No, the silent nature of hybrid cars does not create a safety hazard, because hybrid vehicles are just another vehicle on the road. People are generally careful when they approach roads. People know that hybrid cars exist, and they are more careful to look out for them. People are generally responsible, and the silence of hybrid cars is not likely to be a large problem.
I do not believe that the silent nature of hybrid cars create a safety hazard because most of toady's standard cars have little to no sound when running . Unless you have an off road vehicle, or a vehicle with an engine designed specifically for the purpose of being loud. There is little to no difference between the sound of toady's standard vehicle when running , and that of a running hybrid vehicle.