Portable changeable letters should be a minimum size of how many inches?

The minimum size for portable changeable letters being 18 inches is crucial for ensuring visibility and legibility from a distance, especially in emergency situations where quick recognition is essential. Larger letters help drivers and pedestrians clearly see the information being displayed, which can include critical messages related to road conditions, detours, or safety alerts.

This size requirement also accounts for various environmental factors, such as distance from the sign, lighting conditions, and potential obstructions. When letters are 18 inches tall, it minimizes the risk of misunderstanding or overlooking important messages that may affect safety and response time during emergencies.

Choosing a size smaller than 18 inches could compromise the effectiveness of communication, particularly in high-traffic areas or during adverse weather conditions, where visibility may be further reduced. Therefore, the choice of 18 inches is specified to maintain optimal safety and ensure that information can be conveyed effectively to motorists and the public.