Even though ice melts at 32f there are factors like roadway salt, heat from car tires, the heat absorption of asphalt, etc that should cause deicing at lower temps. What concentration of salt do road crews shoot for when salting and what is a general area of temp that icy roads clear up.
Way too many variables to give a single temp or even a small range.
Air temp, sunlight, ground temperature, traffic, road surface, moisture content of the snow; all factor into when ice will melt. Roads can be pre-treated with salt to prevent ice from forming. They are scraped by constant plowing during storms to prevent a buildup of snow which is then compressed into ice. Black ice can form during specific conditions, water running across roads, temps rising or falling, etc.
You can have clear roads at 20 degrees F (or 0 F for that matter) if the road was treated right, plowed regularly, and has regular traffic. It can be 50 F and the roads might still be covered with a messy mix of ice, slush, and standing water.
The rule of thumb that seems to come up in these parts (Ontario, Canada) is that normal road salt (NaCl) will work to keep roads ice-free down to about -18 C, or about 0 F. Below that you need another solution (pun intended).