Transportation PE Practice Problem: Intersection Sight Distance Example

A common roadway design topic tested on the Transportation PE Exam is Intersection Sight Distance (ISD). Candidates are often required to determine the minimum sight distance needed for a vehicle entering or crossing a major roadway from a stop-controlled minor road.

In this Transportation PE practice problem, we'll calculate the required intersection sight distance for a passenger car making a left turn onto a four-lane highway and discuss a common mistake that can cost valuable points on exam day.

Practice Problem

A passenger car is stopped at a stop sign on a minor-road approach to an intersection. The major road is not stop controlled. The car intends to turn left onto a 4-lane major highway. The design speed of the major road is 55 mph.

What is the required Intersection Sight Distance (ISD) along the major road?

A. 605 ft

B. 645 ft

C. 685 ft

D. 725 ft

Solution

Per Ch. 9 of the AASHTO Green Book this scenario falls under Case B – Intersections with stop control on the minor road, and more specifically Case B1 – Left turn from minor road. Use the corresponding AASHTO Green Book guidance for this scenario.

Step 1: Determine the required time gap

Base time gap for a passenger car turning left:

Adjustment for a 4-lane major road:

Total time gap:

Step 2: Calculate the intersection sight distance

Use the AASHTO equation for a vehicle turning left from a stop-controlled minor road:

Substitute the given values:

Answer: B. 645 ft

Transportation PE Exam Tip

A common mistake is to use the base time gap of 7.5 seconds and forget the adjustment for the 4-lane highway.

If you do that:

ISD=1.47(55)(7.5)

ISD=606.4 ft

which would lead you to select A) 605 ft instead of the correct answer. On the Transportation PE exam, watch for adjustments to the base gap time due to additional lanes, grades, and design vehicles before plugging values into the ISD equation. For Case B1, these adjustments are listed in the fine print below AASHTO Green Book Table 9-6 (Time Gap for Case B1, Left Turn from Stop).

What Factors Can Affect Gap Time?

When solving intersection sight distance problems, check whether adjustments are needed for the below items. These adjustments can be found in section 9.5 of the AASHTO Green Book.

Number of Lanes

Additional lanes on the major roadway can increase the time required for a vehicle to complete its maneuver safely.

Grade on the Minor Road

Vehicles starting on upgrades may require additional time to accelerate into traffic.

Design Vehicle

Larger vehicles such as single-unit trucks or combination trucks require larger gaps than passenger cars.

Intersection Control Type

Different intersection configurations and maneuver types may require different time gaps and procedures.

Transportation PE Exam Strategy

When solving Type B intersection sight distance problems:

1. Identify the maneuver being performed and the relevant AASHTO Green Book case that applies.

2. Determine the correct base time gap.

3. Check for lane, grade, or vehicle adjustments to the time gap.

4. Calculate the total time gap.

5. Apply the AASHTO intersection sight distance equation.

A large percentage of Transportation PE calculation mistakes occur before the intersection sight distance equation is ever used. The challenge is often determining the correct time gap.