you have no idea what your talking about. Those guys are getting paid the same whether they are "standing around" assisting "grandma" or sitting in their station doing nothing. You THINK they are doing nothing, but you have absolutely no idea what they are actually doing. It could range from providing lighting or traffic control to awaiting the arrival of an environmental agency, or maybe you didn't see the child that got hit prior to your arrival. It's standard for multiple officers to be on any scene where criminal charges may be required. Maybe "grandma" was intoxicated. Maybe there were drugs or guns or whatever present in her car. The cost of fuel is negligible in the overall budget of any public safety entity.
It's people like you who criticize these situations that you know nothing about, that will be glad that the police, fire, or ems departments bring their full weight when its your family in danger.