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Questions and Answers
The overtaking sight distance is measured along the edge of the road.
The overtaking sight distance is measured along the edge of the road.
False (B)
The gradient of the road does not affect the passing sight distance.
The gradient of the road does not affect the passing sight distance.
False (B)
The passing vehicle travels at the same speed as the vehicle being passed while in the left lane.
The passing vehicle travels at the same speed as the vehicle being passed while in the left lane.
False (B)
The spacing between vehicles is independent of the speed.
The spacing between vehicles is independent of the speed.
The calculation of passing sight distance required for a given roadway is best accomplished using a complex model.
The calculation of passing sight distance required for a given roadway is best accomplished using a complex model.
The model used to calculate the passing sight distance incorporates two vehicles.
The model used to calculate the passing sight distance incorporates two vehicles.
The total reaction time of a driver can be split into more than four components based on PIEV theory.
The total reaction time of a driver can be split into more than four components based on PIEV theory.
The sight distance required is less when the efficiency of brakes is lower.
The sight distance required is less when the efficiency of brakes is lower.
A higher speed of the vehicle results in a shorter sight distance.
A higher speed of the vehicle results in a shorter sight distance.
The brake efficiency of a new vehicle is always 100%.
The brake efficiency of a new vehicle is always 100%.
Reaction time is the time taken from the instant the brakes are applied to the instant the object is visible to the driver.
Reaction time is the time taken from the instant the brakes are applied to the instant the object is visible to the driver.
The perception-reaction time of a driver is typically around 0.5 seconds under normal conditions.
The perception-reaction time of a driver is typically around 0.5 seconds under normal conditions.
The lag distance is the distance traveled by the vehicle during the braking operation.
The lag distance is the distance traveled by the vehicle during the braking operation.
The braking distance on a horizontal plane is given by the formula D = V^2 / (2gf).
The braking distance on a horizontal plane is given by the formula D = V^2 / (2gf).
The sight distance on a vertical summit curve is only calculated when S is less than L.
The sight distance on a vertical summit curve is only calculated when S is less than L.
The standard value of h1 for stopping sight distance is 6 inches.
The standard value of h1 for stopping sight distance is 6 inches.
The safe stopping sight distance is the sum of the lag distance and the braking distance.
The safe stopping sight distance is the sum of the lag distance and the braking distance.
The formula S = vt + V^2 / (2g(f-G)) is used to calculate the sight distance on a horizontal grade.
The formula S = vt + V^2 / (2g(f-G)) is used to calculate the sight distance on a horizontal grade.
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Study Notes
Overtaking Sight Distance
- Overtaking sight distance is the minimum distance required for a driver to overtake a slow vehicle safely against oncoming traffic.
- It is measured along the center line of the road, taking into account the driver's eye level (1.2m above the road surface) and the height of an object (1.2m above the road surface).
Factors Affecting Passing Sight Distance
- Velocities of the overtaking vehicle, overtaken vehicle, and oncoming traffic
- Spacing between vehicles, depending on speed
- Driver's skill and reaction time
- Rate of acceleration of the overtaking vehicle
- Gradient of the road
Calculating Passing Sight Distance
- A simple model incorporates three vehicles and six assumptions:
- Constant speed of the vehicle being passed
- Passing vehicle follows the slow vehicle into the passing section
- Time required for the driver to perceive a clear opposing lane and begin accelerating
- Passing vehicle travels at an average speed 10mph faster than the vehicle being passed
- Oncoming traffic in the opposite direction
- Computation of sight distance depends on reaction time
Reaction Time
- Time taken from when an object is visible to when the brakes are applied
- Can be split into four components: perception, intellection, emotion, and volition
- Typically around 1.5-2 seconds under normal conditions, but may vary depending on driver characteristics
Speed and Sight Distance
- Higher speeds require more time to stop the vehicle, increasing the required sight distance
Brake Efficiency
- Efficiency of brakes affects sight distance, with lower efficiency requiring more sight distance
- 100% brake efficiency is impractical, so more sight distance is required
Safe Stopping Sight Distance
- The sum of lag distance (vehicle travel during reaction time) and braking distance
- Lag distance: d = vt (velocity x reaction time)
- Braking distance: depends on velocity, gravity, and friction (moving on a horizontal plane or grade)
- Safe stopping sight distance: S = vt + (V^2 / 2gf)
Sight Distance on Vertical Summit Curve
- When S < L: L = AS^2 / (100 * (√2h1 + √2h2)^2) (metric) or L = AS^2 / 1400 (english)
- When S > L: L = 2S - (200 * (√h1 + √h2)^2) / A
Standard Values in Road Design
- Stopping sight distance (SSD): h1 = 3.75 ft (1.14m), h2 = 6 in (0.15m)
- Passing sight distance (PSD): h1 = 3.75 ft (1.14m), h2 = 4.50 ft (1.37m)
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