If you want to create a one-way road, you can add the following tag to a road config.txt file.
isfreeway and set that to 1
This forces the traffic to go in one direction. When you go to use the freeway/highway, you drag the spline in the opposite direction the traffic is supposed to go in.
You can increase the number of cars by increasing the number of carrate above the number already there. I can't remember the settings for what the numbers mean.
There's a way to increase the traffic speed, but I can't remember that parameter offhand.
Your best bet is to consult the Trainz Wiki here:
http://online.ts2009.com/mediaWiki/index.php/Main_Page