It’s a huge round for the company, which is much loved for its community spirit and pink “carstaches,” but has also been much questioned for its practice of having regular people essentially act as city taxicab drivers in their own cars.

The deal was a speedy one; it was negotiated over the course of the past week, said Lyft co-founder and president John Zimmer.

And it brings Lyft to $83 million in total funding, having just raised a round about six months ago.

Why so much money so fast? It’s to try to get Lyft to the next level, given the expansion, competition and regulatory challenges it faces. “We can scale globally in the next 12 months,” said Zimmer.

For those paying close attention, Lyft was formerly known as Zimride, and it continues to operate a long-distance ride-sharing service under that brand, but it changed its name a month ago to reflect the momentum of the Lyft business, which is a year old.

Andreessen Horowitz partner Scott Weiss, who led the investment, contended that Lyft differs from competitors like SideCar and Uber, which are both based in San Francisco as well and have been expanding nationally. He likes that Lyft’s mission is around “taking cars off the road,” not replacing or augmenting existing systems.

Weiss said Andreessen Horowitz was motivated to swoop in and make a deal because Lyft “looked and smelled so much like Airbnb.”

Weiss noted that he’d been following Lyft closely, and saw the service grow from 14,000 rides per week two months ago (mostly in San Francisco, but expanding in other newer cities like Los Angeles and Chicago) to 30,000 rides per week now. Plus, he said, more than half of the company’s passengers are women, which says something about safety.

“With the risk of the Samwer brothers [who infamously replicate consumer Internet businesses for the European market] and the copycats, you can’t let grass grow under your feet,” Weiss said. “You have to grow it as quickly as you can, and that’s where we come in.”

It was way less weird than I expected.

The guy who gave me the ride was an incredibly friendly fellow named John Robertson, who was driving for a start-up called SideCar, which has built an alternative to taxis, with cars driven by regular people and hailed using smartphone apps. SideCar launches to the public today in the San Francisco Bay Area.

At a SideCar employee’s suggestion — I should admit that my Friday afternoon meeting had actually been at SideCar’s office — I sat in the front seat of Robertson’s car and chatted him up.

Robertson told me he recently retired after 30 years as a project manager at UPS, and quickly got bored. He now generally drives five or six hours per morning with SideCar, and golfs in the afternoon.

Robertson said he lives in the East Bay, and has used the driving as a chance to get to know San Francisco better. He only makes $80 or so per session — which isn’t great, minus the cost of gas — but he doesn’t do it for the money, but rather because he believes in what SideCar is doing.

And Robertson meets all sorts of interesting people — during the beta period, lots of Twitter and Google employees, where word about SideCar apparently spread quickly through their SF offices.

SideCar is run by Sunil Paul, the serial entrepreneur and investor behind companies like Brightmail, who in recent years has been exploring cleantech and now the “cleanweb.” Paul first worked on getting California legislation changed to allow car sharing, and tested a start-up in that space called Spride, but then decided the more transformative and potentially lucrative opportunity was around sharing rides within an online community rather than borrowing people’s cars.

If you ask Paul, the idea for a ride-sharing start-up first occurred to him in 1999 — and he actually received a patent on some aspects of it in 2002.

After sitting on the idea for some 10 years, Paul recently invested and became CEO of a University of Michigan transportation research spinoff called Shepherd Intelligent Systems — now SideCar.

SideCar has been in testing since February, with more than 10,000 rides to date. In San Francisco, SideCar isn’t nearly as fast as Uber, but rides can be booked in “minutes to 10s of minutes,” as Paul described it.

As seems to happen with many timely ideas, there is a direct SideCar competitor that also just launched — it’s called Lyft, and is made by the start-up Zimride. I tried Lyft yesterday, and from a user’s perspective, it seems almost identical.

There’s one key difference between SideCar and the competition, at least according to Paul. After all the work he did with car sharing, he knows the regulations around collaborative consumption and transportation backward and forward, and SideCar’s payment system is apparently more strictly built to abide by existing laws.

The way it works is that SideCar automatically and dynamically computes a suggested payment that averages what other people have paid for the same kind of ride. This sidesteps the company and the driver setting a price for the ride, in order to avoid commercial vehicle restrictions. SideCar takes a 20 percent cut.

Users don’t have to pay for the ride, but if they don’t, drivers are unlikely to give them a good rating, and they’re less likely to be picked up the next time they request a ride. (Lyft has the same policy.)

Of hinging his company’s business model on an optional donation, Paul said, “The capitalist in me is scared silly of that idea, but the human being says you can trust people.”

Another nice feature of SideCar is an easy way for riders to share their ETA with a friend and send them a live online map of their progress in the car. This looks a lot like Glympse, and helps people reassure themselves that someone knows where they are while they’re in a stranger’s car.

Paul is being somewhat secretive, even as SideCar opens to the public, and won’t disclose how much funding he’s raised or how many employees he has. He did give me a long list of investors, which includes his own Spring Ventures, Huron River Ventures, SV Angel, Lerer Ventures, First Step Fund, Jeff Clarke, Lisa Gansky, Robert Goldberg, Jared Kopf, Konstantin Othmer, Mark Pincus, Martin Roscheisen, Josh Silverman and Thomas Varghese.

With an interface like Uber, Lyft users will be able to find nearby drivers and hop in for a ride, paying a suggested donation that’s approximately 30 percent less than a cab fare.

Drivers will be vetted by Lyft, and both riders and drivers authenticate via Facebook. Lyft is only in San Francisco and on iPhone for now, where there’s also a competing service called SideCar in beta testing.

Asked about the licensing and insurance issues that come with effectively turning regular cars into taxis, Zimride COO John Zimmer said, “I’m sure people will get upset about more competition. But our understanding is that when it’s ride-sharing, you can use your personal insurance policy. As for regulation, a lot of state laws are supportive of carpooling and ride-sharing and want to make that work.”