From Daniel Stern Lighting
There's no such thing as "DOT approved". DOT does not "approve" products as the European regulatory body does. Rather, the manufacturer of an item of motor vehicle equipment is legally obligated to self-certify that his product complies with all applicable regulations. For some items of equipment, such as headlamp bulbs, the certification takes the form of a "DOT" marking on the bulb base. However, there is no legal obligation for the manufacturer to submit [the] product for government testing before applying the marking, and many companies go ahead and apply the marking even to bulbs that do not comply with the law.