Here is a great video from CNET about working with Siri to read and reply to texts.

When you are traveling in your car in feel the need to check a text message that just came into your iPhone I urge you not to because you could endanger the lives of others. Having stated that, you can use Siri to read your text messages and enable you to reply to them without taking your hands off the wheel. If you have your iPhone on Bluetooth you can actually do it right through your cars Bluetooth system.

At the end of the day it is more important to get you home safely and to keep those around you safe as well. Learn these Siri tricks to be a safer driver, or simply pull over to the side of the road to read your text messages and emails.Use these commands to get your virtual assistant to read your new text messages, hands-free.