Heading South On A White Bronco

 is a secret Achievement/Trophy in Red Dead Redemption that is earned by evading US Marshals while riding a Hungarian Half-bred horse in Single Player or Free Roam.

US Marshals seem to be called out once your bounty reaches $2500, however lower bounties have been confirmed if the player's fame status cuts bounties in half.

Tips and Tricks

 * You will need to have broken a Hungarian-Half Breed found in Mexico. You can find them wild near the "Coronas" of Diez Coronas or south east of Escalera (marked on the in-game map by three galloping horses). Once you have broken one you can either take it to a hitching post to save it or purchase one from a general store.
 * Now that you have your White Bronco, accumulate a bounty of approximately $2500 (or $1250 if your fame is maxed out) or higher and evade local law. Equipping the bandana will prevent you from losing honor without ruining this achievement.
 * The easiest way to do this is to simply enter a town and start slaughtering the townsfolk. Shoot one person to get the attention of the local law enforcement, and then proceed to take them all out. Eventually the town will run out of officers and your wanted level will disappear, leaving only your bounty. This works VERY well in Blackwater as the town seems to never run out of officers, but they do not come at you in large groups.
 * A good way to survive the onslaught of lawmen is to take cover behind the counter of the doctor's office in Armadillo. There's only one entrance to the office and it's in plain sight. When a lawman appears, shoot him and go back to take cover.
 * In at least one case a player reported being chased by Marshals with a bounty of only $560 (with fame maxed out).
 * Next, fast travel to Mexico. Then, with your horse at your campsite fast travel back to America. By doing this the U.S. Marshals will automatically spawn on top of your position.
 * Immediately start running away from them, using horse pills, apples, and health restoring items as necessary. Once you have evaded the Marshals, the trophy/achievement should unlock.
 * You can go to MacFarlane Ranch, because there are a lot of routes to scape and less officers to chase you, get their attention and go out from the route they're not coming, use one horse pill and you'll get the achievement/trophy.

Player Provided Tips

 * When you have the horse and the Bandana ( if you don't care about your honor, no need for bandana) go into Armadillo, walk into the Gunsmiths shop, walk past him into the little room in back, equip the bandana, come back to where he is and rob him. Then go back to the back room and just wait for the law. The Pump-Action shotgun is the best way to clear out the lawmen. Only a few will try to come from the back door and that is very few and far apart, most will come through front ( 2 at a time) and will just stand there by counter, peek around and kill them. Doing this until last lawman is dead. Keeping your bandana on go out side and start killing people until you are wanted again, then run inside and repeat...unless the Marshals show up then get on the mount and haul butt. Once you get away and your wanted is gone, poof, trophy. Also a good and simple way/place to get the Friends in High Places trophy.

Trivia

 * The name of this achievement might be a reference to the police pursuit of OJ Simpson, after being charged with the murder of Nicole Brown and Ronald Goldman. Al Cowlings drove a white Ford Bronco SUV, with Simpson lying down in the back seat, and attempted to evade the following LAPD Officers. Interestingly, the SUV was driven northbound on Interstate 405, not southbound.