When are they going to install a traffic light at the entrance?

There is a I-95 on ramp North and South of the museum. If you feel unsafe making a left you can turn right out of the museum & use that 95 enterance or the overpass to turn around. As this is still within the DC commute area and the base employs a large population traffic can be extremely congested during peak travel times and on the weekend. VDOT probably does not see justification to increase the congestion this close to a major intersection for the sporadic museum traffic. The slight adjustment to your travel plans will usually work to reduce a great amount of stress.

You shouldn't have trouble unless it's a Holiday. It was easy ingress & egress for us on a week day at 95 N to Quantico & following the signs.

Depends on VDOT. Might not happen for a good long while.