The following quote is from an article in theSan Francisco Chronicle:

"Driving a taxi continues to be a rough ride in the age of Uber and Lyft. Many San Francisco cab drivers say they are mired in debt paying off their $250,000 medallionspermits allowing them to run their own cabs. Almost half of the city's 1,476 medallion holders want to sell, including about 300 who paid for their medallions and 391 who earned free medallions after years on a waiting list and now are older than 60 or disabled...

Taxi drivers nationwide are facing similar crises as customers defect to cheaper, more convenient ride services. Now that Uber and Lyft are public, they may raise rates to try to make money, but they may still be cheaper than cabs. The ubiquity and convenience of app-hailed ride services make them a formidable competitor."

what kind of market structure is this and why?