The roar of a jet engine, the vroom of a car, the vibration of a moving ship.
These sounds and sensations, commonly associated with travel and motion, share a common source: fossil-fuel-powered engines.
But as regulators and businesses around the world try to reduce carbon emissions, airplanes, automobiles and ships are going electric. It's good for the environment, but it also means travel itself may be changing. Here's a look at a few of the initiatives underway.
This year, Mokulele Airlines, of Hawaii, and Ampaire, a California-based electric airplane startup, will begin testing a hybrid aircraft over Maui on a commuter route between the Kahului and Hana airports.
The flying public won't be able to buy tickets for those early test flights, but the companies behind the tests say it's an important step in proving that electric service is viable.
"You uncover a lot of gremlins when you start flying in a true operational cadence," said Kevin Noertker, Ampaire's co-founder and chief executive. He added that the plan is to introduce commercial hybrid service by the end of 2021.
The planes have a range of around 200 miles, but because they are hybrids, test pilots don't need to worry if their batteries run out of juice midair.
"We've designed the system with resilience in mind," Noertker said.
Hybrid and fully electric airplanes can also have lower operating and maintenance costs than fossil-fuel-driven aircraft, because they use less fuel and have fewer moving parts. Those savings may ultimately be passed on to passengers. They also mean that more rural airports could one day see commercial service as the economics of flying to low-population destinations improve.
"Scheduled air service can come back to communities that have lost it in all the consolidation with the airlines," said Rob McKinney, the president of Pacific operations for Southern Airways Express, which operates Mokulele Airlines. "Rural communities will have air service again that haven't had it in 20 or 30 years."
Nevada has been undergoing a yearslong project to build out its electric vehicle charging infrastructure. It has deemed a stretch of U.S. 95, the primary route between Las Vegas and Reno, along with portions of other major roadways in the state the "Electric Highway," and it is encouraging visitors to see the state by taking an emission-free road trip. Nevada is also home to the Tesla Gigafactory, which in some ways has jump-started its electric vehicle efforts.
"We have so many cultural and historical sites that people are interested in, and we promote the state through the road trip," said Chris Moran, a public relations specialist at Travel Nevada. "It's great to do it in a sustainable and respectful way to the environment."
In some ways the new technology also allows people to travel in a more old-fashioned style. Because charging takes longer than filling up a gas tank, it means electric vehicle drivers are more likely to spend time and money in small communities along their route.
"If you think of what the interstate highway system is, how it so dramatically changed the nature of road travel and road trip tourism, this is a different way to travel and see places," said David Bobzien from the Nevada Governor's Office of Energy. "We have a lot of car enthusiasts who love their electric vehicles and they love to drive them, so it's the road trip and the joy of the road trip that lures them to use the highway."
Similar infrastructure can be found in the Northeast of the United States, with Tesla charging stations along the Interstate 91 and 95 corridors.
Hamlet Aguilar relies on those stations as the anchor for his new business, Bound. The car service shuttles passengers between New York and Boston in Teslas for prices that begin at $99 each way.
"The electric cars allow us to provide a price point that the other transportation companies cannot do," Aguilar said. "The high cost of maintaining a fleet and the gas, it was never cost-effective. Being able to use these Teslas enables us to offer an attractive price point."
Water and electricity are finally beginning to mix.
Hurtigruten, a Norway-based expedition cruise company, is investing heavily in sustainably powered ships. The company focuses on exploration cruises, with its ships traveling places far from the usual Caribbean and Mediterranean destinations that most cruise passengers visit.
"This is something that has been a natural part of the development of the company and driven by a lot of very engaged people," said Daniel Skjeldam, Hurtigruten's chief executive. "One of the reasons they have been so proactive on it, since we've been sailing in these areas, they've been seeing the changes," like glaciers retreating, firsthand.
The company's efforts include investing in an all-new hybrid-powered cruise ship, which will launch in September. That Alaskan cruise is already sold out.
In the next few years, the company will also convert its older diesel-burning ships to liquid natural gas and biogas-fueled hybrids, and will launch a battery-powered hybrid catamaran for expeditions near the Norwegian coast this year, with more coming in 2020.
The company is working to develop solar chargers for those vessels.
None of these first-generation ships will be fully electric, but Skjeldam predicts that in the not-too-distant future, some vessels will be able to sail entirely under battery power for hours at a time.
He also said he thinks his company isn't too far ahead of industry trends.
"I think we'll see, to be honest, a revolution on sustainable travel in the next decade. I think a lot more ports worldwide will be much more demanding to the industry on what kind of ships they will accept into ports. The local emissions from a cruise ship are huge if they're running on heavy fuel oil," he said, adding that Norway will require ships in its fjords to be emission-free by 2026.
He admits though, that battery-powered vessels are still technologically limited.
"We don't think batteries can be the only source for power, but it's the perfect way to operate ships into sensitive areas."