- Decision sight distance is longer than stopping sight distance. Its use is appropriate in road environments where motorists (a) make complex or instantaneous decisions, (b) have difficulty perceiving information, or (c) must execute unexpected or unusual maneuvers.
- Decision sight distance time affords motorists the opportunity to detect an unexpected or difficult-to-perceive information source or condition in a visually complex road environment, recognize a potential threat, select an appropriate speed or path, and efficiently and safely execute the necessary maneuver.
- Table 5.4 provides guidance on calculating decision time.
| Table 5.4 Guidance for Calculating Decision Time | |||||
|---|---|---|---|---|---|
| Avoidance Maneuver | Stop on Rural Road | Stop on Urban Road | Speed, Path, or Direction Change on Rural Road | Speed, Path, or Direction Change on Suburban Road | Speed, Path, or Direction Change on Urban Road |
| Time (t) | 3.0 sec | 9.1 sec | 10.2 – 11.2 sec | 12.1 – 12.9 sec | 14.0 – 14.5 sec |
| Equation | d = 1.47Vt + 1.075 (Vᵗ / a) where: t = time V = design speed (mph) A = motorist deceleration (ft/s2) | d = 1.47Vt where: t = time V = design speed (mph) | |||
| Examples |
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* The time component (t) is the sum of perception-reaction time and maneuver time
- The use of decision sight distance may be appropriate if a site has any of the following characteristics:
- Motorists experience high workloads due to concurrent tasks (e.g., merging, reading signs)
- Truck traffic and bridge structures intermittently block views
- Visual clutter adjacent to the road that can potentially distract motorists
- Poor weather that increases driver workload and reduces the visibility of cues (e.g., markings)
- High traffic volumes
CONTACT:
Chris VanDyke
Research Scientist | Program Manager
chrisvandyke@uky.edu
