Transcript for Bikers Dash Across Mexico City in Illegal Alleycat Races

You're about to meet a group of adrenaline junky bikers so hard core, they make the tour de France look like a practice run. On this race, there's no course, just chaotic streets and nerves of steel. ABC's Mariana van Zeller went along for a hair-raising ride. Reporter: We're in Mexico City, riding in one of the craziest bike races you've ever seen. There's few rules here. No set race course. And certainly no officials closing down the streets. Making cars and pedestrians fair game. Dubbed alley cats, these illegal races are the invention of bike mes sjengers. "Nightline" got a special invitation to go inside this punk rock tour de France. To see what compels these guys to risk life and limb for mere bragging rights. Getting onto a motorbike right now. What do you think of this guy? Loco. Reporter: Look at him. Look at him. Meet Lucas, the king of the alley cats. Who, with his homemade helmet cam in towel has been traveling around the world, filming these races for more than a decade. He asked to be interviews where he's most comfortable. Yeah, this is great. It's like really comfortable. Reporter: What happened there? Well, just avoiding potholes. It's Mexico. Reporter: Lucas once competed in organized bike racing. The type that occurs on closed courses without speeding traffic or blown red lights. But always the rebel, he felt constrained by the rules. He quit. And soon fell in love with the alley cat race. Huh does it feel to be so close to danger all the time? Feels good. You know, I'm familiar with danger. You have a buzz with your mind, saying, you know, I'm not totally in control of what's going on right now. Reporter: On race day, we meet up with Lucas and some of the other top riders. Bit nervous. Reporter: Chris from New York City, considered one of the fastest in the world. And Pancho, one of the best Mexico City riders. Ready? Think so. Reporter: He's not nervous. I'm nervous for him. He says I'm getting him nervous just by talking like this. We are starting very soon. Reporter: Organized to mimic a day of delivers packages, the race is made up of a series of checkpoints scattered throughout the city. Racing in teams of two, riders much check in at each location before heading to the finish. Woo! ? Reporter: This is some crazy stuff. Oh, my god. Oh, my god. Even though we're on bikes, it is almost impossible to keep up with these guys. And some of the maneuvers we're doing here are, like, crazy dangerous. Oh, my god. In 2008, an alley cat rider in Chicago ran a red light and was struck and killed by an SUV. It's the only known death attributed to an alley cat. People watching the video might say, my god, these guys are on the verge of killing somebody or whatever, but it's just the opposite. 99.9% of accidents are not caused bid us. Reporter: I totally get your point. There is the idea, though, that there's not a lot of respect for traffic. Incidents are rare. But as far as breaking laws, I mean, that's like every 30 seconds, you know, or less, I mean -- Reporter: That's what you guys do. Get off! Use your head. Reporter: Back in the race, the riders are finding the checkpoints as they go. Their race directions are stamped at each stop to prove that they were there. But even finding the checkpoints is an immense challenge in this crowded city. Chris, Lucas and poncho are lost. They end up in this sprawling marketplace. Finally, we catch up with Lucas and the other riders, but our path is suddenly blocked by a massive highway. Lucas just hops off his bike and playing crossing guard, holding his hand out while well scurry across six lanes of traffic. Chris and krponcho arrive at one of the checkpoints. They do not complete a task. They were supposed to take a cell phone photo with their teammate. Ay, Yi, Yi. Reporter: It's now a mad dash to the finish. Usa versus Mexico. With his legs spent, poncho eagerly hops a ride. This puts Chris just out of reach. Poncho beats Chris and Lucas to the finish. But it doesn't even matter, as Chris finds out he's been disqualified. This is why I asked in the beginning in English what the rules are, you told me something different. Idiots. Reporter: How was it? It was intense. Reporter: It was a crazy rush for us just to be following on motorbikes. I understand why you guys like doing this, crazy adrenaline. At the beginning, it was really dicey, but going through the market, that was crazy. And then going on the back roads, like, just -- real Mexico. Reporter: Except for some bumps, bruises and flat tires, these alley caters have survived to race another day. For "Nightline," I'm Mariana van Zeller in Mexico City.

This transcript has been automatically generated and may not be 100% accurate.