You can exchange currency at Johannesburg International Airport or at the larger branches of South Africa's banks, such as ABSA, Standard Bank, and Nedbank's operations in Rosebank and Sandton. Look for the bureau de change signs at these banks. ATMs are all over the city, especially at shopping centers. Be careful when using them, though. Don't let anybody distract your attention, and avoid ATMs in quiet spots at night. Traveler's checks are welcome, but more and more businesses are switching over to credit cards.
Johannesburg is notorious for being South Africa's most dangerous city. It's quite common to hear about serious crimes such as armed robbery and murder. Even South Africans fear it, regarding it as some Americans regard New York City: big and bad. That said, it's safe for visitors if they take reasonable precautions. These include not leaving bags or valuables visible in a car and keeping the doors locked, even while driving (to minimize the risk of smash-and-grab robberies or being hijacked), not carrying large wads of cash, and not walking in the city center or any of the townships with flashy jewelry or expensive equipment.
