According to the Associated Press, the decision came because record levels of air pollution. The government made public transportation free during the ban, and police monitored vehicles coming into the city. Hybrids and EVs were still permitted to enter, no matter what their license plate read. Officials also decided not to extend the measure into a second day to affect those with odd-numbered plates, according to the AP. Parisian authorities have tried this pollution-reducing strategy before. Last March, there was a similar 24-hour ban, and in that time the police reportedly issued some 4,000 tickets for not complying, each with a $30 fine. By the next day, things were back to normal.
Obviously, these one-day license plate bans can only have a temporary effect on reducing smog levels. However, the government in Paris has been working on longer-term solutions. For example, it has suggested lowering the speed limit there to less than 20 miles per hour and keeping old cars off the road.