Why do concrete roads have compression expansion joints?

Concrete pavement always develops cracks. Unless expensive additives are included, the pavement block will shrink slightly during curing - and soon crack. Pavement expands when heated and contracts as it cools so, even when reinforced with steel mesh, a sufficiently long block of pavement will eventually contract enough to crack, or expand enough to buckle (explode up).

The solution is to intentionally make cracks exactly where intended, at expansion and contraction joints, and design them not to leak, nor shear vertically, nor fill up with grit.

There are far more contraction joints than expansion joints in highway pavement. The expansion joints are made with a gap all the way through the concrete. The contraction joint has no gap at first, but is designed with a deep slot and shallow groove to invite a crack. With proper construction techniques and frequent contraction joints, the pavement will not develop troublesome stray cracks.

Both types of joints are reinforced with a sturdy set of parallel steel rods spanning the joint. The pavement blocks are kept in alignment with the rods, yet can slide along them. The rods are coated with epoxy for rust-proofing. The epoxy is coated with an oil-soap film so the rods can slide in the concrete.

Every joint is sealed closed just below the surface with an elastic glue, or springy structure. The seals job is to keep out water, road salt, and grit.