CHICAGO — Travellers spent some US$2.4 billion on Airbnb lodging in the U.S. alone in 2015, according to a new study by CBRE Hotels.

Five cities – New York, Los Angeles, San Francisco, Miami and Boston – dominated the Airbnb stats, accounting for more than half (55%) of the $2.4 billion spent. The amount is “a significant portion of the lodging revenues in these markets,” noted the study.

The study also compiled an ‘Airbnb Risk Index’, highlighting which traditional lodging markets were most at risk as a result of Airbnb’s popularity, based on a comparison of Airbnb’s average daily room (ADR) rates compared to traditional ADRs, the scale of Airbnb inventory to the supply of traditional hotels and the overall growth of Airbnb supply. New York was deemed most at risk using these metrics, followed by San Francisco, Miami, Oakland and Oahu.

While it’s not the only option in the short term rentals category – competitors include HomeAway, VRBO and FlipKey – Airbnb is far and away  the most dominant. Founded in 2008, it now counts 60 million users providing access to locations in 34,000 cities in more than 190 countries, and averages half a million stays per night.