purpose of roof lights?

Whitenight wrote:
I think its so at night oncoming and following traffic will know that the vehicle is wider (80" wide or more).

Yep on the 80" and wider needing to have the outer marker lights plus the center ID marker lamps.

FMVSS 108 and CMVSS 108 cover lighting.

Here's where you can read all about it: http://www.gpo.gov/fdsys/pkg/CFR-2011-title49-vol6/pdf/CFR-2011-title49-vol6-sec571-108.pdf

If you like pictures, read this: http://www.nhtsa.gov/cars/rules/standards/conspicuity/TBMpstr.html

In summary: "To Indicate the presence of a wide vehicle"

(read #3 and #7)

Here's #3:
Front Identification Lamps (ID) (P2 or P3) Indicate presence of a wide vehicle

#7 deals with the rear lights.

Wide is defined as over 80"

mileshuff wrote:
What purpose do they serve?

I think its so at night oncoming and following traffic will know that the vehicle is wider (80" wide or more). My dually has DOT cab lights, fender marker lights and DOT lights on the tail gate. It is most likely illegal driving without the tail gate. All RV's have DOT marker lights on the rear too.

I guess there are number of reasons for cab lights. Younger brother with Ram 3500 dually says clearance lamps when hauling tall equipment.

I've also noticed it simply provides additional lighting at night when you're working outside your truck or idling on a dark road.

What purpose do they serve?

its a 1-3-1 pattern frt & rear tell approaching trk's that u are a wide body, or towing , but is for the other guy to see whats coming at him

interesting - thanks everyone for the background

If you pull a fifth wheel or travel trailer, the 3 center ID lamps and outer marker lamps are required as well.

All deals with the width of the vehicle. By DOT regs dually has to have them due to the wider vehicle. Other 3/4 and 1 tons are optional but will have them from time to time as they haul wider trailers. 1/2 ton typically do not haul these bigger trailers so rarely have them. You will see them on trucks like raptor or the Chevy quadrasteer as they are wider as well. Nothing to do with weight. It also why you have three red lights across the back of your rv.

GordonThree wrote:

What purpose do the lights serve, other than working-man's "bling"?

To be a source of cab roof leaks as the truck ages. :B

As far as I know, any vehicle with a body width of over 80" requires them. That's why the F150 Raptor and the Hummer H2 have them. Not sure about weight requirements, but most 250/2500s and SRW 350/3500s I've seen do not have them. They are an option, at least they were when my dad bought his 2000 F250.