Brake Reaction Distance

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Brake reaction distance, abbreviated as $$B_r$$, is the distance traveled from the point of detecting a hazard until you begin braking.

Brake Reaction Distance Index

Brake reaction distance, abbreviated as $$B_r$$, also called reaction distance, is the distance a vehicle travels while a driver reacts to a perceived hazard and initiates the braking process.  It is a crucial component of the overall stopping distance and plays a significant role in ensuring safe driving and collision avoidance.  When a driver observes a hazard or an unexpected situation on the road, there is a delay between perceiving the danger and physically reacting by applying the brakes.  This delay is known as the reaction time and is influenced by various factors, including the driver's alertness, visibility, cognitive processing, and physical coordination.

It's important to note that the brake reaction distance is the distance the vehicle travels during the time it takes the driver to initiate braking.  The total stopping distance, on the other hand, includes both the brake reaction distance and the braking distance, which is the distance the vehicle travels while coming to a complete stop after the brakes have been applied.

To ensure safety, drivers are encouraged to maintain a safe following distance from the vehicle in front of them to account for reaction time and braking distance.  Reaction time can vary based on individual factors and external conditions, so having a cushion of space helps mitigate the risk of rear-end collisions and provides sufficient time for the driver to respond effectively to changing road conditions.

Brake reaction Distance formula

$$B_r = 1.47 \; V \; t$$     (Brake Reaction Distance)

$$V = B_r \;/\; 1.47 \; t$$

$$t = B_r \;/\; 1.47 \; V$$

Symbol English Metric
$$B_r$$ = Brake Reaction Distance $$ft$$  $$m$$
$$V$$ = Design Speed $$ft \;/\; sec$$ $$m \;/\; s$$
$$t$$ = Brake Reaction Time $$mi$$ $$km$$

AASHTO Exhibit 3-1 Stopping Sight Distance

Brake reaction distance predicated on a time of 2.5 s; deceleration.  Rate of 11.2 $$ft/s^2$$ used to determine calculated sight distance.

Design Speed
Design Speed Distance
Braking Distance on Level Stopping Sight Distance
Calculated Design
15 55.1 21.6 76.7 80
20  73..5  38.4  111.9  115
25 91.9  60.0  151.9 155
30 110.3  86.4 196.7 200
35 128.6  117.6  246.2 250
40 147.0  153.6 300.6 305
45 165.4  194.4  359.8 360
50  183.8  240.0 423.8  425
55 202.1  290.3 492.4 495
60 220.5  345.5 566.0 570
65 238.9  4.505 644.4 645
70 257.3  470.3 727.6 730
75 275.6  539.9 815.5 820
80 294.0  614.3 908.3 910