This last reaction can go in either direction, depending on temperature and the amount of sunlight. If there is a lot of sunlight, the equation moves to the left, and more ozone is produced. If nothing else gets in the way, an equilibrium is reached, and the ozone level stabilizes. However, there is something else involved. Remember that the cars are also emitting hydrocarbons as well as oxides of nitrogen.
Hydrocarbons are the other main ingredient in photochemical smog.
When hydrocarbons are present, nitric oxide reacts with them instead of the ozone. One is that a lot of volatile, reactive organic compounds are generated directly. So the amount of ozone in the air increases. With nitric oxide reacting with hydrocarbons, ozone may accumulate to damaging levels. These are referred to as secondary pollutants, because they are formed by the reaction of primary pollutants, nitrogen oxides and hydrocarbons, emitted by burning fossil fuels.