If you're visiting Texas from another state, you must comply with all the Texas laws for ATV riders, including age, supervision, helmet and eye protection, and the size of the ATV you ride. You must also carry the safety certificate from your home state with you when you ride in Texas. If your state doesn't issue a similar certificate, you'll need to complete the Texas course or ride with another adult who does have the safety certificate.
Check out these Mertens, TX ATV/OHV resources:
