Driving near a large tanker truck might make you nervous, and I’m here to tell you: You’re right to want distance. It turns out that despite several different designs, fuel tankers are just never fully stable at freeway speeds. In fact, two liquid transport tank designs are several times more prone to tipping over sideways than passenger cars. Why? It all comes down to physics.

You might think that all tanker trucks are manufactured to equally meet full safety parameters while transporting liquids. Actually, there are three basic types of liquid tankers: smoothbore, baffled, and bulkhead. What’s more, while some decrease dangerous freeway transport, none fully address the risks.

Smoothbore tanks are the simplest design. Built like their name suggests, these are smooth, empty cylinders. Smoothbore tanks fill up via a single port at the top of the tank. While simple, this design can create large, forceful waves inside the tank. These waves can push or pull on the whole tanker truck, leading to instability at higher speeds. Sudden turns or engaging the brakes in an emergency situation can cause the liquid inside to surge, pushing the truck unintentionally. This force can move the rig forward to an area the driver doesn’t mean to go, including into obstacles or other vehicles.

Baffled tanks are especially prone to tipping over. This design uses tilted dividers spread evenly through the tank’s interior. The dividers have holes in them to allow liquid through, which slows the wave effect. Baffled tanks also fill via a single port at the tank’s ceiling. While the baffles prevent the large front-to-back wave effect that smoothbore tanks can suffer from, they can still host “side splashing.” This behavior can develop into a strong, coordinated side wave. As you can imagine, a strong enough side wave can cause tipping. If you see a single tanker truck turned over on the highway, it might have a baffled tank.

Bulkheads are the third fuel tanker design. This tank has several isolated sections, each filled individually via its own port. Like the baffled design, it prevents front-to-back waves but doesn’t protect against the side waves described above. What’s more, it’s difficult to ensure each section is equally filled. If the front or rear section is overfilled, the risk of tipping can be even greater.

So, even if you aren’t sure which tanker truck design you’re driving close to, it’s a good idea to keep a safe distance.