Seeing an accident from the sidelines often means that, you noticed someone else making a mistake during the day. Did you notice someone making an error the day before your dream?
If there was someone in the car, it usually means that you might have some resentment or anger towards the person driving.
If it was empty, and crashed by the side of the road, you might feel this way about your car. Is it old? Cost you a lot of money lately? Does it feel unsafe when you drive it? You might have a negative attachment towards your car if it has caused you some financial problems or has inconvenienced you lately.
Another viewpoint is this is what could happen to you if you are a particularly careless driver. It might be your subconscious warning you to be more cautious when on the roads.
If a car crashed into yours, it suggests that your lifestyle, beliefs and goals may differ considerable from other people’s. If there has been conflict in your personal or professional life, this could be symbolic of this clashing of personalities or viewpoints.
It can also an indication that you have received some news that has jolted you or injured your pride if seeing the car being hit shocked you in your dream.