While walking my dog and humming the Starship Troopers theme tune (my brain is like a broken and expectionally nerdy iPod) today, I started thinking about chase scene. I love them in movies, but I've yet to see very many RPGs handle them in a fun way. While many systems have quite reasonable mechanics for such things, mostly they just boil down to a series of opposed rolls, with little or no choices made on the part of the players. This sucks, because RPGs are really all about making choices. So, here goes a simple mechanic for d20 games (Microlite20 mostly, but it's adaptable) for chases and races.
Step 1: Setup
During the chase, distance between people (or horse, or starships, or whatever) will be determined by points of Lead. Pick some increment of distance that Lead will represent for this chase. For a thief running through the streets chased by guards, 10 feet is good, while an all-day overland chase on horseback might have a point of Lead represent several miles. Generally starting the two parties (pursed and pursers) about 5 points of Lead apart is good, but do whatever fits the situation.
Step 2: Determine Initiative.
Unlike in normal combat, chase initiative is rolled at the start of every round. Each individual or group in the chase rolls a d20 and add their Mind bonus. If the individual or group is being pursued they get +2. If a group is pursuing and outnumbers the pursued party, then they get +2. Highest roll goes first this round.
Step 3: Determine conditions.
Whoever won initiative for this round gets to describe the conditions that the chase will be going through this round. If they choose to play it safe, each participant in the chase must make a DC 10 check (usually Physical + Dex, but whatever suits the type of chase), with failure losing them a point of lead, and success winning a point of lead.
Alternatively, the initiative winner can choose different conditions, increase the DC, and add other effects to winning or losing (such as damage), as suits him.
Example: Claude the Clandestine has stolen a horse from a lord's stables and is fleeing through the woods on it. The lord's men are chasing him. Claude, having won initiative, decides he'll ride through the thickest part of the wood, braving the low-hanging tree branches. He tells the GM that for this round, the DC for the chase will be 17, and failure will deal 2d6 damage, in addition to losing anybody who fails 2 points of Lead from riding into a tree-branch and being knocked off their horse.
Step 3.5: Roll the Dice
Once the initiative winner has worked out the conditions, everybody rolls their check (the GM may choose to roll individually or as a group for grouped creatures), and applies the effects. Additionally, for any such check where the speed of the participants would aid them (i.e. pretty much every check), the faster party gets +2 on their check, or +5 if they're twice as fast, or +10 if they're three times as fast or more. Any party can voluntarily fail their check if they so wish.
Example of being tricky with these rules: Claude has stolen a magical trinket from a dragon's horde and is now being chased by a huge and rather unhappy giant lizard. Claude, having won initiative once again, decides to repeat his old trick and rides into some trees. However, this time, Claude says that the DC will be 10, with failure causing the loss of a point of Lead (like normal), and success causing the gain of a point of lead, and 6d6 damage unless you also make a save to avoid smacking into a tree.
Claude then voluntarily fails his check, and loses a point of lead. The dragon, hoping to catch up to the thief and roast him, decides to roll to succeed (getting a +5 bonus on the roll because dragons are twice as fast as horses). Easily rolling high enough, the dragon slams into the treeline, and now has to make a save to avoid braining itself with a conifer...
Step 4: Are We There Yet?
After Lead points have been lost and gained, work out what happens. If the pursuer has caught up to the pursued, they can they attack or catch their prey, as appropriate. If the pursued have got 10 points of lead ahead of their attacker, they've managed to get away. If neither such circumstance has occurred, go back to step 2 and repeat for the next round.
Optional Variants for Races:
For races, usually everybody starts at the same point, and thus nobody has a Lead point advantage. Additionally, give every participant two extra points of lead every turn, in addition to however many they won or lost form their skill checks, and whoever gets to the finish line (say, 20 points of Lead) first is the winner.
Also, as neither party can really direct the course of the racetrack, use a straight d20 roll for initiative. For added fun, have any players who are just spectating roll as well, and determine the conditions of that round is they win.