Transportation PE Horizontal Curve Practice Problem: Stopping Sight Distance with an Inside Obstruction
- Andrea R
- Jun 1
- 3 min read
Updated: Aug 5
If you're preparing for the Transportation PE exam, you can expect to see 8-12 horizontal design problems on exam day per the NCEES CBT exam specifications. Horizontal curve geometry is a foundational topic in highway design and frequently appears in Transportation PE practice problems.
One frequently overlooked Transportation PE topic is determining how a sight obstruction on the inside of a horizontal curve affects available stopping sight distance. This type of Transportation PE question requires more than simply calculating a curve radius or degree of curvature. Instead, you must evaluate whether roadside features such as rock cuts, retaining walls, embankments, or vegetation limit the driver's available stopping sight distance.
Understanding how to solve Transportation PE horizontal curve sight distance problems is important because these questions combine multiple highway design concepts into a single problem. To answer them correctly, you must recognize when the horizontal sight line offset (HSO) equation applies and know where to find it in the reference materials provided during the exam. The horizontal sight line Offset equation is found in Chapter 3 of the AASHTO Green Book.
The following Transportation PE practice problem demonstrates how to determine the additional horizontal clearance required when an obstruction on the inside of a horizontal curve restricts available sight distance.
Problem:
A two-lane rural highway has a horizontal curve with a radius of 1,200 ft.
The required stopping sight distance is 645 ft.
A rock outcrop on the inside of the curve currently provides a horizontal clearance (middle ordinate) of 34ft between the centerline of the inside lane and the obstruction.
How much additional horizontal clearance must be provided to achieve the required stopping sight distance?
A. 5 ft
B. 9 ft
C. 27 ft
D. 43 ft
Solution:
Use the horizontal sightline offset equation:
Where:
HSO = horizontal sightline offset (ft)
R = curve radius (ft)
S = stopping sight distance (ft)
Step 1: Calculate the required horizontal sightline offset
Given:
R = 1,200 ft
S = 645 ft
Substitute into the equation:
HSO = 1200[1 − cos(28.65 × 645 / 1200)]
HSO = 1200[1 − cos(15.40°)]
HSO = 1200[1 − 0.9641]
HSO = 1200(0.0359)
HSO = 43 ft
Therefore, the required horizontal sightline offset is:
HSO ≈ 43 ft
Step 2: Determine the additional clearance required
The existing horizontal sightline offset is:
34 ft
The required horizontal sightline offset is:
43 ft
Additional clearance required:
43 ft – 34 ft = 9 ft
Answer
B. 9 ft
Roadside obstructions can significantly reduce the sight distance available to drivers and must be considered during the design of horizontal curves. When solving Transportation PE horizontal curve problems, pay close attention to mentions of the following in the problem statement:
Rock cuts
Retaining walls
Vegetation
Roadside obstructions
Horizontal sightline offset
Stopping sight distance
These clues often indicate that the horizontal sight line offset equation from Chapter 3 of the AASHTO Green Book is required. Many Transportation PE candidates spend time learning formulas but struggle to identify which equation applies to a given problem. The candidates who consistently score well are usually the ones who can recognize the design concept being tested and quickly locate the appropriate formula in the provided references.
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