Three of the four seats are empty

The supply is a journey somebody is making anyway.

• Three of the four seats in that car are emptyAverage occupancy on a long-distance car journey is close to one and a half people, which is the entire supply side of the category and the reason it looked easy.
• Cost-sharing, and why the wording matteredPricing a seat at a share of fuel and tolls rather than as a fare is what kept the model out of taxi regulation in most of Europe.
• The density problemA marketplace needs enough drivers on one corridor on one afternoon; below that threshold the product is simply empty.
• The detour nobody pricesEvery pickup is a diversion, and the minutes it costs the driver are the hidden term in the arithmetic.