FE Civil Practice Question Solution included after investigation
CASE E-105 · FE

The Lane That Carried 1,800 veh/h, Not 1,227

The spacing was 132 ft per vehicle, but 45 mph was treated as 45 ft/s. Correct unit conversion gives density 40 veh/mi and flow 1,800 veh/h/lane.

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Case E-105 engineering failure visual
HOW THIS WORKS

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Follow the investigation process used in the field — in five guided steps.

1

Inspect

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2

Commit

Choose your hypothesis.

3

Prove

Run calculations and test your idea.

4

Fix

Select and validate a safe correction.

5

Sign Off

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Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A uniform traffic stream has average space headway of 132 ft/vehicle and average speed of 45 mph on one lane. A note treats 45 mph as 45 ft/s, calculates a 2.93 s headway, and reports about 1,227 veh/h. Determine the correct density, time headway, and lane flow.

Transportation Engineering Civil Design Verification Root Cause Analysis Safe Correction
E-105
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

Miles per hour was treated as feet per second.

What was missed

The required unit conversion between miles and feet.

Corrected model

Use consistent units and q=kv, or compute time headway with speed in ft/s.

QUICK ANSWER

k=5280/132=40 veh/mi/lane. Therefore q=kv=40×45=1800 veh/h/lane. The equivalent time headway is 2.0 s/veh.

ENGINEERING TAKEAWAY

Use consistent units or convert spacing to density in veh/mi before applying q=kv.

CASE QUESTIONS

Questions this case should settle.

What is the fundamental traffic-flow relation?

q=kv, where flow equals density times speed with consistent units.

NEXT FE PRACTICE QUESTION

The PM Peak That Sent 199 Cars In, Not 306

Transportation Engineering · 5–10 min

Continue Practice →
WHAT YOU’LL LEARN

Real failures. Lasting skills.

Each case is designed to build the judgment, analysis, and confidence you need for engineering exams — and beyond.

  • Apply core engineering concepts to real-world problems
  • Practice structured troubleshooting and analysis
  • Strengthen exam-ready thinking through realistic scenarios
COMMON QUESTIONS
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What happens after I complete a case?

The sealed debrief unlocks with the root cause, corrected reasoning, fix, and takeaway.