Tech industry CEOs regularly lament what they say is the scarcity of qualified coders. But another worker shortage may be on the horizon in Silicon Valley: not enough bus drivers to take those coders to work.

In San Francisco, the so-called Google Bus – the cushy coaches that ferry employees from the city to the company's Mountain View headquarters – has become a divisive symbol of gentrification and economic stratification. One group of protesters went so far as to smash a Google Bus piñata to vent their outrage over skyrocketing housing prices.

But among employees themselves, the Google Bus is apparently a victim of its own success. Appearing at a San Francisco conference on ride-sharing, Google transportation specialist Anna Walter said the popularity of the buses is making it a challenge to scale the program.

"We own most of the buses themselves, but we contract out with the drivers. And even right now we're feeling the strain of that," Walters said. "It's just a resource scarcity issue."

Google has 100 buses in its fleet ridden by about 5,000 riders per day from seven Bay Area counties, Walters said. That amounts to more than one-quarter of the total number of Google employees on its main campus.

The buses have become a point of contention in San Francisco because of the ease with which they allow well-paid Googlers to travel back and forth to work. Instead of concentrating Google workers' wealth in one area near the office, the buses make that money mobile, which drives up rents, especially near bus stops.

From Google's perspective, offering their workers flexibility to live where they want is exactly the point. "We want to make sure we can get our employees to work safely, quickly, and stress-free," Walters said.

The other big goal is to reduce Google's commuter footprint overall by minimizing the number of employees traveling to work by themselves in their own cars. Walters said about 40 percent of the workers in Mountain View drive to work alone. Along with the buses, employees also participate in van pools and car pools, she said. And as the Google Bus system itself starts to strain against demand, the company is looking at other options.

"We would definitely look at ride-sharing solutions," Walters said. She wouldn't go into detail about what those solutions would look like, but with Lyft co-founder John Zimmer on stage as a co-panelist, it's easy to picture Google availing itself of a similar app-based ride-on-demand model.

Already, Google has a fleet of 60 Chevy Volts at its main campus to serve as a private Zipcar-esque service for employees (6,000 Googlers are signed up for the free program, Walters said). Google also has shuttles that loop around campus, as well as an on-demand taxi service and 1,000 of its famous clown-colored bikes for traversing the Googleplex.

In any discussion of Google as a transportation provider, it's obligatory to remember its aggressive effort to invent a self-driving car. Predicting the potential uses of autonomous vehicles is something of a Silicon Valley parlor game – will they do deliveries for Google's shopping service? Will they solve traffic once and for all?

One possibility I haven't heard, however, is drone-buses to ferry tech workers to their jobs. But with bus rides from San Francisco to Silicon Valley becoming an expected perk, the competition for qualified drivers could grow even more acute. In that case, Google might be that much more motivated to invent a bus that needs no driver at all.