Disclosure: I work at ACOREL , a French company that builds automatic passenger counting (APC) systems for public transport. This post is about the engineering problem, not about our products.

A bus door is about a metre wide. People go in, people go out. Counting them sounds like a weekend project with a camera and an object detector.

Then you look at what the numbers are used for. Transit agencies plan timetables with them. In integrated fare systems, operators split ticket revenue based on how many passengers each one carried. Public service contracts pay per passenger-kilometre. At that point, 95% accuracy isn't "pretty good". It's a 5% error in somebody's revenue.

Getting from 95% to 99% is not about buying a better sensor. It's about the whole chain: geometry, tracking, edge processing, validation and data integration. Here's how that chain works.

1. Why the doorway is a hard place to count

A doorway is a narrow, crowded, constantly changing scene.…