L.A. County residents are preparing to vote on a measure approved recently by the county, state legislature, and governor to extend the most recent transportation-dedicated 1/2 cent sales tax measure for an additional thirty years beyond it's current expiration date, in 2039, all the way out to 2069. Rather than adding any new taxes, the intent is to expedite the construction of currently planned transportation infrastructure by bonding against revenue to be collected in the future - in this case, the pretty distant future. Read more about what L.A. County has accomplished so far in Yonah Freemark's article, linked above.
If this measure fails some of the major rail projects in the region will require decades to complete rather than years, but Freemark brings up some valid objections to this plan (although he seems generally supportive of it on the merits). I'll address these in the order in which they appear, starting with the question of what happens if in 2040 L.A. County residents decide they're actually not happy with the infrastructure that's been built.
Firstly, based on the trajectory of urban areas over the past few decades it seems very likely that people will appreciate these transit investments and they'll get plenty of use. As cities have grown up, particularly in their central/downtown regions, transit has become a more and more integral element of transportation and there's no sign of this changing. Even with the advent of driverless cars there's no changing the fact that cars take up a lot of space, and to accommodate the number of people who want to live and work in cities we need a way to move more people using roughly the same amount of space. That's transit, and barring some incredible and unforeseen innovation in moving people from place to place, it's going to remain transit. The only really plausible reason that a region that once wanted better transit might change its mind seems to be that it suddenly started becoming less dense and less populated, which I doubt any part of L.A. County is planning for. Building out this infrastructure (and building it quickly) actually speeds up this densification and development process, so the chances of this happening anywhere in L.A. become even less likely as the projects move forward. And let's not forget that L.A. residents have already voted to increase their sales tax by 1/2 cent three times in the past 32 years, most recently just four years ago, so they seem to approve of what's been built so far.
He also notes that 'The referendum would extend the tax “for another 30 years or until voters decide to end it.”' So, what if voters decide to end this tax? Well, I don't think it really matters. The same thing happens as if any other source of revenue suddenly disappears: you find another way to pay for it or you make cuts elsewhere. If it's a matter of finding another way to pay for it this might actually be a good thing, given that sales taxes are one of the most regressive ways for a government to raise revenue. If it's a matter of making cuts then you find a way to make them, but I think this is all semantics anyway. The odds of this being overturned in the future seem exceedingly low. For one, although sales tax measures for transportation investment don't always pass, I'm not sure there's any precedent for one actually being repealed after being approved by voters (especially by a 2/3 vote). If you know of one I'd like to hear about it, but I suspect it's extremely uncommon if it's happened at all.
This also fails to recognize the increased revenues that always accompany major transportation projects - between the increased values of properties near rail lines and the billions of dollars of new development that's likely to accompany it, this isn't just an extra cost for the county and it's residents - it's also a way to bring in more tax-paying residents, more businesses, and more investment. And by expediting these projects all of the attendant benefits accrue to to the county earlier (to say nothing of decreasing the amount of time that construction disrupts business and mobility in the area). You can finish the subway in 2020, or you can finish it in 2036 and miss out on those 16 years of increased revenue.
The measure also "does not specifically guarantee that the projects promised back in 2008 will actually be delivered," but seeing this as a negative is an implicit expectation for incompetence on the part of the county's leaders. As he notes, so far L.A. has done a good job of staying roughly on time and on budget, and just like planning for a city to decline in population even though you don't want it to, this would be a very strange way to handle future planning. If said leaders are being poor financial stewards of the county's major projects, we have elections to replace them with people who will (hopefully) get things right. Failsafes and backup plans are completely justified, but everything has risks and a no vote based on that fact is a recipe for stagnation and decline. This may also be viewed as a sign of adaptability and a retort to those who worry what we'll think of these projects in thirty years - it provides room to modify and optimize plans as circumstances dictate.
As Freemark notes, if L.A. wants to get these projects done anytime soon they don't have many other options, particularly with declining assistance from the federal government. And given the current state of the labor market and the reduced cost of contracting in this depressed economy there's no better time than now.