The purple PTSS card is worth substantially more in an accessible vehicle
If you hold a wheelchair-subsidy PTSS card, the purple one, you receive 75% of the fare up to a maximum subsidy of $35 when you travel in your wheelchair in a wheelchair accessible taxi. In a conventional taxi the same card pays only 50%, capped at $25.
Across roughly 150 return journeys a year, that gap is not a rounding error. If you're a purple cardholder currently taking whatever vehicle turns up, this one detail is worth more than any discount a transport company could offer you.
