Ride-sharing services provided by companies such as Lyft, Sidecar and uberX have become popular, if somewhat controversial, lower-cost alternatives to traditional taxicabs in many cities and at many airports.
The services match people who need rides with mobile app-dispatched citizen drivers willing to provide rides and accept a fee.
But, citing an aggressive stance by authorities at Los Angeles International Airport for issuing citations to drivers picking up passengers there, Uber and Sidecar have recently pulled the plug on that part of their LAX service.
"Although we look forward to working with the authorities to resolve these issues quickly, this unwarranted action by authorities to punish drivers and riders cannot continue," Uber spokesman Andrew Noyes wrote in a company blog post a week ago. "That's why we're temporarily halting uberX pick-ups at LAX effective immediately."
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Noyes told CNBC there were no projections on when the uberX pickup service might resume, but that for now uberX drivers are still dropping off passengers at LAX. The company's other services, UberBLACK and UberSUV, which work with licensed commercial drivers, continue both pickups and dropoffs at LAX, he said.
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Sidecar spokeswoman Margaret Ryan said via email that because the company has heard of the increased enforcement action at LAX, "we've advised Los Angeles drivers to avoid picking up passengers at LAX as well."
In an email, Los Angeles Airport Police spokeswoman Sgt. Belinda Nettles said "no special enforcement is taking place" against uberX, Sidecar or other ride-share drivers. Only that "airport police officers are enforcing airport rules and regulations, as well as any violations pertaining to the penal code, vehicle code and the Los Angeles municipal codes as appropriate."
At issue are the first round of rules issued by the California Public Utilities Commission for regulating companies such as Uber, Sidecar and Lyft, which the commission calls transportation network companies. "The question of picking up passengers by TNCs is still under review" by the commission, and TNCs wishing to serve the airport also need licenses or permits, and insurance, to do business at LAX, Nettles said.
Nettles said Thursday she was unable to provide information on what types of citations were issued to uberX drivers. "We cite for airport rules and regulation violations and California vehicle and penal code violations as appropriate daily," she said.
LAX is not the only airport that has taken action against ride-sharing companies.
In April, San Francisco International Airport issued a cease and desist order to ride-sharing services operating there. "These were enforced primarily through admonishments, and some citations were also issued," said SFO spokesman Doug Yakel.
Like many other airports, San Francisco has rules stating that each business that provides ground transportation, rental car or airport parking services must get an airport permit .
In response, Uber published a blog post in August with tips for riders at SFO noting that pickups by Uber services were unaffected, but that "SFO has taken an aggressive stance against uberX and has begun citing some drivers." The company suggested fliers instead use another Uber ride service, such as UberBLACK or UberSUV.
Ryan said Sidecar is working with the state utilities commission to work out a solution but that in response to the cease and desist order, "we've advised San Francisco drivers to avoid trips to SFO until we've figured it out."
Lyft has not yet responded to a request from CNBC for the status of its services at LAX or SFO.
For its part, SFO airport, which recently came to an agreement with car-sharing service Relay Rides, remains "open to new business models that provide our customers with a variety of transportation options," said Yakel.
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He said while the decision by the California Public Utilities Commission to regulate transportation network companies provides a framework to move forward with a permitting process at SFO, "we have yet to receive word of any TNC attempting to operate at SFO being permitted through the CPUC."
—By Harriet Baskas, special to CNBC.com. Baskas is the author of seven books, including "Hidden Treasures: What Museums Can't or Won't Show You," and the Stuck at the Airport blog. Follow her on Twitter at @hbaskas. Follow Road Warrior at @CNBCtravel.