Speed limits are set by local laws and regulations, and they vary from place to place. In many jurisdictions, there are both maximum and minimum speed limits posted on highways. However, some highways might not have a posted minimum speed limit.
If there is no minimum speed limit posted, it might be technically legal to drive at 30 mph in a 70 mph zone, as long as you are not obstructing the flow of traffic. However, you should always exercise caution and consider the safety of yourself and other drivers.
In practice, driving significantly below the speed limit on a highway where other vehicles are traveling at higher speeds can be dangerous and lead to accidents. It's essential to be aware of your surroundings and adhere to a safe and reasonable speed that allows for smooth traffic flow.
Keep in mind that local laws can change, and it's always best to consult with local traffic laws or seek advice from a legal professional for the most up-to-date and accurate information on speed limits in your area.