Navigating the Paris Metro is notoriously difficult. The official maps are helpful only if you already know your way around the network. The official interactive map by RATP is feature-complete but not very user friendly, and sometimes not working (for reasons we discuss below).
There used to be a fantastic device in the Paris Metro, called "PILI" for "Plan Indicateur Lumineux d'Itinéraire" where one could push a button for a start station, and a button for a destination, and the best way between the two would light up on a big metro map above.
These machines were built in the 1930 and at some point there were around 80 of them in the metro; but they were very difficult to maintain and so they eventually fell into disrepair; the only ones that are left don't work anymore.
Nowadays, it should be possible to make an interactive map that would replicate the ease of use of a device built... more than 80 years ago; and yet the existing transport map (for Paris or other cities) are usually very complex to use (when they exist at all)
In a bold and very welcomed initiative, the RATP (the public body that runs the Paris Metro) recently published its data about stations, routes, schedules, etc.
And so I decided to use this data to build an online version of the "PILI".
The originality of our approach, I think, is that pathfinding is done in the browser; all the other apps or services (that I'm aware of) call a server to do the pathfinding, resulting in delays (Google Maps is by far the fastest, but it's still not immediate; others can be quite slow) or complete non-response if the service is down (the official RATP Plan Interactif is often down on week-ends for instance).
Doing pathfinding in the browser has many advantages
The last point sounds trivial but it's actually a very important part of the UI; I can afford to make pathfinding as easy as 2 clicks (or even one click) because every path is calculated on the client. If a child decides to spend an afternoon clicking on the map (and my kids love it!) I have absolutely no problem with that. This addresses scalability issues in a big way.
The application was first implemented in French, but today I'm happy to say that the English version is out! Language is not hugely important, since it's mostly visual, but directions should be easier to understand for English speakers now.