The fundamental relationship of traffic flow states that flow equals density times speed, and it ties together the three quantities every highway capacity question is built from. Flow is vehicles per hour past a point. Density is vehicles per mile of roadway, a snapshot of how tightly packed the traffic is. Speed is the space-mean speed of those vehicles. Given any two, the third follows, and the FE Civil exam usually gives you exactly two. The units are what make or break these items. Density in vehicles per mile times speed in miles per hour yields vehicles per hour, which is why the relationship works with no conversion factor, but only if the speed is in miles per hour and the density is per mile of the whole roadway or per lane consistently with the flow you are asked for. Mixing a per-lane density with a total-facility flow is the most common failure. The relationship also explains the shape of the fundamental diagram: flow is zero both when density is zero, because there are no vehicles, and when density reaches jam density, because nothing is moving. Capacity occurs somewhere in between, at roughly half the free-flow speed under the linear Greenshields model.
Commit to an answer before opening the solution. Each problem shows the governing model, every substitution, a unit check, and why each wrong choice is wrong.
Original question written by FE Exam AI Prep from the public FE Civil exam specification. Before release, a solver model re-derived the answer twice from the question alone, without its answer key, and a critic model checked the result. No licensed engineer reviews these questions. It is not an NCEES question and does not come from any NCEES practice exam. FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES.
FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES. All practice questions are original and generated for study purposes based on publicly available exam specifications.