Here in Massachusetts, tailgating is illegal.
According to 720 CMR 9.06(7):
The driver of a vehicle shall not follow another vehicle more closely than is reasonable and prudent, having due regard to the speed of such vehicle and the traffic upon and condition of the highway.
This regulation, issued by the department of transportation, gives no further details regarding what distance is “reasonable and prudent” between two vehicles.
Moreover, as far as I know, there’s no Massachusetts case law on the issue.
The RMV’s Massachusetts Driver Training Manual (which is not binding law) suggests the “three-second rule” when driving behind another motorist.
Use the “three-second” rule to keep a safe distance from the vehicle in front of you. Pick an object in front of you, like a sign post or a tree. When the vehicle in front of you reaches that object, count out “one one-thousand, two one-thousand, three one-thousand”. If you reach the object before you count three, you are too close. Slow down until you’ve put enough distance between you and the other vehicle.
The manual then emphasizes that the “three-second rule” is the minimum distance for safe driving. Certain types of vehicles (e.g., motorcycles and tracker tailers) should be given more distance. Additionally, snow, rain, and other inclement weather would also require more space between you and the driver in front of you.
The fine for tailgating in Massachusetts can be up to $100.