Is it time to rethink the car, an iconic symbol of 20th-century economic prosperity? The notion of disrupting Detroit certainly isn't new — there's been a rising tide of interest in transportation from Silicon Valley pillars and newcomers alike for several years now. But 2015 felt like a transformational year: there are the inescapable rumors of an Apple car, the expansion of Google's testing, and newcomers like Renovo and Faraday Future, all of which made news this year and call California home. And then, of course, there's Uber. The San Francisco unicorn had another banner year marked by nearly unthinkable leaps in valuation, gangbusters expansion, and key policy wins. As part of our end-of-year series, Chris Ziegler and Andrew Hawkins discuss Silicon Valley's challenge to the transportation industry.

Here's how the Valley took on Detroit in 2015.

Chris Ziegler: Detroit’s reputation as a global force in the automotive industry has been challenged harder recently than at any point since the 1973 oil crisis — an event that gave rise to affordable, fuel-efficient imports at a time when Americans were gorging themselves on muscle cars. This time around, the "imports" are a little closer to home: the heat is coming from Silicon Valley, where Google and Tesla have dominated headlines this year.

Google remains the poster child for autonomous driving

It’s not just that Google and Tesla have decided to make cars, of course; it’s that they’re legitimately shaking things up. Google remains the poster child for autonomous driving, rolling out its adorable little two-place vehicles near the company’s Mountain View headquarters and expanding to Austin, Texas; Tesla, meanwhile, has done more to push the electric vehicle agenda than any other firm. (It doesn’t hurt that Elon Musk, a larger-than-life personality and perhaps the single most recognizable Silicon Valley executive working today, is Tesla’s co-founder and CEO.) The company’s Model X crossover finally started shipping this year, and it has repeatedly affirmed that the Model 3 — a far more affordable model — will be unveiled in early 2016. Thanks at least in part to Musk’s efforts, countless other automakers have recently committed to making reasonably priced long-range EVs, including industry giants like General Motors and Volkswagen.

And then there’s Apple, which has had a banner year — if not in duplicating the iPhone’s global success, then certainly in expanding to new market segments. But Apple’s biggest single product of the year doesn’t even exist yet: the car, which The Wall Street Journal first reported in February. The lukewarm reception of the Apple Watch proves that the world’s richest company isn’t bulletproof, but regardless, an Apple Car could have a disruptive impact on transportation — an iPhone for the road, if you will.

Andrew Hawkins: Not everyone thinks that Detroit will be cast aside by the disruption, though. In an essay posted on LinkedIn, General Motors CEO Mary Barra predicted that 2016 would be the year the automotive industry would overtake Silicon Valley in the race to innovate transportation. "Even disruptors are being disrupted, as customers demand more efficient transportation solutions," she said. "So, I have committed that we will lead the transformation of our industry." (Emphasis hers.)

It’s classic frustration-disguised-as-optimism

It’s classic frustration-disguised-as-optimism. After all, the major transportation innovations — autonomous driving, ride-hailing, supersonic train travel — were all being spearheaded by technology companies, not traditional carmakers. And that trend has shown no sign of slowing down in 2016.

Chris: And of all of those, ride-hailing seems like the most imminent threat. Car companies like GM and Ford are trying to get into it, and both Uber and Lyft have taken on tremendous amounts of funding.

Andrew: Totally. Uber won several major political victories in 2015, most notably in New York City and Las Vegas, and continued to grow its driver- and customer-base. Travis Kalanick’s ride-hailing company introduced several new products — UberEats, UberPool — and set its sights on new markets in China and India, while continuing to raise borderline-offensive amounts of cash. Meanwhile, Lyft and three of Asia’s biggest ride-hailing companies — Didi, Grab Taxi, and Ola — joined forces in the hopes their combined assets could help slow Uber’s expansion.

And then there's the Hyperloop

But Elon Musk isn’t just attacking transportation with EVs: this was the year the startups invested in Musk’s fantastical Hyperloop vision, doubling down with major hires, land purchases, and promises of imminent, proof-of-concept tests. How effective (and realistic) these supersonic tests are will be among the top stories to watch in 2016.