Avoiding Emission
Every vehicle has an Emission value, the higher the Emission value the more it affects a city. Emission affects a city by lowering its growth rate by a percentage based on the amount of emission produced by your vehicles. If you have too many vehicles passing near the Residential Area of a city, the growth rate will be affected. Industry and Commercial areas don’t care about emission.
A little bit of Emission is inevitable if you want to connect the entire town with transportation. In order to minimise the effect of Emission, you can use the vehicles with the lowest amount of Emission. Remember, you don’t necessarily need super fast vehicles to transport people around. You will still get a growth bonus from the number of destinations.
One of the biggest culprits of emission is aeroplanes. They produce the highest emission and can pass directly above your cities. Always make sure your aeroplanes aren’t passing above your cities and try to keep airports away from them. You can use the Emissions Overlay button by clicking on the top-left to bring the drop-down menu and clicking on the Emissions Layer button. Find out where your emission is coming from and make adjustments to reduce the amount of emission from your cities.
An easy, but expensive way of lowering emission is to increase your vehicles Maintenance budget, this will increase the Maintenance Cost, but it will maintain your vehicles in good condition. When vehicles are in bad condition, they will suffer from a penalty that increases the emission they produce.