Expert Review: Redesigning City Bus Networks - Frequency vs. Coverage

Recently, several major cities have announced comprehensive redesigns of their bus networks. The public discourse often centers on specific route changes—why is my stop moving? Why are we losing the direct bus to downtown?

But from an optimization perspective, these redesigns represent a fascinating (and difficult) mathematical challenge: solving the Network Design Problem (NDP) under shifting objectives.

The Frequency vs. Coverage Trade-off

Most redesigns aim to shift the balance between two competing goals:

1. Coverage: Ensuring everyone is within walking distance of a bus stop. This maximizes social inclusion but spreads resources thin, leading to infrequent service (e.g., every 30-60 mins).
2. Frequency: concentrating resources on high-demand corridors to provide "turn-up-and-go" service (e.g., every 5-10 mins). This maximizes ridership and efficiency but may force some users to walk further.

In many recent cases, agencies are pivoting towards Frequency, creating "high-frequency grids" that rely on transfers.

Why Optimisation Matters Here

This isn't just a policy decision—it's an optimization problem. We can model this as maximizing accessibility to jobs and services within a bounded travel time budget.

Where $d_{ij}$ is demand and $A_{ij}$ is accessibility.

When we look at news reports of "angry residents losing their stop," we are often seeing the local cost of a global optimization that improves system-wide efficiency. As experts, our role is to quantify these trade-offs and ensure that the "global optimum" doesn't hide "local inequities."

This post is part of my OptiTransit Commentary series, where I apply academic rigour to public transport news.