The introduction of licence-plate-based driving restrictions on Saturdays in Mexico City have not led to an improvement in air quality, according to a study published in Scientific Reports this week.

Driving restrictions were introduced in Mexico City in November 1989. The program - Hoy No Circula - bans drivers from using their vehicles one weekday per week based on the last digit of their licence plate. For almost two decades the restrictions applied on weekdays only. However, in July 2008, the programme was expanded to include Saturdays.

Lucas Davis analysed hourly air pollution data from 29 pollution monitoring stations distributed throughout Mexico City, from 2005-2012. By analysing levels of eight pollutants, including carbon monoxide, ozone, sulphur dioxide and nitrogen dioxide, the author found that there was little evidence to suggest that the expansion of the program had improved air quality. The author also investigated if driving restrictions induced people to use different forms of public transportation. By looking at the number of people using subways, buses and light rail over the same period, the study concludes that there was no evidence drivers were switching to lower-emitting forms of transportation because of the driving restrictions.

The author notes that the effectiveness of driving restrictions depends on the availability of alternatives and the willingness of drivers to switch to lower-emissions forms of transportation. However, as incomes continue to increase, so does the value that people place on their time and thus, the author suggests, the preference for private transportation.