This is something you will
notice if you ever have the opportunity to be above a
delivery truck (e.g.
FedEx,
UPS, etc.) or, better yet, inside one.
The problem: it's hard to find what you need in the back of the truck if it's dark. Light is necessary. Small lightbulbs are inadequate, and powerful lightbulbs draw too much power and/or are too expensive.
The solution: a translucent, white plastic roof. Lest we forget, there's that great, huge, free lightbulb up in the sky otherwise known as "the sun".
You get a nice, bright, even light in the back of your truck on a sunny day, and a less-bright though equally even light on a cloudy day. No electricity required. No bulbs to buy and/or change.
As a bonus for the delivery companies, installing one of these devices does not usually mar the uniformity of the mobile advertisement that is their truck. How often does the person in the street see the top of the truck?
This concept is so brilliant, yet so simple, that I'm sure it took decades to discover.
At night, of course, you're back to square one.
Or perhaps not. brassmule, who has a much greater degree of intimacy with this subject than I do, says: "At night, there are cargo lights - really nice, bright ones on newer trucks, not so bright/not working on older trucks. Some drivers, at least from my experiences with FedEx Ground (all of whom are independent contractors) carry a flashlight, both for finding packages in the dark, or for finding street numbers under the same conditions. FedEx Ground rarely worked past dark, so it was not too big a deal. UPS and FedEx Express and Home Delivery all do - and I pity those poor souls."