Images - 1) - empty road & 
2) road blatantly full of goats

If you stick exclusively to tar roads you will be missing out on some of the most incredible scenery Namibia has to offer. Some roads are only passable to high-clearance vehicles, or require 4x4; there are also a number of gravity-defying mountain passes. You are advised to check locally for up-to-date information on conditions before getting too far off the beaten track. For suggestions about outfitting for a proper off-road expedition, go here.

South Africa's road network is fantastic, and most people will never have to leave tar behind until they go round Kruger, and even then many stick to paved roads. 

National (N) Roads are South Africa's motorways. Some of them (most of the better ones) are now toll roads  - as a result, nearly all of the Johannesburg-Durban road which used to be a complete bottleneck is now four lane and relatively free-flowing. In some rural areas you take your life in your own hands whenever you go off the more beaten track; to get to farmhouse B&Bs you may have to scrape your way slowly up a deeply rutted track. 

(As it is currently almost impossible to buy fuel in Zimbabwe, this is somewhat irrelevant). Be aware that the roads are in a mess, and it is hugely difficult to get any form of practical support. And the politics cannot be escaped from - by going to Zim, your money will find its way back to Mugabe and the stats of foreign tourists used to justify his policies. If you do choose to go, be aware that Mugabe's supporters - and others driven to desperation - may well report any political statements made. But by showing commonsense, being prepared to leave the tourist hot-spots, and having an awareness of the past 30 years, Zim could once again become an incredible destination.