Where I live, we are about to get a bunch of snow (5”-8”). This has brought about the question of which is more dangerous an icy road or an icy road with snow? Does the snow help traction on the ice, or does it make it worse?
Both are dangerous, and as James says both require drivers to be extra cautious and adjust their driving. However from my experience living in Alberta, Canada where we spend ~5 months of the year in the snow with temperatures as low as -25C ice with snow on top is more dangerous.
Ice with snow on top once your vehicle begins to slide your tires are not in direct contact with the ice and you have only the friction between the snow and the ice to rely on rather than the friction from your tires and the ice. Additionally snow on top of the ice can hide ruts in the ice that you may otherwise have a better chance of seeing and anticipating, if your tires hit these ruts at higher speed or in a lighter vehicle you may find your vehicle being thrown the side.
The roads are not the danger, the drivers on the road are the danger!
Think about your question inversely (antonym); "Which is safer, icy roads or icy roads with snow on top?"
It is not the presence or lack of ice/snow that creates the danger/safety, it is how the driver responds to the presence of the snow/ice. For example, drivers with 4x4 trucks occasionally believe the four wheel drive makes driving on snow and ice safer, and it does... Except when the 4x4 driver starts driving too fast for conditions, and crashes.
The biggest danger is not adjusting your driving for conditions.
Which leads us to black ice this ice can develop on roads and be difficult or impossible to see. Any road condition that is not apparent to the driver is going to be the most dangerous.
So given your question, (black) ice is more dangerous than (white) snow. But even this can be anticipated and adjusted for. If there is moisture and road temperatures below freezing adjust your driving for slippery roads.
Lastly, Aquaplaning or hydroplaning is just as dangerous in warm weather as ice and snow in cold weather.
Getting rid of the subjectivity of danger, I am going to answer this for two different basic scenarios. There are so many variables that are possible within scope of the question, but I am going to leave it to 2 scenarios. Plain ice, and snow over plain ice and the. apply studded winter tires or plain all season or 3 season tires to those.
Under normal circumstances ice is more slippery than ice with snow on top. Snow being crushed under the tires creates a friction element that is not present with bare tires on ice.
Now, studded winter tires flips this over. The metal studs directly pressed hard on the ice grips the ice better than it would with a layer of snow between the tire and ice.