Traffic operations: The distance a vehicle travels during the driver’s perception and brake-reaction time is known as what?

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:

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.