How did the roman transportation system keep the empire strong?

Because in a time when hand-carried cards were the only form of communication, good roads made sending and receiving messages much faster (or rather, a lot less slow). Sending messages was the only real way to keep a government intact. Also, good roads made trade faster and easier, which meant more money and more efficient collection of taxes, which in turn made the Empire strong.
Hope that I helped!

C. congress was able to pay debts run up in the war