Despite their deeply held love of cars, millions of Americans ride trains and buses each year. But popular modes of public transport are often curiously lacking one of the most common safety mechanisms associated with travel: seat belts.

The absence of seat belts on trains and buses can seem confounding, but the reasoning spans financial concerns, accessibility and comfort, and even, perhaps paradoxically, safety considerations.

Alternative modes of transportation are more relevant than ever in the push to cut carbon emissions, and Americans appear to already be relying more on modes of mass transit such as trains and buses. In fact, 2013 saw a public transit boom in the U.S like nothing since the mid 1950s, according to the American Public Transportation Association.

Still, a number of factors keep the majority of the country from relying on public transit, and chances are perhaps weakened when flaws within the system are exposed. Safety, for example, is one factor that was recently highlighted after an Amtrak train traveling between New York and Philadelphia derailed this year, killing eight people.

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Among other safety issues, the question of seat belts was raised: would mandating them in trains prevent deaths in crashes? It's a question that is also regularly applied to buses, which, with trains, represent two of the most common modes of public transit on which Americans rely, but are also not always required to have seat belts. Their absence begs the question: why, if strapping in is so strictly enforced in cars, are seat belts not always provided — let alone mandated — on buses and trains?

On trains, seat belts could actually be more dangerous.

The Amtrak train that derailed earlier in 2015 caused a flurry of media coverage around rail safety precautions. On the issue of seat belts, articles pointed to research that put forth some surprising reasoning. The thinking is that not only would sufficient safety belts be costly, and the stiffer seats to support them pose greater safety concerns — lap belts have been shown to increase spinal injuries in crashes — but having any belts at all might have lessened the chance of survival for passengers by locking them place, exposing them to "structural intrusion." Here's a snippet from one widely cited report on the benefits of no safety belts on trains.

"Analysis suggests that restraining passengers in seats, whilst reducing the likelihood of ejection, may have other more serious consequences and create significant numbers of additional casualties (or fatalities) as a result of loss of survival space."

And as CityLab noted, seat belts in trains could even pose another, slightly more abstract danger: commuters, who would otherwise use rail, would be deterred by the lost comfort of seat belt-less trains and driven "from the relative safety of riding trains to the relative danger of driving cars." On roads and highways, where seat belts are generally the law, accidents are much more likely.

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Some buses require seat belts, others do not.

In late 2016, buses that provide service between cities and tour buses must be equipped with lap-and-shoulder seat belts, according to a federal rule laid out in 2013. But that rule does not apply to school buses or city buses. The National Highway Traffic Safety Administration said the measures are meant to bring down the annual number of deaths and injuries reported in bus crashes each year — 21 and 8,000, respectively.

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Still, the fact that these federal regulations do not necessarily cover city and school buses, is a sore spot for some, such as parents of children killed or injured in school bus crashes. Only school buses under 10,000 pounds are required by U.S. law to have seat belts, and just six states require their fleets to be equipped with belts, according to a recent ABC report. Federal regulators say that even though school buses don't always have safety belts, they are still a much safer option for the 25 million children ferried to and from schools every day in them, ABC reports. They also note that expensive seat belts could reduce the number of buses on the road, causing more children to be driven in cars, where they are many times more likely to die in a traffic accident.

When it comes to school buses, officials say that they are designed to be one of the safest ways students can get to school — even without seat belts. Other research, however, indicates that seat belts on buses could reduce injuries and death significantly, especially in the event of a rollover.

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Overall, the logic seems to be that buses — and trains — are much less likely to be involved in a crash, and even when they are, these are structurally much safer than, say, a sedan.