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GATE 2026 CE (CE1) – Question 42

Traffic Engineering and Transportation Planning · Fundamental relationships of traffic flow · 2 marks · Multiple choice

A road is divided into four sections having varying widths as shown in the figure. Section-2 (S2) represents a capacity constrained condition with respect to the traffic flow passing through Section-1 (S1). Section-3 (S3) and Section-4 (S4) do not face any such capacity constraint with respect to the flow. A flow-density relationship for unconstrained and constrained flow conditions is shown in the figure.

If Section-1 observes density D2, the option representing the correct state of density in Sections 2, 3 and 4 is:

A road with a wide section S1, a narrow section S2 and wider sections S3 and S4, above a flow-density graph. The unconstrained curve peaks at density D2 and the constrained curve peaks lower, at density D3. A dashed horizontal line at the level of the constrained peak meets the unconstrained curve at densities D1 and D4 and the constrained peak at D3.
  1. S2 – D4 ; S3 – D3 ; S4 – D2
  2. S2 – D3 ; S3 – D2 ; S4 – D1
  3. S2 – D3 ; S3 – D4 ; S4 – D2
  4. S2 – D4 ; S3 – D3 ; S4 – D1

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Correct answer: (B) S2 – D3 ; S3 – D2 ; S4 – D1

Explanation

Section 1 is at density D2, where its flow is at its own maximum, which is more than the constrained section can pass. So section 2 operates at its own capacity, at density D3. This answer follows the usual reasoning of a bottleneck, and picks the option that agrees with it for sections 2 and 4.