Depending on how you look at it, the "Solar Roadways" proposal is either a brilliant solution to America's energy woes — or totally insane.

The idea is simple enough: The United States would replace the asphalt and concrete in its roads, sidewalks, and parking lots with a type of industrial-strength glass that contain solar panels. Like so:

The result? The United States would have plenty of space for solar panels generating clean, carbon-free electricity. Those glass roads might also be able to do neat things like provide their own lighting or LED signs for drivers.

This isn't entirely fantastical: For the past decade, Julie and Scott Brusaw's Idaho-based startup Solar Roadways has been developing a type of glass that can withstand the stress from cars and heavy trucks driving over them.

The company has already received a grant from the US government

The company has already received a grant from the U.S. Department of Transportation to build a crude prototype of this glass. It was selected as one of Google's "Moonshots" in May 2013.

Currently, as Rob Wile reports at Business Insider, the company is trying to raise $1 million in funding to move the technology from the prototype phase to production. (They've raised $145,000 so far.)

It's a fun idea, and worth a shot. But there are still reasons to be skeptical. Yes, solar power — particularly rooftop solar power — has been making some impressive gains in the United States in recent years. But solar roads are likely still a longer ways off.

Why solar roads are so enticing

At a very abstract level, it's easy to see why solar roads might be attractive. On their website, the Brusaws offer a detailed FAQ and some rough numbers on this score.

In theory, solar roads could generate three times as much electricity as the US uses

They note that there are about 30,000 square miles of roads, parking lots, driveways, playgrounds, bike paths, and sidewalks in the contiguous United States.

Assuming we could replace that pavement with glass-covered solar panels that have an 18.5 percent efficiency rate, we could generate up to 14 trillion kilowatt-hours of electricity per year — more than three times what the United States uses.

Now, in practice, we almost certainly wouldn't be able to harvest that much power. For the time being, there still aren't many practical ways to store solar-generated electricity for hours when the sun isn't shining. What's more, electricity generated on remote roads would have to be transported to where it was needed. That would all require a lot of infrastructure.

Still, the concept might be useful on a local scale — say, starting with parking lots and driveways inside cities and towns. Or perhaps one day it could be combined with wireless technology to charge electric vehicles as they move along the roads.

But that still leaves harder questions about cost…

The case for skepticism

Right now, the Brusaws say they're working on updated estimates of how much these solar roads would actually cost to install and maintain. Yet earlier back-of-the-envelope calculations offered plenty of reasons for skepticism.

Covering the entire country with these panels would cost trillions

Back in 2010, the company assumed that a 12' by 12' glass panel would cost around $10,000. At this rate, covering all of our roads would cost $56 trillion — nearly 20 times the annual federal budget. Even on a smaller scale, these panels are at least 50 percent more expensive than regular roads, and possibly more.

Back then, the Brusaws argued that an investment in solar roads could pay for itself within 22 years. After all, the solar panels would cut down on electricity bills. And, of course, most of the pavement in the nation's roads needs to be replaced over time anyway — so we wouldn't need to pay for asphalt. (What's more, if solar panels continue to get cheaper and more efficient, the price could come down further.)

But as Aaron Saenz pointed out at the time, many of those estimates seemed awfully questionable. For one, asphalt wasn't nearly as expensive as the company assumed. And the maintenance costs for these solar roads is still wildly uncertain.

Solar Roadways is currently trying to rework its calculations to take into account a new hexagon design for their glass. It's also possible the roadways could find other ways to pay for themselves, like offering advertising. But, right now, the cost estimates are pretty fuzzy.

Meanwhile, there are other questions that haven't been answered satisfactorily yet. How will the roads stay clean, for instance? (The company's answer here — tinkering with self-cleaning glass or maybe employing street sweepers — is a bit vague.) How much will it cost to heat the panels so that they stay free of snow, as seems to be the plan?

Right now, the Department of Transportation is asking for smaller demonstrations in, say, store parking lots. If that actually works, perhaps the idea could scale up slowly over time. But we're still a long way off from covering all our roads in solar panels.

Further reading: While we're dreaming about solar roads, it's worth checking out the gains that actual rooftop solar has been making over the past year. It's still a small part of the US energy supply, but prices have been tumbling fast — and there's now enough installed capacity to power more than a million homes.