The Van Ness expansion part is only supposed to take a year, the rest of the previous 4~5 years was for a sewer replacement. It was done this way because of San Francisco's new policy requiring transit improvements to be coupled with large infrastructure maintenance (to minimize digging up the street multiple times) though in hindsight might not have been the best idea as it lengthens the amount of time the street is under construction.

But try paying for the cheaper road with the tax revenue of the 5 people helped by the broader road.

Not much difference? Widening the SF street is running into billions.

Which can be paid for on a $12.6 billion yearly budget (if it is wise to do so is another question).

Well, the budget in 2009 was $6.5 billion. It seems the taxes are growing for SF at a very high rate, and it would eventually become unsustainable. The taxes are certainly going up faster than population and inflation combined, and it sure seems like we're getting less for more money.

I think the reasons are very different though. For small towns it seems to be poor planning, for larger cities it seems to be corruption.

Not sure a $1B street update (literally 2 miles of street in a city with thousands of miles) could be regarded as "affordable" with a $12.6B city budget.

The overruns are because they decided to replace a hundred year old sewer system as part of the project. Along with a corruption scandal in the DPW.

Once they've gotten to the actual street widening part of the project things seem to move (relatively... to the 10+ year timeframe) quickly, a month or three to build the new bus lanes and repave the remaining lanes.