Here’s how it would work:

• Alt-transportation users would log their shared rides/bicycle rides/walks using a service like RideSpring, which may provide rideshare matching service. From these logs as well as the type of car that would be used otherwise, carbon emission savings would be computed. Certification might involve a mutual process or a device such as the Freiker or the Zap tracking devices for walkers/bicyclists. Users with would be able to withdraw these carbon offset certificates as printed stickers with a barcode.
• Non-alt transportation users buy carbon offsets certificates issued in proportion to the carbon emissions saved by the tracked usage of alternative transportation. This certificates would be valid for a year and issued as bumper stickers that buyers can proudly stick on their car.
• Businesses would accept payment of their products/services in part with the carbon offset certificate stickers.
• In addition, thanks to the recent Commuter Bicycle Benefit bill, bicycling commuters riding at least 3 times a week would be able to get $20 pre-tax income every month from their employers in the form of additional carbon offset dollars (that could only be spent at bicyclist shops).
• The overall system would be operating by a non-profit charity with a specific additional mission of lobbying for additional tax breaks, in particular for ride sharers not using vanpools.

I need to do the maths, but in the meantime, would love to hear what you think.