Great idea Phil !

I don't think you need a new device to do this though - you could build an iPhone application to do it right now (and presumably an Android one, once devices using it appear on the market).

On a related (but less ambitious) note, Tech Review has an article today on carpool web sites:

Robert Gilliland didn't think much about carpooling until gas prices got out of control. Now, he's happy to trade his motoring freedom for $120 in weekly savings.

Gilliland found one rider through the classifieds Web site Craigslist and another using the carpool-matching service eRideShare.com. Thousands of commuters like him have turned to the Internet to arrange shared rides as average gas prices hover around $4 a gallon.

Each day, Gilliland picks up Brian McKenzie near his home in Lakeland, Fla., and Mike Rogers in Seffner on his way to work. He drops off McKenzie in Tampa and Rogers in Clearwater, where Gilliland works as a construction designer. He reverses that for the commute home.

The extra stops add up to an hour in round-trip commuting, meaning he could be spending three hours a day in his Ford Taurus, but the alternative is to shell out $150 a week out of his own pocket. With carpooling, his two passengers contribute $60 a piece; he pays only half of that to factor in wear and tear on his car.

"We've become friends," Gilliland said. "It's been a good experience for me socially as well as economically."

In Israel it is called sherut and it have been in use since Israels foundation.Between cities and in cities,usually the same rounds busses are going. Fram 7 seaters to 10 or 15 seater minibusses and it is working just fine.You just stop the taxi or minibus and pay about the same fare as the bus would cost.You can get off wherever you want.

I've seen, and occasionally used, this sort of operation in action in a lot of countries in Africa and Asia too - works great (although not always as safe or comfortable as a regular bus service).

Ah, it seems you're nigh on two years late Phil: High-tech Hitchhiking — Sightline Daily (formerly Tidepool)

Posted Dec. 11, 2006.

So, if someone can figure out how to broker the rental of some of those unused seats, she or he will be rich, and the driver and rider will save money by the oil barrel.

Up to now, the biggest obstacle to such a market has been information. How can drivers and riders find each other? How can they know whether to trust each other? How can they ensure payment?

That’s where the Seattle start-up Goose Networks comes in. Goose has built a real-time ridesharing system that links riders with drivers by combining text-messaging mobile phones, mapping software, a clever database, and a billing system for splitting the cost of fuel.

Some 200 Microsoft employees who live in central Seattle are currently testing the system. Here’s how it works: a goose member sends a text message about her impending departure and location to the central computer, which instantly looks for matches with others going the same way. The service is free for both rider and driver (except for the cost of the text message). Goose’s costs are paid by the employer. (The phone company also makes money on the text messaging. In fact, cell phone companies might do well to underwrite digital hitchhiking just for the texting it generates.)

At some time in the future, Goose Networks believes it can move beyond commute trips, once text-hikers are commonplace and text-hiking is ubiquitous. For now, though, Goose’s whole business plan pivots on employers and commuters. The company aims to grow by adding one large employer after another, capitalizing on the trust among these firms’ employees; their similar commute schedules; and these firms’ obligation under state laws in California, Oregon, and Washington to plan for reduced solo-driving commuters. (Compared to the cost of vanpools, employee transit passes, and showering facilities for cyclists, paying for a corporate Goose program is likely to be a good deal.)

Check the story out for links and more info. Covers all manner of approaches.

I've used a similar service in Thailand, where you flag down the "bus" (converted pickup truck) and tell them where you want to go - if your destination isn't in the opposite direction to the current passengers they will take you right to your destination, altering the route as they go to get the shortest route between all of the destinations.

It was great.

How do you cope with the jostling to get a window seat so you can take pot shots at the no-such-people-as-the-Palestinian/terrorists as you speed to your settlement ?

I think a key aspect of this might be vetting the membership. A clean record and a recomendation from a current member needed to become a member I think would give people a good feeling about who is stopping to give them a lift. Also you could limit it to people who own cars by making it a "pay it forward" system where each time you give a member of the club a ride you get a credit for a ride yourself. I don't know how the tax man would handle the "pay it forward" system where only once you are clearly a dead beat are you kicked out. No money would be changing hands for the service and there wouldn't even be a non monetary trade involved.