My mother-in-law is a pharmaceutical company's worst nightmare - fit, healthy, and medication free. But I think she'd be happier if she took a taxi once in a while to go out in the evening with friends. And I don't understand: why won't she?

In three years, how many times has my mother-in-law taken a taxi? Excluding trips to the airport or train station - journeys she made by taxi even in her car owning days - none. Zero. Zip. She walks or takes the bus to the grocery store or up the town.

Three years ago my mother- and father-in-law decided to sell their car. They worked out that they would save enough on maintenance and insurance to pay for a taxi at least once a week.

The economic approach to the question considers marginal costs and marginal benefits. A car involves high fixed costs, but low marginal costs. Using a taxi involves no fixed costs but high marginal costs. People will take trips as long as the marginal benefit of the trip is greater than or equal to the marginal cost. Car owners face low marginal costs, so take low marginal benefit trips, such as driving to the mall to buy a can of tuna. Taxi takers face high marginal costs, so won't undertake a journey unless the marginal benefit is high, such as taking a taxi to the mall to buy a week's groceries.

The economic approach explains why taxi users take fewer trips than car owners. But it's hard to explain why someone would take no trips at all by taxi. One study I found reported that just 1 percent of seniors in Britain regularly used taxis. Yes, taxis are expensive - but so are cars. The cost of insurance alone would pay for dozens of taxis.

I have a few totally unsubstantiated theories about why people don't take taxis more often, and some policy suggestions.

Unsubstantiated theory 1: habit/routine. If every time I sent to the grocery store, I gave profound consideration to every purchase decision, it would take me 2 hours to get around the store. People simplify their lives by using rules - I just grab the brands that I know, from experience, will satisfy my needs and are within my budget. Many people have lived life according to the rule "I can't afford to take taxis." It's hard to get accustomed to new routines.

Unsubstantiated theory 2: the fitness club problem. Why do fitness clubs sell memberships that provide for unlimited use? Going to the gym makes you feel good in the long run. But it hurts in the short run. Having to pay each time you go to the gym would just exacerbate the short run pain and make one even less likely to go. Taking a taxi has a similar dynamic - short run pain (the fare) for long run gain (freedom, connection with friends and family). Because we focus too much on the short run when making decisions, we don't take taxis, even if getting out would make us better off in the long-run.

Unsubstantiated theory 3: Taxis aren't set up to make old folks feel comfortable.

So what's the answer?

More flexibility in taxi pricing structures would help a lot.

One solution is to reduce the marginal cost of taking a taxi with off-peak fares, "season passes" that would allow seniors to pay a lump-sum for unlimited free or low-cost rides, or by encouraging drivers to make multiple pick-ups/drop-offs on a single route so two or three seniors could share a given fare.

An alternative is to take the pain out of paying the cost of a taxi, with pre-paid taxi vouchers, automatic credit card billing, and so on. Handing over five $20s for five different rides hurts more than a single $100 credit card charge.

It would be easier to make the decision to sell the car and take taxis instead if one could pre-commit to spending, say, $400 or $500 a year on alternative forms of transportation.

But comfort matters, too. In a taxi both passenger and driver are in a vulnerable situation, alone with someone they've never met before. There's numerous ways in which taxis could be made more comfortable for passengers:

- making it easier for passengers to get to know and to request a certain driver, especially when booking ahead of time. "You had Jamie last time, would you like the same driver again?"

- taxis that are easier to get into and out of - a Toyota matrix, rather than a low-slung vehicle.

Much of the policy debate seems to be focus on publicly providing or financing services to seniors, for example, through para-transpo. But that's a price solution (low cost transport) to an income problem (inability to afford to take a taxi).

And not all seniors are poor. Mrs A, living in multi-million dollar Vancouver home, doesn't need para transpo.

She just needs to get her act together and dial a taxi, so she can get out and enjoy life.