Scott Olson/Getty ImagesA secondary crash is one where, say, you get rear-ended in stop-and-go-traffic on the highway. That causes your car to be pushed into the adjacent lane, where traffic is moving faster. Then, someone barrels into the side of your car, because you were shoved into that lane. Yep, that would be very bad.A secondary crash can also be caused by someone not obeying the move-over law, meaning, a crash has already taken place, the cars are already off on the shoulder, and EMS or a tow-truck is attending to that accident. The secondary crash is a driver barreling into a person attending to those folks. This is also terrible and means the towing industry is one of the deadliest forms of employment in the country.Fortunately, most secondary accidents, according to a 2023 Federal Highway Administration study, are non-fatal. The administration examined 5,461,044 primary crashes across 10 states. It sorted through all of those accidents, then checked where the states had reported secondary crashes. Researchers tried to prove the secondary wrecks occurred on the same route, within a close timeframe and within about a mile of the actual primary accident, since it's not a real secondary accident if it didn't coincide with the first crash. That analysis showed that only 15,488 secondary crashes resulted from those primary crashes. This means that secondary crashes are incredibly rare. No state studied had a rate higher than .81 crashes per primary wreck. AdvertisementAdvertisementPlus, one of the study's main conclusions was that "Most of the secondary crashes (71%) involved no apparent injury, and less than 1% of the secondary crashes involved fatal injuries." That said, there are contributing factors that can make even that rare secondary accident far more severe, like higher speeds, the involvement of large trucks, or fewer lanes.Read more: These Are The Deadliest Cars On The RoadSpeed kills, and so do trucksPetemuller/Getty ImagesIt's not hard to understand that a car accident, or a secondary accident, involves kinetic energy. Or, simply, speed kills. A 2020 study published by the International Journal of Environmental Research on Public Health found that raising speed limits can cause more deadly accidents. Every 1% increase in the speed limit increased the probability of a severe secondary crash by 2.6%. The researchers attribute this not just to the obviousness of all that added force, but also to reduced reaction times for drivers.But there's another factor to consider: traffic. Both this study, and a separate, 2019 article in the journal, Physica A, show that higher traffic volume is much more likely to lead to secondary accidents. Both of the former studies looked at Chinese traffic. That could be different from American traffic. However, a 2025 study from the Eastern Transportation Coalition, which eyeballed secondary accidents in Maryland, Virginia, Tennessee, and Florida, also found that more traffic drove up the chance you'll get into a secondary accident. Researchers found a roughly 5% to 6% odds increase for a secondary collision, based on data for Virginia and Florida, though only a 2.5% odds increase in Maryland. Still, more volume equals more secondary pileups.AdvertisementAdvertisementAnd there's another risk, which is what happens when an 18-wheeler is involved in the traffic accident. While limited to just the aforementioned study in the International Journal of Environmental Research on Public Health, when a truck is in the mix of a secondary accident, the outcome for anyone in a passenger car is far more likely to be severe.More lanes and wider shoulders lead to fewer secondary crashesBilanol/ShutterstockYou don't have to be a genius to know that, sometimes, you really shouldn't do "your own research." Legitimate science requires rigor. But in the case of what reduces secondary crashes, common sense makes the formula relatively straightforward. And research bears that out. A 2017 study in the journal, Accident Analysis & Prevention, found that a broader shoulder is safer, not just for EMS personnel attending to accidents, but also for reducing the likelihood of secondary crashes. And there's a knock-on effect that's related, too, which is that wider roadways with more lanes are safer. Again, this is pretty logical. Think of a divided highway. If there are two lanes in each direction, an accident in one of those lanes suddenly reduces the carrying capacity by 50%. But with three lanes in the same direction you've only lost a third of the capacity. On a three-lane highway, if you're just behind that accident, you likely have more options to avoid piling into the crash ahead of you. The aforementioned study found that roads with more lanes see fewer secondary crashes: Two-lane highways had 73% more secondary crashes than roads with three or more lanes. A 2014 Journal of Safety Research study of the New Jersey Turnpike found that closing more than one lane of traffic increased the risk of a secondary crash by a fairly terrifying 4.8 times. That is a yikes statistic that should encourage you to always heed the life-saving three-second buffer driving rule. AdvertisementAdvertisementWant more like this? 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