Are there any add-ons to allow the vehicles to Turn Right/Turn Left/Straight Ahead?
You mean a signal? have a look at addons.simutrans.com (http://addons.simutrans.com/?pak=128&cat=other&todo=search)
You mean something that restricts which way a vehicle may turn? No, there's nothing that does this directly. Indirectly however, you can prevent certain turns by making the roads there one-way (by sign) - but that's about all there is right now.