The rule is, when stopped, you should still be able to see the rear tires of the car in front of you. Why?
1. As the car in back of you usually stops within inches, if you were inches from the car in front, you would not be able to get out of line and escape in the event of an emergency in the car in front of you, e.g., such as if it's on fire.
2. Many times the lead car stops in the cross walk and/or beyond the white line. If a truck is trying to turn left from the street on the right, the car in front of you will have to back up. If you're pinned in, it will take a long time to get everyone to back up. Without thinking, he might just back up into you.
3. If you're up against the car in front of you and get rear-ended, the force of the collision will push your car into the one in front of you. This will cause your car to be damaged both in the rear and in front. The damage caused to the car in front of you by your car may be considered to be your fault. You may get a ticket, For Failure to Maintain a Reasonable Distance and a hike in your insurance premiums.
4. To save time. This will actually save you time in leaving the intersection. When the car in front of me starts to move, so do I, at nearly the same time. If I was within inches of the car in front of me, I'd have to wait until it gets a reasonable distance away before I could even start to move; therefore, I don't save any time by being closer.
Rule: If you can't see the rear tires, you're too close.