How Pizza Tycoon simulated traffic on a 25 MHz CPU

Context

Pizza Legacy — an open-source reimplementation of the 1994 DOS game Pizza Tycoon — finally got its little street-level cars to behave like a city. The project’s author says they started the close-zoom map back in 2010 and only recently cracked the driving logic after multiple rewrites and dead ends. Why did such a small feature take so long? Because the first instinct was modern: scene graphs, pathfinding, collision detection. All that stuff. On a 25 MHz machine? It didn’t add up.

How the traffic worked

The secret was embarrassingly simple and beautifully old-school. It has been reported that the original game encoded routing in the tiles themselves: each road tile carries a permitted direction, so a car only needs to know what tile it sits on to decide where to go. No graph search, no A*, no destination lists — just tile types, a coin flip at corners for a left or straight, and one pixel of movement per tick while checking for blockage. Earlier reimplementations tried complex grid locking and per-tile reservation systems; those designs turned into a shared-locking nightmare. The developer says LLMs helped decode the original assembly, revealing that the classic game relied on tiny, local rules to fake a living street.

Why it matters

This is a neat reminder that convincing complexity can be cheap if you pick the right constraints. Retrocomputing isn’t just nostalgia — it’s a lesson in elegant engineering: small rules, careful map design, and a bit of randomness give the illusion of life without modern computational heavy lifting. Want to simulate a bustling city on a chip from the 1990s? Turn down the abstraction, and let the tiles do the talking.

Sources: pizzalegacy.nl, Lobsters