If you hit a car from behind are you legally at fault?

Most likely you are, but it also depends on what the person you hit was doing In most circumstances, yes ... it's called "failure to control vehicle" and is usually caused by tailgaiting the car ahead or not paying attention to what is happening. Too many people only see one car ahead when they are driving - they need to be aware what is happening well beyond that. Professional bus drivers are taught to constantly be aware of everything happening about them at all times ... despite having air brakes does not mean that 17 tons can stop on a dime.