Depending upon where you live and what freeways you are in the habit of driving upon, you may or may not have come upon variable speed limits. These variable speed limits are not controlled by whether it is daylight or nighttime but basically are "smart" speed limits which adjust their speed according to the density of traffic on the road, to which, their point in existing is in theory by giving advanced notice to drivers that there is heavy congestion ahead, that these drivers will then adjust their speed lower far in advance as opposed to driving too fast, only to have to slam on their brakes, creating a stop and go movement that slows down traffic even more.
While variable speed limits might seem to make sense to Department of Transportation, or traffic engineers, the actual usage of them, seems to be the wrong tool for the wrong place. For instance, if the normal speed limit is 65 mph, and the variable speed limit for signs further down the freeway drop the speed to 55 mph, than 45 mph, and finally to 35 mph, you might well conclude how brilliant this variable speed reduction was in reducing the vehicle speed on the roads in a conscious and cognizant manner, thereby reducing vehicle speed variance by quite a bit, but that isn't even close to the truth in reality. In reality, the variable speed limit signs, don't do much of anything, as first off because of traffic congestion you aren't able to drive close to the variable speed limit, and secondly, the traffic ahead has already made it abundantly clear to you that you aren't going to be traveling too fast anyway.
The bottom line is that there are few, if any, drivers that believe that they can always, without exception, irrespective of traffic, weather, and other factors, drive the speed limit on the freeway to begin with. As a basic law, you are not permitted to drive faster than what is safe for current conditions, so if the traffic is at a standstill ahead, you can't actually drive at 35 mph, or 45 mph, or even higher speeds, because it isn't physically possible. For those, that just want to stop all the sudden accelerating, relentless lane changing, and sudden braking, a better method is to have available to drivers, an app to their phone, which tells drivers in real time the average speed one mile ahead of them. In addition, instead of useless variable speed limits, variable message signs would be of far more value, stipulating either the average vehicle speed ahead, and/or giving travel times for exits up ahead, to which, the driver can thereby calculate about what the speeds are going to be and intuitively know how fast or slow traffic is ahead.
While it is good that traffic engineers want to experiment with tools to help with traffic flow, variable speed limits just seem annoying, useless, and stupid, as in almost all cases, the driver is clearly aware already that traffic is slow ahead and thereby the posting of such a variable speed limit, doesn't inform the driver of the most valuable information which would be the actual average speed of the traffic directly ahead of him. If, the Department of Transportation truly wants to come up something to improve traffic flow, variable speed limits, are a rather poor attempt to do so.