Forget religious fanatics, gun-toting kidnappers or any other threats you have associated with Iran, you are more likely to get into trouble with the traffic than anything else. Iranians will tell you with a perverse mix of horror and glee that Iran competes for the highest per-capita number of road deaths on earth - in 2010 that was more than 22,000 people, with another 200,000 plus injured.
Noone pays any notice of road rules and the willingness of a car to stop at a busy intersection is directly proportional to the size of the vehicles in its path. Playing on this, some cunning motorists have fitted deafening air horns, usually found on trucks and buses, to their Paykans and Prides. A quick blast sees other traffic screech to a halt, fearing they have been outsized. Meanwhile, the modest little Paykan/Pride sails through the intersection. Size (or at least the perception that you are big) matters.
Be aware of contraflow bus lanes (along which buses hurtle in the opposite direction to the rest of the traffic), and motorbikes speeding through red lights, along footpaths and through crowded bazaars.
Vehicles never stop at pedestrian crossings so don't underestimate the possibility of dying a horrible death while crossing the road. It may be little consolation, but the law says that if a pedestrian is hit the driver is always at fault and is liable to pay blood money to the family of the victim. Until you have got your head around the traffic, perhaps the best advice comes from one pragmatic traveller: 'Cross a busy street with an Iranian person, but make sure the Iranian is closest to the approaching traffic'.