U-Drive-It, also known as UDI, is a feature found in My Sim Mode that allows you to take control of various transit vehicles in a city. They include cars, trucks, buses, trains, planes, ships, tanks and monorails.
UDI came out in the Rush Hour expansion pack. There are two modes of driving: Scenario Mode, and Free Drive. Both modes can be accessed either by clicking on a UDI mission bubble (the blue circles hovering over vehicles in your city), or by selecting a vehicle from the UDI menu in My Sim mode. UDIs will automatically end if you cross your city's borders. If you are in a Scenario Mode mission, and crossing the city border is not the goal of the mission, leaving the city will result in failing the mission. UDI vehicles have health diamonds (similar to The Sims) above the vehicle that indicate the condition of the vehicle, starting out green, and turning increasingly red with damage to the vehicle.
In Scenario Mode, you are given instructions to carry out certain tasks in a limited time. Successfully completing the specified tasks in the given time will result in an award. The award may consist of unlocking certain rewards, a boosted mayor rating, or Simoleons (money). The tasks may include traveling to specific parts of the city, covering yellow squares, and using special vehicle-specific abilities. For example, you may be asked to travel to a particular location and destroy the building, or hand out money, or write patterns in the sky with a skywriting plane. Some missions come with both an award and a penalty for completing the mission, for example, successful completion of a mission may award you Simoleons at the cost of reducing your mayor rating. Additionally, many Scenario Mode missions are paired with an alternative mission that uses the same vehicle, but produces an opposite combination of awards and penalties. The screen the pops-up when you select a UDI mission provides background story, describes the potential awards and/or penalties for completing the mission, and has a button to view the alternative mission.
All Scenario Mode UDIs can be failed by either running out of time or destroying your vehicle. Additionally, some scenarios can be lost by the occurrence of scenario-specific activities (being caught by the police, for example). Failing a mission may result in a penalty as well, such as loss of Simoleons or a damaged mayor rating.
In Free Drive Mode, you select the vehicle you want to drive, and are free to travel around your city and use all of the vehicle's special functions. You cannot "fail" a Free Drive, however the Free Drive will end if you destroy your vehicle. Awards are not given for Free Drives, however they are useful as practice with particular vehicles, or simply as a unique way to view your city.