Housing No Backplate | 0.53 mi (extrapolated) |
| KK- | Black Housing Black Backplate | 0.57 mi (extrapolated) |
| YKY | Yellow Housing Black Backplate Yellow Border | 0.72 mi |
| KKY | Black Housing Black Backplate Yellow Border | 1.2 mi |
ADVANTAGES AND DISADVANTAGES OF EACH DEVICE
| DEVICE |
ADVANTAGES |
DISADVANTAGES |
Highway yellow signal
housing |
- Visible at night in headlights if the signals are dark
- Existing equipment
|
- Hard to see the signal face at a distance
- Hard to see the signal color at a distance
- Might be aesthetically unpleasant
|
Flat black signal
housing used alone |
- More visible during the daytime
- Easier to see the signal face at a distance
|
- Totally invisible at night if the signals are dark
- Hard to see the signal color at a distance
- No improvement at night
|
Flat black signal
housing with black
backplate |
- Much more visible during the daytime
- Easier to see the signal face at a distance
- Easier to see the signal color at a distance
|
- Totally invisible at night if the signals are dark
- No improvement at night
|
Black signal housing
and backplate with
highway yellow
retroreflective border |
- Most visible during the daytime
- Most visible at night
- Most visible at night if the signals are dark
- Easier to see the signal face at a distance
- Easier to see the signal color at a distance
- Signal face is visible much farther away
- Signal color is visible much farther away
|
- Might be aesthetically unpleasant
|
| Battery backup |
- Signals stay on during a short power failure
- Traffic accident power failure doesn't cause traffic jam
- Signal goes to flashing if it can't operate normally
|
- Other failures can still make signals go dark
- Battery backup only delays signals going dark
- Battery backup usually lasts about 24 hours.
- Dark all-black signals are invisible at night
|
THE DANGER OF SIGNAL HEADS THAT ARE TOTALLY BLACK
The factors combining to cause the danger:
- Battery backup can't keep the signals going forever. If power is not restored,
the signals eventually go dark.
- Electrical failures inside the signal control box can make signals go dark even
though the battery backup is there.
- When an all-black signal is dark (no indications are lit), it becomes totally
invisible at night.
- When drivers see all signals dark (no indications are lit), the signal legally
becomes a stop sign,
- If a darkened signal is totally invisible at night, the driver
can't see it to know he is required to stop for it.
Signals that are totally black are dangerous and should not be used.
|
CONCLUSIONS
- Signals that are totally black are dangerous and should not be used.
- Battery backup is not a substitute for signals that are visible at night when
unlit.
- The black housing with black background and retroreflective highway yellow border
outperforms all of the others.