The levels in most Carmageddon games are arranged into groups. Usually, groups are consisted of around 5 levels, all of which must be completed to unlock the next one; thus progressing through the game. After all groups are completed, the player is free to choose which group to play on, but until then, (s)he must first complete each one in order, being unable to go back to a previous group. The player is free to replay races on the current group, though.
Carmageddon II has 10 groups in total, with three 'race, wreck or wreak havoc' levels and one mission level in each. All 3 of the racing levels must be completed to unlock the mission, and the mission level must be finished to unlock the next group of levels. The initial times for each race on the first group are 2:00, 1:30 and 1:00 in each difficulty. After the first group, all races have a time of 1:30, 1:00, 0:30 in each difficulty.
Carmageddon TDR 2000 features the 'race, mission, race, mission, race, mission' scheme in almost all of its groups. Each level (mission or race) must be completed in order to unlock the next, unlike Carmageddon II, which already has 3 races to choose from at the start of each group. The last missions on each group generally show how the player ends up on another location.
Carmageddon 64 works like Carmageddon II, but the player is able to change group at any time, from the ones (s)he's completed. The groups can be changed by pressing the C-up (previous) and down (next).