Yes—cedar is usually a good choice for an urban planter box, and it does not rot too fast when the planter is designed and maintained correctly. The key is to use the right cedar and keep moisture from sitting against the wood. Western red cedar is the most common choice for wooden planter boxes because its heartwood contains natural oils that resist moisture and insects. Alaska yellow cedar is also excellent. Aromatic cedar is not the same thing and is less reliable outdoors.
Cedar is not rot-proof, so rot resistance depends on the grade and the conditions. It can fail at the soil line, joints, and base if the box holds water. In an urban setting, reflected heat, automatic irrigation, and tight courtyards can make a planter stay wet or dry out too fast. Use a free-draining soil mix, drainage holes, feet to lift the base, and an air gap around any liner. Avoid sealing the inside completely. If you use a liner, make sure it drains and does not trap water against the wood.
A well-built Western red cedar planter box can last 10–15 years or more in a well-drained setting, and longer with simple wooden planter care such as annual inspection and occasional oiling. It will weather to a silvery gray, which many people like, or you can re-oil it to keep more of the warm color. It will last less time if it sits in standing water or is made from sapwood rather than heartwood. Composite or plastic planters may need less maintenance, but cedar offers a natural look and is easy to repair.
Bottom line: cedar is a good choice for an urban planter if you choose rot-resistant heartwood and design for drainage and airflow. It does not rot too fast in most urban planter boxes when basic care is done. Avoid it only if the planter will sit constantly wet or poorly ventilated.