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Tailgating is the practice of driving on a road too closely behind another vehicle, such as less than the travel distance in two seconds or equivalently, one vehicle-length for every 8 km/h (5 mph) of the current speed. In most cases this is done out of frustration toward, or in order to help get past, a slower moving vehicle which is supposedly, but not always, impeding the progress of the vehicle behind it. Some slower vehicles will not speed up or pull over when the situation arises; this is seen as selfish behaviour by tailgaters, and may also be illegal in some jurisdictions due to its increased potential for causing a car accident. On the other hand, tailgating could also be seen as not only selfish, but unnecessarily risky, as it could cause the vehicle directly in front of the tailgater to crash.

There can be several reasons for tailgating:

• a driver travelling at a higher speed reaches a car traveling in the same direction at a lower speed, and the faster driver may tailgate while awaiting the first opportunity to overtake. Such expressions of impatience may be conscious, unconscious, and/or dangerous.
• in its worst form, it can be a particularly virulent form of road rage and a form of intimidation. This is where the tailgating driver (the driver in the following vehicle) can threaten damage to the leading vehicle and its occupants by driving aggressively (perhaps also with use of headlights and horn) to encourage the leading vehicle's driver to do something to placate the tailgater (often illegal, like increasing speed beyond the speed limit or changing lanes without due regard for safety). However, it can also be dangerous to the tailgater as well, especially if he or she is driving closely behind a large vehicle (such as a tractor-trailer, or gas tanker). If the leading vehicle decelerates suddenly (such as when encountering a traffic jam, avoiding pedestrians, etc.), the tailgater has a high risk of causing a rear-end collision.
• Tailgating can also occur because of a lack of perceived risk in so doing. Thus, it is done unconsciously or negligently, very often by people who consider themselves safe drivers and generally obey the other rules of the road. Approximately one third of rear-end collisions involve tailgating.^[1]
• a form of deliberate tailgating known as "draft-assisted forced stop" or "draft-assisted forced auto stop" (D-FAS) is a technique which has been used by people known as hypermilers to achieve greater fuel economy. D-FAS involves turning off the engine and gliding in neutral while tailgating a larger vehicle, in order to take advantage of the reduced wind resistance in its immediate wake.^[2] Note that this practice is highly dangerous: while tailgating itself is inherently risky, the danger of collision is increased with D-FAS as power for power brakes can be lost after a few applications of the brake pedal and, with older cars, the pressure that causes power steering to function can be lost as well.^[3]

Cyclist tailgating in a keirin.

• There are cycling disciplines in which the cyclist pedals around the velodrome track tailgating a motorbike that removes air resistance, enabling much higher speeds than in normal cycling. The most popular discipline is Japanese keirin.

In finance, tailgating means the action of a broker or adviser purchasing or selling a security for his or her client(s) and then immediately making the same transaction in his or her own account. This is not illegal like front running, but it is not looked upon favorably because the broker is most likely placing a trade for his or her own account based on what the client knows (like inside information).

In the context of building security, the term "tailgating" is used to describe the situation where one or more people follow an authorized person through a secured door or other entrance when the authorized person opens the door legitimately. This can be either with or without the authorized person's knowledge and/or consent. A "tailgater" can be an unauthorized intruder, but can also be a normally-authorized person who has forgotten or lost their access key, pass or token, or finds the access procedure inconvenient. High-security buildings typically use secure revolving doors in order to prevent tailgating. Such doors may have smaller segment space between the door leaves, and can also be fitted with electronic sensors which cause the door's powered rotation to reverse if more than one person is detected in a segment space.

• Tailgating Information (Government of South Australia)
• TRAFFIC AND CRIMES AMENDMENT (MENACING AND PREDATORY DRIVING) BILL (New South Wales parliament Hansard transcript where tailgating is noted as being an offense under the new legislation).
• Tailgating Information (Wisconsin Department of Transportation)
• "Two Dots to Safety" campaign (operating in the United Kingdom and Europe, but also in Pennsylvania, Minnesota, Maryland, and Washington State)
• Police Force safety advice (Singapore)
• Report on an anti-tailgating operation (by the Hong Kong police)
• A petition to the British Prime Minister asking that tailgating be made a criminal offense (registrations close 20 February 2007)