Boston residents who used Uber to hail a ride recently may be surprised to see a murdered out Tesla Model S roll up to the curb. And they would probably be doubly surprised to learn their driver is the millionaire co-founder of the popular travel website Kayak, who is driving for Uber undercover while he researches his next startup.

Paul English, who founded Kayak in 2004 and sold it nine years later, started driving for Uber on Halloween night. (He was still in costume as a vampire after attending an "epic party" thrown by his current company Blade, a tech incubator, not the Uber for helicopters.) Since then, he's completed 26 trips, and he plans to keep driving until he completes a total of 100. But why would the Boston native, whose 3 million shares of Kayak netted him $99 million when the site went public in 2012, be sweating it out in the trenches of the gig economy, when he could be wining and dining angel investors?

"I don't know how much you know about the travel industry, but half of travel is still booked offline," English told The Verge. "So either you call 1-800-Marriott or American Airlines or a travel agent. And we are thinking a lot about the role of humans in travel booking. I've been very inspired by Uber in terms of how they handle making sure you have a driver who's polite and car that's clean. It's a very simple thing that they do, and they didn't invent it."

English is of course referring to Uber's famous — or infamous, depending on how you look at it — rating system, in which drivers and passengers rate each other on a scale of one to five stars. He won't say how exactly, but being able to see this system at work, up close and personal, is helping to inform his new startup, code named Blade Travel, which he says is currently in "stealth mode."

"It's a very interesting dynamic," he said, adding that he's taken over 500 rides as a passenger, but wanted to experience the rating system as a driver too. "And that's going to be helpful to me as we continue to develop our own travel company."

English says he keeps his identity as a tech millionaire under wraps while driving. He tells people he's merely a "programmer." Besides, his passengers care less about the driver and more about the vehicle in which they're riding: an all-black Tesla Model S P85D with a dual engine and all-wheel drive (base price: $105,000). One woman was unfamiliar with Tesla and assumed the dashboard display was a giant iPad. Another passenger insisted on riding in the passenger seat so he could see the vehicle's new Autosteer feature in effect. (English obliged because, after all, this is about ratings.)

One surprise for English was learning more about people outside his immediate, tech-centric world. "Driving for Uber, I'm encountering parts of the population that I wouldn't normally encounter," he said. "And if I have a captive audience in my Tesla for 10 or 15 minutes, you get to have in-depth conversations. So that's really interesting to me."

Uber and its rating system have drummed up a substantial bit of controversy over the years, the former for its tendency to flout regulations, and the latter for, as The Verge's Josh Dzieza recently wrote, for turning "customers into unwitting and sometimes unwittingly ruthless middle managers."

English, dressed as a vampire, driving for Uber on Halloween night.

English says he admires Uber's disregard for regulations, because "to make change, you have to break the rules." And he thinks the rating system helps "curb bad behavior." Uber drivers who fall below a certain threshold can get booted from the platform, and English says that's a good thing. Those with good ratings thrive.

"They know if their car is clean and they're polite and they drive well, they get a higher rating, which means they get more passengers," he said. "And in the case that you have a bitchy passenger who just trashes every driver, I think the Uber algorithm takes into effect that someone who only gives out ones or twos, their rating is less important than someone who always gives a five and occasionally a one or two."

He admits there are likely "aberrations" in Uber's rating system, but argues those flatten out if drivers log enough trips.

After a stint in hospital IT, English founded Kayak with his partner Steve Hafner, who had previously helped found the travel site Orbitz. Time magazine listed it among its 50 best websites in 2009, and Travel + Leisure named Kayak's app in its list of the Best Apps for Business Travelers in 2013, as well as its list for the Best Apps and Websites for Travelers. The company went public in 2012, and a few months later sold to Priceline for $1.9 billion.

Since then, English has been working on other projects, including Blade, LLC, his tech incubator in Boston. After raising $20 million in equity financing from Kayak's investors, Blade booted its incubator tenants, took over the space, and grew the staff from nine to 27 in a few months. Next month, English plans to announce the details of his new "stealth" travel company.

Until then, he'll continue to drive for Uber, picking up a diverse array of passengers like the eighth grader who wants to go to MIT, or the chess champion from Jackson, Mississippi. And he'll be keeping a close eye on his own rating, which right now is exactly where he wants it: five stars.