Robotaxi companies Waymo and Zoox said Tuesday they were expanding into new cities.

Amazon's Zoox said its self-driving vehicles will now be on the streets of Houston and San Diego, bringing its total markets to 12.

Zoox said it chose the two cities as its next launch regions because they both have heavy reliance on driving and high demand for ride sharing.

Waymo said it is launching driverless rides in Denver, San Diego and Tampa, Fla., marking 14 cities where it now offers autonomous trips.

Tens of thousands of riders in each of Waymo's operating cities have signed up and expressed interest in using the platform, said the company, which is primarily owned by Google's parent Alphabet.

Amazon is a more-recent entrant in the robotaxi industry. It acquired Zoox in 2020, and gained regulatory approval in July to offer paid robotaxi services.

Meanwhile, Waymo has been an early leader, with roughly 4,000 vehicles operating in various cities. Tesla also is producing the Cybercab, a vehicle that, similarly to Zoox, lacks a steering wheel or pedals, but has not gotten the same regulatory approval yet.