Cities are marketing the programs as experiments in using demand-based pricing to reduce traffic congestion — the kind caused by circling drivers desperately seeking parking spots — and to make more spaces available at any specific time. Drivers are encouraged to use mobile apps to check parking availability and pricing, though coverage is not universal. Parker, for example, from Streetline, gives detailed information about on-street parking for Los Angeles, but not for San Francisco.

Image In smart-parking systems, street sensors tell parking enforcement workers if a space is occupied. Credit... Streetline

SFpark is using “smart pricing” to achieve a target of having one parking space available most of the time in the areas it covers, says Jay Primus, the SFpark program manager. SFpark, he says, “de-emphasizes inconvenient time limits and instead uses smart pricing” to achieve those targets. The same spot, for example, may have different parking rates for different times of day. That intraday pricing is adjusted at multimonth intervals, but theoretically, it could be altered on the fly, depending on availability at any given hour.

One way to increase availability would be to use sensor technology to deter drivers from the tactic of staying parked in one place and repeatedly feeding the meter. Or a system might reset the meter automatically when a vehicle leaves a spot, so any remaining time is zeroed out. But meter resetting is “a fairly politically charged issue,” says Zia Yusuf, the C.E.O. of Streetline, and his company doesn’t provide it. (Many cities have found that midblock payment kiosks that replace individual meters solve this problem handily.)

In San Francisco, the sensor technology installed by Streetsmart Technology has been bedeviled by electromagnetic interference from overhead trolley lines. Mr. Primus says the vehicle-detection sensing is only about 90 percent accurate.

Daniel E. Mitchell, a senior transportation engineer who manages LA Express Park for the City of Los Angeles, says the accuracy of his program’s sensors has been in the mid-90 percent range, but he sees that as inadequate for automatically issuing tickets. Even if the sensors were 97 percent accurate, Mr. Mitchell says, “you’d have 3 percent of your customers experiencing a problem and that would be too many.”