What are these things for?

February 21, 2007 10:32 PM

Why do the traffic signals at a particular intersection have light-blocking louvers on them?

These traffic lights were installed about six months ago. There are two lights in quick succession, then an overpass, then two more lights. The first set of lights is about 30 to 40 yards apart, the same distance for the second set.

These louvers, or slats or whatever , prevent the light from being seen until you are very near the first signal in the set--then the light becomes visible quite abruptly. The slats are not effective at night, though they dim the light somewhat.

What is the purpose of these slats? Why would you want to make a traffic signal less visible?

No idea if this is relevant, but the replacement lights with these louvers are LED-based. The previous ones were old-style lights and none of the signals had these louvers.

posted by aerotive to Travel & Transportation (10 answers total)

I may not be fully understanding your description, but typically the purpose of the devices that obstruct the light until you're at a certain angle and/or distance are there to avoid confusion, because if you could see them from afar, you might be confused about which light is for which street or location.

For example, in Chicago there are some really nasty 6-way intersections where a diagonal street meets the crossing of 2 perpendicular streets, and it can sometimes be easy to confuse stoplights from a "shallow diagonal" street to the street you're on.

Alternatively, if there are two stoplights very close to each other (say 100ft away from you and 125 ft away from you as an arbitrary distance) and the first one is red and the second one is green, the slats could be there to prevent you from confusing the green (far) light from the red (close) one and not stopping.
posted by twiggy at 10:37 PM on February 21, 2007

i think the purpose is to keep people from speeding through intersections, and to keep aggressive drivers (like myself) from predicting when the other direction's light turns red.
posted by phaedon at 10:52 PM on February 21, 2007

I agree with twiggy about having them so that you dont get the signals confused. Which I have seen happen so many times. I think it was just a few days ago when my friend was driving and he started going at a red light, because the next intersection had turned green and he thought it was our intersection. So really, that seems like the most reasonable answer.
posted by rubberkey at 11:10 PM on February 21, 2007

I've seen something similar, although probably not blocking the light from being seen as much as you're describing, for lights which are directly in line with the rising or setting sun so as to reduce glare. But that's always on the older style lights as glare isn't such an issue with the LED ones (which is one of the reasons why they get used), so in this case it sounds like twiggy's suggestion is right on.
posted by shelleycat at 11:45 PM on February 21, 2007

I also think it's more important to have these on the new LED type lights as they can be seen from a much further distance than the old-style incandescent.
posted by medium format at 2:34 AM on February 22, 2007

I would wager a guess that it is exclusively to limit the view to only a specific angle so that other lanes do not misinterpret the light for their lane. You can check this for yourself by noting when you see one of these signals, whether it would be potentially confusing and visible from another competing lane if the louvers were missing.

Signals that have a rising/setting sun behind them are bordered by a black frame extending around them. Louvers would not be any more effective than non-louvers at avoiding backlit glare from the sun, me thinks.
posted by qwip at 3:03 AM on February 22, 2007

Lots of time fresnel lenses are used over the lights to restrict the view until you get near to the signal so that there is no confusion for drivers which signal corresponds to which intersection. LED lights are much brighter than old style lights so this could lead to more confusion when consecutive signals are closely spaced.

Other times hoods are used to block the view from the intersecting street. This prevents people on side streets from getting a jump on the signal change.
posted by JJ86 at 6:19 AM on February 22, 2007

Also forces people to slow down as they approach the intersection.
posted by unixrat at 11:30 AM on February 22, 2007

unixrat mentioned: Also forces people to slow down as they approach the intersection.

Not typically. I came up to an intersection late last year where the signals were all off due to some localized power outage. Nobody on the cross street was even slowing down. I had to ease the car out and start honking at some idiots. Green means go and off means go faster?
posted by JJ86 at 1:30 PM on February 22, 2007

They're there to eliminate the level of anticipation from drivers waiting to enter the intersection from the opposing direction, thereby lessening accidents with those that find it necessary to stomp on it when the light turns yellow or is turning red.
posted by SoftSummerBreeze at 2:04 PM on February 22, 2007