While road conditions are probably correlated with surge it's really more closely tied to the effect bad weather has on someone's willingness to walk (often to distant public transport). It turns out that when it's raining or very cold a lot more people choose to take a car. It's not about risk/reward it's about supply/demand. When supply (drivers) is low relative to demand (riders) higher prices will incentivize more drivers to go online and thus fix the scarcity of drivers.
