DOT and the Insurance co's actually say very little about DRL's. But there is a lot of data showing they are highly effective - especially in bright sunny conditions on dry pavement, when most accidents occur.

It's not a scam by lamp manufacturers, though they would obviously stand to benefit.

Volvo's implementation of DRL's is terrible. They make the driver run all their lights - headlights, tail lamps, instrument cluster lights. What a waste. (GM does it best, I think, with a pair of low power forward lights only.) In my case, I wired my foglights for independent use, so I can use them as my DRL's.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)

Well, I can say with all honesty that I do not believe what the DOT/USGOV/INsurance tells me because I do not live in the US. I live in Canada where DRLs have been mandatory for years and where I have had first hand experience comparing cars that had them and cars that did not. What I have found is that a DRLs alert a driver to oncoming traffic more than vehicles that do not. When you see the lights, you actually are on a higher level of alert. These days, drivers need all the alert that can receive being distracted by cell phones, sat nav systems and anything else a car company can squeeze onto the dash and centre console.

Hella has published date (it's somewhere on their website I believe) attesting to their efficacy; the Canadaians also have data. The local police chief here once showed me an eye-popping statistic about how effective they are in reducing daytime accidents. I won't quote the number, but if it's even a third of what I saw it's a strong case for DRL's.

Some countries that require DRL's (certain Scandinavian countries and Canada come to mind) have a lot of twilight and two-lane rural roads. It seems intuituve that DRL's make an awful lot of sense there.

Though you don't exactly become invisible if you run without DRL's, they aid in early visual acquisition (detection), depth perception and visual tracking. In winter conditions with extreme solar glare with the sun behind you, they become very useful (as you CAN become invisible in these circumstances).

I've wired my lights so my foglights (cheap H1 bulbs) are my DRL's - sparing my tail lamps, instrument panels lights and VisionPlus capsules from the burden of full-time operation.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)