---
title: "Short note: reverse or tidal flow operation"
subject: "Transportation Engineering II"
chapter: 1
key: "semester_7/transportation_engineering_ii:short-note-reverse-tidal-flow"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/short-note-reverse-tidal-flow
site: BCT Engineering
---

# Short note: reverse or tidal flow operation

Transportation Engineering II · Chapter 1: Traffic Engineering

- Asked in: 2066 Bhadra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/short-note-reverse-tidal-flow
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-1

## Question

Write a short note on reverse or tidal flow operation.

## Short answer

Reverse tidal flow operation shifts lane directions to match peak demand using overhead signals or movable barriers. It needs a 65:35 directional split and three-plus lanes, improving capacity cheaply, but introduces head-on conflict and driver confusion risks.

## Answer

**Reverse (tidal) flow operation** is a traffic management technique in which the number of lanes available to each direction is changed with the time of day, so that more lanes are given to the heavier direction at peak hours. It is used where the peak flow in one direction is much larger than in the other (for example inbound in the morning and outbound in the evening on a commuter corridor).

```text
 Morning peak (to city):     Evening peak (from city):
  --> --> -->  (3 lanes)      --> (1 lane)
  <--          (1 lane)       <-- <-- <--  (3 lanes)
```

**Conditions for use**
- A directional split of about 65:35 or more in the peak period, and a road with at least three or four lanes.
- Roadside access and turning movements limited.

**Methods of control**
- Overhead lane-control signals (green arrow / red cross).
- Movable (zipper) barriers, cones or lane-markers placed by crews.
- Signs and markings; enforcement by Traffic Police.

**Advantages:** uses existing road width more efficiently, increases capacity in the peak direction without new construction, and is cheap compared with widening.

**Disadvantages:** safety risk of head-on conflict if drivers do not follow signals, cost of signals and staff, confusion of drivers, and difficulty in turning movements at junctions.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is reverse (tidal) flow operation?
   A. Banning all vehicles in one direction permanently
   B. Changing the number of lanes given to each direction with time of day
   C. Making a two-way road one-way for ever
   D. Reversing the speed limit during peak hours
   Answer: B. More lanes are given to the heavier direction at peak hours, then switched as the peak direction changes. (Key point: Definition)

2. Reverse flow is justified mainly when:
   A. Traffic is equal in both directions
   B. The road has only one lane
   C. The peak directional split is about 65:35 or more and the road has three or four lanes
   D. There are many side accesses and turning movements
   Answer: C. A strong directional imbalance and at least three or four lanes are needed, with limited access and turning. (Key point: Conditions for use)

3. Which is a method of controlling reverse flow lanes?
   A. Overhead lane-control signals or movable barriers
   B. Removing all road markings
   C. Speed humps
   D. Rotary islands
   Answer: A. Lane-control signals, zipper barriers, cones, signs and enforcement are used. (Key point: Control methods)

4. A main risk of reverse flow operation is:
   A. Reduced capacity in the peak direction
   B. High cost of widening the road
   C. Head-on conflict if drivers ignore the signals
   D. Increased pavement thickness
   Answer: C. Safety risk of head-on collision, plus signal and staff cost and confusion at junctions, are the main drawbacks. (Key point: Disadvantages)

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- [Tidal versus wind power plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Atidal-vs-wind) — Power Plant Design, chapter 1
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