What Those Gaps Along Highway Bridges Are Really Designed To Do

Those gaps on highway bridges are more than just jarring moments that make your car go ka-thud (and can exacerbate the symptoms of wonky suspension), they are crucial to maintaining the structural integrity of the bridge. So how does purposefully putting bone-rattling gaps in a road actually improve infrastructures reliability while providing an additional thrill by scaring the hell out of drivers? Allow us to explain.

Those gaps are how bridges and other structures deal with thermal expansion so they can remain structurally reliable. This has to do with how concrete, steel, and other materials react to extreme temperature changes, and those gaps (commonly referred to as expansion joints) help compensate for the materials' extension or reduction. Bridges can stretch oh-so-mildly in hot weather, but they can also shrink or contract when the mercury drops. Without expansion joints, there's little room for the concrete slabs to expand or contract, and that's not good, considering that expansion joints bear the most strain in any structure, particularly long-span bridges. 

When expansion joints fail, and the concrete expands due to excess heat, terrible things can happen, like the concrete slab buckling and launching a poor Corolla in the air like it was driving on a rally circuit. But when they work, the bridges are fine. That's why the Texas Department of Transportation responded to social media posts from residents about gaps in the U.S. Highway 290 West flyover to South I-35 in February 2026 by on X, "The flyover is structurally sound," and the gaps are nothing to worry about since they're there for a purpose.

Concrete bridges inevitably expand or contract, and expansion joints are there to offer breathing room when they do, preventing the slabs from cracking and smashing into each other. Moreover, the expansion joints help absorb the movement or flexing that occurs when the bridge is supporting the weight of cars, trucks, and other vehicles. Expansion joints come in many types, designs, materials, and forms, but all share the job of spanning gaps between structures and enabling the two ends to cope with thermal expansion and deflection under load.

Short- and medium-span bridges typically have sliding-plate or compression-seal joints that use elastomeric foam and internal grids. Long-span bridges usually have finger-plate or finger-expansion joints that use steel finger plates, rubber sheets, and anchor bolts. Engineers select expansion joints based on the materials' anticipated "movement," or thermal expansion, and are classified accordingly as small-movement (under 45 mm of movement), medium-movement (45 to 130 mm), and large- movement (over 130 mm).

As for that slightly unnerving "thud" you hear when driving over bridge expansion joints, it turns out that some are better than others in providing a quieter, almost-seamless transition between the concrete surfaces. Tests have shown that wider gaps between bridge expansion joints produce more noise, and modular joints used in large-movement bridges tend to be noisier than the finger-plate expansion joints in medium-movement ones.

Roads and bridges in the U.S. have been in decay for years, but things are looking up since the American Society of Civil Engineers upgraded its 2025 Report Card for America's Infrastructure from a C- in 2021 to, umm, just a C, which is still better than the D+ from 2017.