Undoubtedly some of the most daring, exciting, fastest, scariest and always packed bus rides I have gone through - and not just once. The Guatemalan "chicken buses" are the main form of transportation for locals - after having taken more than 10 I have yet to see a tourist on any of them. I now know why. They speed at tremendous velocities through tight hairpin curves where even the driver has to grab a rail to hold himself in his seat.
Some are run down, most are pimped up to the max - all shined up - and with powerful sound systems inside.
They call them chicken buses - not because of chickens - but because the passengers are packed in like - chickens. Average 6 to a row and sometimes as many as 9 - somehow! And yes, to get in or out, one literally must crawl over other passengers.
If you visit Guatemala, don't get talked out riding a chicken bus - you are in the ride of your life. As someone wisely told me: it's the one transport system where you either arrive on time - or not at all.