Self-driving taxis are spreading across the country. Waymo, the robotaxi company owned by Google's parent Alphabet, now operates in 15 U.S. cities with roughly 4,000 vehicles, TechCrunch reported. About 80% of the fleet is in California and Texas.

Nashville is one of the newest markets. Waymo launched service there in April through a partnership with Lyft, and rides became available in the Lyft app in September, according to Lyft. Now a new 80,000-square-foot depot will bring hundreds more self-driving cars to the city, WSMV reported. Lyft's Flexdrive unit handles fleet operations, such as cleaning, charging and maintenance.

The goal is shorter waits. Robotaxi services need enough cars spread across a city so riders do not wait long. Depots let the companies charge and service cars close to where they are needed.

Waymo's cars have no human driver. Riders summon them with an app, and the vehicles use cameras, radar and lidar sensors to navigate. The company says its cars are involved in fewer injury crashes than human drivers over the same distance, though critics say more independent data is needed.

Here is the background. Waymo began as Google's self-driving car project in 2009. It launched the first public robotaxi service in the Phoenix area in 2020 and expanded to San Francisco and Los Angeles. Rivals include Tesla, which has started a limited robotaxi service, and Amazon's Zoox.

Growth brings challenges. Robotaxis have blocked emergency vehicles and confused other drivers at times, and some cities have pushed back. Labor groups also worry about the effect on ride-hailing drivers.

For riders in Nashville, the Lyft app will show when a Waymo is available for a trip.

What we know: Waymo runs about 4,000 robotaxis in 15 cities, and Nashville is getting a new depot.
What we don't: How fast Waymo can grow outside California and Texas.