Which feature is NOT typically associated with emergency vehicles to enhance their visibility?

The streamlined design of emergency vehicles is primarily focused on aerodynamics and performance rather than enhancing visibility. While a more aerodynamic shape may improve fuel efficiency and speed, it does not contribute to making the vehicle more noticeable to the public or other drivers in emergency situations.

In contrast, warning lights, retro-reflective striping, and sirens all serve the specific purpose of increasing visibility and alerting others to the presence of an emergency vehicle. Warning lights often include bright flashing lights that catch attention, retro-reflective striping helps the vehicle stand out, especially in low-light conditions, and sirens produce distinctive sounds that alert surrounding drivers to give way. Thus, streamlined design stands out as the feature that does not focus on enhancing visibility.