Transit is of course more efficient than private cars, many people on one vechicle. But because a round-trip for a couple or family involves buying 4 to 8 single tickets, couples and families who have cars will often take their cars unless parking is going to be a problem. For example, for us to go downtown it's $6 within SF. For people taking BART from Berkeley or Oakland it's $13.40 for 2 people. Makes it very tempting to take a car, even if it costs a similar amount (at 35 cents/mile, 15 of those for gasoline in a city) for the convenience and, outside of rush-hour, speed.

So even if transit is the winning choice for one, it often isn't for 2. And while 2 in a car is better than 1, an extra 2 on transit during non-peak hours is even better for traffic and the environment.

Many transit agencies offer a one-day family pass, mostly aimed at tourists. There may be some that also offer what I am going to propose, which is a more ordinary one-way or return ticket for groups of people living at the same address, that is sufficiently discounted to make them do the transition from car to transit.

This isn't trivial, we don't want drivers to have to check addresses on IDs as people get on the bus. They can check a simple card, though. For example, people could get a simple, non-logged card with their photo and some simple word, symbol or colour combination, so that the driver can tell right away that all the cards were picked up together. (For example they could all have the same randomly chosen word on them in large print, or 3 colour stripes.)

The household/family fare would be only available outside of hours where the transit cars get loaded to standing room. Past that point each rider should pay, and driving is usually rough anyway. Passengers could board, show their matching cards, and get reduced, or even free fares for the additional people. The driver could look at the photos but probably only needs to do that from time to time. (Mainly, we would be trying to stop somebody from getting a set of household cards, and selling cheap rides to random people at the stop with them. Not that likely an event anyways, but random photo checks could stop it.)

It's harder to manage at automatic fare stations as found on subways. There you could get more abuse, but it might not be so much as to present a problem. The main issue would be homeless people "renting" card sets to groups of people who arrive at a turnstile. (At fancy pay-to-pee public toilets in SF, the homeless were given unlimited use tokens. Better that than have them urinate on the streets for lack of a quarter. They promptly got to renting these tokens to tourists wanting to use the toilets.)

If you're not too worried about abuse, family tickets could simply be purchased in advance from a desk where they can check that everybody is in the same household. The adults would have to show (easiest for couples) but they need not bring the kids, who already get reduced fares as it is, though in the household ticket they would probably be free.

I presume some transit agencies already do this since the one-day passes are common enough. How do they work it out? Is it aimed at locals rather than tourists? Do they assume locals close to the transit line get monthly passes?