As of tomorrow, August 4, 2012, Chicago's new marijuana ordinance takes effect. Rather than arresting someone who has 15g of pot or less, Chicago Police can now issue tickets with fines from $250-$500 instead. First offense fines are $250 but the fine would double for a second citation within 30 days of the first.
Before the change in the law, anyone caught with cannabis, no matter how small of an amount, was charged with a Misdemeanor. They were arrested, their mug shot and fingerprints were taken and they were placed on bond. The person was then given a court date which they were required to appear at, along with the officer who arrested them.
The new law, however, is simply an ordinance violation which carries only a fine, so long as the person has less than 15 grams of cannabis. The person avoids an arrest and having to go to court, which saves everyone time.
However, it is still an arrestable offense to smoke pot in public. You can also be arrested if you have any amount of cannabis on you in a park or on school grounds, or if you are under 17.
Thus, anyone going to Lollapalooza this weekend should not have pot on them at the festival as the grounds are owned by the Chicago Park District and thus you would get arrested for having pot in a park.
The City of Chicago estimates this could bring in approximately $7 million in revenue.