Road condition
I rode all the time on the one main road from Nairobi until Cape Town. Since November 1994, exactly the day when I passed through, even the last stretch of the road, the link between the borders of Tanzania and Malawi north of Karonga MW, is paved. The only potholes I found were in Kenia (all over !) and just south of Karonga in Malawi. All the other streets I took, with some exeptions in Swaziland and Lesotho, were paved and in excellent condition! The main-road connecting Arusha – Dar and Mbeya in Tanzania was then brandnew and in absolutly perfect condition. It was made by german and norwegian constructors in european quality! All other roads are in very poor condition, and you might ruin your bike sooner as you think!
Road safety
The fact, that countries like Tanzania and Malawi, even Zimbabwe, may have traffic every few minutes, makes the overland-roads quite safe. Most of the time, you will be alone on the roads! Unfortunatly, the closer you get to a city, the more traffic you will see, and here the problems begin. The most terrible traffic I encountered was in Nairobi downtown. Getting into a circle traffic ("roundabout" they say) can end in suicide or, if you won’t be knocked down, you will swear never ever entering Nairobi by bike again! In african countries, cyclists don’t count as traffic-members. You always have to get off the way of cars if the car-drivers are in a hurry. And they almost always are!
Tanzania is much better again, here car drivers respect you much better than in Kenia. Malawi, Zimbabwe and South Africa has a kind of european-driving-culture. You shouldn’t encounter too many problems in the southern countries.
But be aware of alcohol: I never rode my bike during the last days of a month! These are the salary-days, here the people get their money and many spend it immediatly on alcohol. This results in many accidents. You better make another programm instead of cycling!
In general, the technical condition of cars and trucks is not too bad. All the countries are enforcing strict regulations, so especially all the trucks must have more or less perfect brakes! I was more afraid of their loads, and I often asked myself when a tower-high load will fall down an overtacking truck and it might knock me down...