Recent interviews of epidemiologists and health experts via Vox have uncovered the safest and riskiest ways of traveling this holiday season, for those who plan to ignore the U.S. Centers for Disease Control and Prevention’s (CDC) advice against traveling for the remainder of 2020.

With COVID-19 raging across the country, there’s really no safe way to get from home to your destination without potentially contracting or inadvertently spreading the virus. But, thinking in terms of health and safety, it turns out that there’s one form of transportation that you’ll definitely want to avoid—lengthy bus rides.

Bus trips typically last longer than other forms of travel, increasing your window for exposure, and occur in a space where social distancing and enforcement of mask mandates can present more of a challenge. "It might be harder on a longer ride to keep your mask on, and it's probably not as enforced as it is on an airplane," Amesh Adalja, M.D., a physician and faculty member of the Johns Hopkins University Bloomberg School of Public Health's Department of Environmental Health and Engineering, told Vox.

While there isn’t much in the way of solid, scientific information available about the risk of COVID-19 transmission aboard long-haul buses, early research has identified this travel method to be riskier than planes or trains. One specific case study discovered that, in China, a single sick passenger managed to infect 23 of 67 other people during a bus ride that lasted less than an hour.

Granted, this example occurring back in January 2020, prior to the placement of COVID-19 precautions, the bus was poorly ventilated and recirculating air, and none of the passengers were wearing masks. Obviously, the world has changed its practices since then.

America’s biggest national bus company, Greyhound , has instituted a strict mask-wearing policy for both passengers and employees, established enhanced cleaning and disinfection protocols, and enhancing ventilation using air filtration systems that don’t recirculate cabin air, replacing it with fresh outside air every five minutes.

Long-haul buses and motorcoaches, along with city buses and subways are ranked as the riskiest form of transportation in terms of COVID-19 concerns. Somewhat less risky, epidemiologists say, is travel by train, for many of the same reasons—typically train rides take longer than, say, a domestic flight, meaning more exposure time to others. Ventilation systems aboard train cars don’t have the same air-replacement rate as an airplane cabin, although Amtrak says clean air is exchanged 12 to 15 times per hour.

Less risky than trains is air travel, for which scientists still have not produced definitive data on the likelihood of COVID-19 transmission onboard an airplane. Planes benefit from high air-circulation rates, fully replacing cabin air with fresh outside air 20 to 30 times per hour. “The data does show that airline travel is fairly safe right now if people are wearing face coverings,” Adalja says.

Although not 100 % risk-free, the safest way to travel at the moment is in your personal vehicle, riding only with members from your own household. One of the keys to lowering your risk is to stay as self-contained as possible, bringing along your own supplies so that you don’t need to make as many stops and minimize contact with anyone outside your group.