Traffic operations: The distance a vehicle travels during the driver’s perception and brake-reaction time is known as what?
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ASight distance
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BStopping distance
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CLag distance
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DNone of these
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EHeadway distance
Answer
Correct Answer: Lag distance
Explanation
Introduction / Context:Stopping sight distance (SSD) is the sum of two components: the distance covered during the driver’s perception–reaction time and the distance required to actually brake to a stop. Distinguishing these components helps in setting safe sight distances and sign placement.
Given Data / Assumptions:
- Driver perceives a hazard and reacts (moves foot to brake) over a standard reaction time.
- Vehicle is initially at design speed.
- No grade adjustments are considered in the definition itself.
Concept / Approach:
The distance traveled during perception and reaction—before effective braking starts—is called the lag distance. After this, the braking distance is added to get SSD. Mislabeling these can lead to under-designed sight distances.
Step-by-Step Solution:
SSD = lag distance + braking distance.Lag distance depends on speed and reaction time (typically 2.5 seconds in many manuals).Identify the asked component → distance during perception–reaction → lag distance.Verification / Alternative check:
Design manuals consistently define lag distance as the perception–reaction component of SSD. Braking distance is computed separately using deceleration assumptions.
Why Other Options Are Wrong:
- Sight distance/Stopping distance: broader measures that include or exceed lag distance.
- Headway distance: spacing between vehicles, unrelated to perception–reaction distance definition.
Common Pitfalls:
- Confusing lag distance with total SSD, leading to insufficient sight lines.
Final Answer:
Lag distance.