Road grade % definition

I am not an engineer but I have a question regarding degree of slope on a road or driveway. I rented a "man lift" to paint the front of my home and was cautioned not to exceed 4 percent of slope! At least I think that was the caution.
I measured my drive with a level and a carpenters square and found it was a 22 degree slope but I don't think that is what they mean on a road surface when I see caution signs for trucks that say to use lower gears for 8 percent ( 8 degree ??)  slope ahead.
How is this figured. A friend said it was the number of feet of drop over 100 feet but logically that doesn't sound right either.
I have searched in vain for the answer. Will someone help?

A 4% slope is one in which there is a 4 feet rise or fall in a 100 feet horizontal distance.  4% is the same as 2.29 degrees.

One way to picture what % slope means is to think about a handycapped ramp, which by code is not to exceed a 1:12 slope, which is 8.33%.

Slope is defined as the rise over the run (length) and then expressed as a percentage (multiply by 100).  Rise divided by the Run.

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Thank you for all your answers. Now I understand.

BigJimFudge