No special access issues
Directions:
From E4; turn off at exit 176 and drive Mälarvägen west. Follow the road for 3 km and turn right towards Runsa (sign). After 6 km, park in the parking lot to the right after the avenue. Then continue on the gravel road for about 500 meters, until a path leads to the left with a sign to Runsaborg. Follow the path past the impressive "stone ship" (which is one of Sweden's largest, and consists of 28 lump shaped stones and a high bow stone) and up to the hill-fort (fornborg). Go straight over the hill-fort and down on the other side and turn left onto a small path towards the water. Soon Flugväggen will appear.
When navigating to the crag, make sure to use the function "Navigate to parking"! If you use the crags coordinates when navigate with google maps you will end up totally wrong.
Public transportation:
Commuter train to Upplands-Väsby, then bus 532 to Runsa. Then follow the road back for about 250 meter to the car park, and follow the directions above to the crag.