34.3.2. Forbes’ model

In this model, the reaction time needed for the following vehicle to perceive the need to decelerate and apply the brakes is considered. That is, the time gap between the rear of the leader and the front of the follower should always be equal to or greater than the reaction time.

Forbes' model emphasizes the importance of reaction time when it comes to following a vehicle. It states that the distance gap between the leading vehicle and the follower should represent not just the physical distance but also factor in how quickly the following driver will react in emergency situations. This means the time it takes to notice the leader's actions and respond with proper braking must be considered. Essentially, if the gap between cars is too small, the follower might not have enough time to stop safely.

Imagine you are walking behind someone on a crowded sidewalk. If they suddenly stop and you are too close, you might bump into them because you didn't have enough time to react and stop yourself. Similarly, in driving, if there's insufficient space between vehicles, a driver may not be able to react quickly enough to prevent a crash.

Therefore, the minimum time headway is equal to the reaction time (minimum time gap) and the time required for the lead vehicle to traverse a distance equivalent to its length.

The model defines 'minimum time headway' as the necessary time gap required to ensure that a following vehicle has enough space to react to the actions of the vehicle in front. Here, the time headway is not just about physical space but is closely linked to the time it takes for the leading vehicle to move a distance equivalent to its own length while taking into account how quickly the followers can decelerate. This conceptualizes a safe following distance that is dependent on speed and vehicle length.

Think of it like a game of leapfrog. If you're waiting for your friend to leap over a line, you need to give them enough space to land safely without crashing into them. If they jump without enough space (which would require lesser time headway), you might collide or cause an accident.

A disadvantage of this model is that, similar to Pipe’s model, there is a wide difference in the minimum distance headway at low and high speeds.

One significant drawback of Forbes' model is that it does not provide a consistent safety gap between vehicles at varying speeds. At low speeds, the model suggests a shorter distance headway, which could be less safe during high-speed situations. This inconsistency can lead to potential safety issues as drivers may not have adequate room to react as traffic conditions change quickly.

Imagine driving at a slow speed in heavy traffic. You might feel safe following the car in front closely. However, if the traffic suddenly speeds up, that close distance could turn dangerous if the lead car has to brake suddenly. It's like a bicycle race, where riders might closely pack together in slow turns but must spread out as they speed along straight paths.

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