Which fits well with dynamic speed limits cropping up in places that get regular bad weather
Whether or not speed variances between vehicles increase accidents doesn't seem to be clear. With a quick search I found this(1) that indicates that speed variances don't play a role in causing accidents. I also found this (2), which says "that the greater the difference between a driver’s speed and the average speed of traffic—both above and below that average speed—the greater the likelihood of involvement in a crash". And I found this (3) which basically says the speed limit should be set at the 85th percentile of what everyone is driving. Kind of like how you should build sidewalks where people walk ("Don’t make any walkways this year. At the end of the year, look at where the grass is worn away. That shows where the students are walking. Then just pave those paths"), set the speed limit at the speed most people drive.
Everywhere in the US I've lived, the max speed limit laws were written like "whatever speed is safe for current conditions up to a maximum of XX Mph" So you can get a speeding ticket for going slower than the posted limit if conditions are bad. I've actually been pulled over going well under the posted max speed limit during a hard snow storm.
(1) https://www.caee.utexas.edu/prof/kockelman/public_html/TRB04... (2) https://en.wikipedia.org/wiki/Solomon_curve (3) http://onlinemanuals.txdot.gov/txdotmanuals/szn/determining_...
There are exceptions, especially in the US West, where interstates may be the only path for miles and miles through some mountain ranges. Of course, they mostly have wide shoulders where people ride.
There is the source for California -- I see it gives exemptions for safety and law (I assume for road works, etc) but generally speed limits are objective based on conditions. It doesn't mean you can drive slow and impede other traffic traveling around you though, which is why there are laws for it. I don't see it enforced as much in San Diego, but when I lived in the Lake Arrowhead area it was enforced quite often on the highway up and down the mountain.
Quick edit -- that law is for highways, I haven't checked whether this also applies for other types of roads.
Note that “highway” in California law has a very broad definition; see https://leginfo.legislature.ca.gov/faces/codes_displaySectio....
“Highway” is a way or place of whatever nature, publicly maintained and open to the use of the public for purposes of vehicular travel. Highway includes street.