Yes, an urban planter box can crack if the water inside freezes solid in winter, but whether it does depends on the container material, drainage, soil moisture, and how much empty space the water has to expand into. Water expands by about 9% when it turns to ice. In a planter, that expansion creates pressure against the sides and base. If the soil is saturated and the planter has poor drainage or no expansion room, the pressure can be strong enough to crack ceramic, terracotta, concrete, or brittle plastic. Wood may split or push apart at the joints, while metal may dent or deform. Flexible plastic and some composite planters are more likely to flex, but they can still fail in a hard freeze, especially if they are old or damaged.
The risk is highest when water collects in a saucer, drainage tray, or blocked drainage hole; when the planter is left outside in repeated freeze-thaw cycles; and when the soil stays wet going into winter. A freeze-thaw cycle is especially damaging because water seeps into small cracks, freezes, widens them, thaws, and repeats. Even a planter that survives one freeze may weaken over several winters.
To reduce the risk, make sure the box has adequate drainage holes and that they are not blocked. Remove saucers or empty them before freezing weather. Avoid keeping the soil waterlogged in late fall, and stop watering when plants are dormant. In cold climates, empty or cover the planter, or move it to a sheltered spot. You can also raise it off the ground, add a drainage layer if appropriate, and choose a container labeled frost-resistant or freeze-proof. If water has already frozen solid, do not pour hot water on the planter; let it thaw gradually and inspect for cracks before reusing it.
In short, an urban planter box will not automatically crack, but frozen water is a real risk. Good drainage, frost-resistant materials, and winter preparation are the best ways to protect it.