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The distance traveled by a moving vehicle during perception and brake reaction time is
Sight distance
Stopping distance
Lag distance
None of these
Lag distance
Lag distance is the distance covered by a vehicle from the instant an object becomes visible to the driver until the driver applies the brakes. It is essentially the product of the vehicle's speed and the total perception-reaction time of the driver.
Lag distance is the distance covered by a vehicle from the instant an object becomes visible to the driver until the driver applies the brakes. It is essentially the product of the vehicle's speed and the total perception-reaction time of the driver.
dL=v⋅t — Lag distance formula where v is speed and t is total reaction time
SSD=dL+dB — Total Stopping Sight Distance where dB is braking distance
When a driver perceives an obstacle, there is a delay (perception-reaction time) before they physically press the brake pedal. During this interval, the vehicle continues to move forward at a constant speed, covering a specific distance known as lag distance.
Perception-Reaction time is commonly taken as 2.5 seconds as per IRC standards.
Lag distance is purely a function of time and velocity; it does not involve deceleration.
Braking distance (dB) begins after the lag distance is completed.
Lag distance is also known as 'Reaction Distance'.
Essential for safe highway design and intersection planning.
Provides a critical safety margin for collision avoidance.
High dependence on human psychological and physiological reaction speeds.
Can increase significantly with driver fatigue or intoxication.
Calculation of Stopping Sight Distance (SSD).
Designing minimum road length requirements for safety signage.
Road marking and signaling distance placement.
Stopping distance is the sum of lag distance and braking distance.
Sight distance refers to the actual length of roadway visible to the driver, which is a broader concept.
Standard IRC perception-reaction time for braking is 2.5s.
C is correct — The distance covered during the reaction time is defined as the lag distance.
Always remember that total SSD is the sum of Lag Distance and Braking Distance; if the question mentions 'perception and brake reaction time', it specifically refers to the Lag component.