The Importance of Correct Stopping and Following Distances

Rear end collisions are the most common accident type in Australia. These accidents typically involve cars driving inappropriately close to a leading vehicle colliding with that vehicle.

While drivers who sit on the tail of other cars are a universally despised demographic of road user, the truth is that not all tailgating accidents are caused by aggressive drivers or those who are knowingly driving too close to a leading vehicle. Instead poor understanding of how long it takes a car to come to a halt, as well what appropriate following distances should be, can cause otherwise responsible drivers to get involved in rear end collisions.

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Stopping distance explained

Before discussing appropriate following distances in various driving scenarios, it is helpful to understand the stopping capabilities and distances for average family cars.

The availability of advanced active safety features like Antilock Braking Systems and Electronic Stability Control Systems can create an unrealistic expectation of how effectively cars react to hard braking. A driver in a modern vehicle might imagine that with a good set of tyres, their vehicle is perfectly capable of quickly coming to a halt in a situation where emergency braking is required.

The reality is that advanced braking systems optimize brake efficiency during braking, but primarily ensure that the driver retains control of their vehicle when forced to brake hard. Furthermore, car braking performance is only one component of stopping distance. The time a driver takes to notice and respond to the need to brake also determines how far a vehicle travels before it comes to a halt.

The following table from the Queensland government’s transport department illustrates typical stopping distances for the average family car.

SpeedReaction distanceBraking distanceTotal stopping distance

As you can see the stopping distances for cars travelling at even low speeds is substantial. A vehicle travelling at the speed of 40/km an hour will cover more than a quarter of a rugby field before coming to a stop. At 100km/h the car will travel the full distance of a rugby field before it stops.

This is on dry roads on a good set of tyres. On wet roads stopping distances increase even further.

SpeedReaction distanceBraking distanceTotal stopping distance

In the wet a car travelling at just 60 km/h will travel half the distance of a rugby field before it comes to a stop.

Understanding following distance

Following distance is best understood in the context of stopping distance. A safe following distance between two cars is one that gives a car sufficient time and distance to come to a halt without a collision in the event that a car in front of it emergency brakes. For optimal safety, a following distance should allow the rear vehicle to come a stop without a collision in the event the lead vehicle collides with an immovable mass and comes to an immediate stop.

While stopping distances are measured in meters, following distances are typically measured in seconds. This is because it is almost impossible for a driver to accurately gauge the distance between themselves and a vehicle in front of them using visual cues. Instead counting the amount of time it takes the two cars to pass the same fixed object can be effectively used to gauge the distance between them.

For example, while travelling on a highway you would begin counting the seconds that passed between a car in front of your passing by a streetlight, up until the moment your own car passes the same streetlight.

The correct following distance

A following distance of two seconds is sufficient to allow a car travelling at any speed between 40km/h and 110km/h to come to a halt in both wet and dry conditions without hitting the lead vehicle after it brakes. However, at a two second following distance there is little margin for error. Any delay in driver reaction timeor inefficiency in the braking system will result in a collision.

Therefore most car safety experts recommend a following distance of three seconds at speeds between 40km/h and 110km/h in both wet and dry conditions.

That said, it is important to note that even a three second following distance does not protect a tailing driver from a collision if the leading car collides with an immovable object (like a stationary vehicle). Furthermore, heavier vehicles will take longer to bring to a stop, and certain road conditions like ice and snow can also impact advisable following distances.

A four second following distance allows a tailing car sufficient time and space to come to a complete halt even in the event the leading vehicle collides with a stationary immovable object. Because many drivers will take advantage of adequate following distances to overtake and cut in, ensure you immediately increase following distance again once you have been overtaken.

Factors that can affect stopping distance

While a four second following distance should be enough to prevent a collision in most situations, the following factors can all affect stopping distances and should be taken into consideration when you’re driving.

Factors that can increase driver reaction time include:

  • level of attention
  • level of fatigue or inebriation
  • visibility issues (like heavy fog, or driving without prescription spectacles or contact lenses when required to do so)
  • the level of experience of the driver and their ability to quickly recognize a road hazard and respond appropriately.

Factors that can increase the distance required for a vehicle to come to a stop once brakes have been applied include:

  • brake conditions and type
  • tyre condition, including both tread depth and tyre pressure
  • towing a trailer or another vehicle
  • overall vehicle weight.

A number of environmental factors beside the weather can also impact stopping distances, including:

  • road surface (tar, concrete or gravel)
  • road gradient, with cars requiring more time to stop on downhill gradients
  • whether braking takes place on a straight or a curve, with corners introducing additional complications which can impact the outcome of emergency braking.

What these various factors mean in combination is that drivers should err on the side of caution if their attention and focus is in any way compromised, or if they have reason to doubt the effectiveness of their car’s brake systems. Additional following distance is also advised if road or weather conditions are unfavourable.

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