What the four-digit PIN is actually for
It is not bureaucracy. It answers the only question that matters at a busy kerb.
At a busy pickup point at night, several cars are waiting. One of them is yours. The registration plate is on your phone, but it is dark, the plate is behind another vehicle, and a driver has just lowered a window and said your name.
This is the moment the PIN is for.
How it works
Every Topsy trip carries a four-digit PIN. The trip cannot start until the driver has it. You give it to them, they enter it, and the trip begins. If the number does not match, the trip does not start.
That is the whole mechanism, and its simplicity is the point. It does not depend on you reading a plate in poor light, or on recognising a face from a small photo, or on the driver knowing your name, which anyone standing nearby could also have heard.
What it protects against
The specific risk is somebody presenting themselves as your driver when they are not. It is uncommon, and it is serious, and the countermeasure has to be something the other person cannot guess. A name can be overheard. A destination can be guessed. A four-digit number that exists only on your screen and in the dispatch record cannot.
It also quietly prevents an honest mistake. Two people waiting at the same hotel entrance, two similar vehicles, two trips booked a minute apart. Without a check, it is entirely possible to get into the wrong car and for neither party to notice for several minutes.
What to do if the PIN does not match
Do not get in. Say the number does not match and check the vehicle and driver details on your trip. Nine times out of ten it is the wrong car in an honest way, and the right one is thirty seconds behind. If something feels wrong, the trip carries SOS and trip sharing, and both are reachable from the same screen.
The rest of the trip
The PIN starts things off, but it is not alone. The route is tracked live for the whole journey. You can share the trip with someone who will then see the car, the plate and its position without needing an account. Trusted contacts and SOS are available throughout.
None of it is complicated to use, which is the standard safety tooling has to meet. A feature you have to think about at the moment you are worried is a feature you will not use.