Flyover not safe for cyclists

I was cycling from Steyning to Shoreham by sea on the A259 after dark, and came to the junction where the road sign offers the two options 'Shoreham Basin and Beach' and 'Shoreham Beach'. Not knowing this area (I live in Bristol) I did not know which to take, so opted for the first. This took me on a huge anti-clockwise circle and UP ONTO THE FLYOVER, which I had not expected, and did not know how to get back to the ground, or where this would take me. It was very alarming !!!! I finally decided that all I could safely do was walk my bike back the way I'd cycled, on the hard shoulder. I then decided to lift my bike over the central reservation barrier and onto the opposite carriageway. I did this safely when there was no traffic and cycled back down to the roundabout leading to Shoreham beach. My concern is that the sign did not indicate that it would lead to a FLYOVER - and what a cyclist should safely do in this circumstance? Luckily I managed to get back safely to Shoreham where I was staying. I am a very competent cyclist, but am 70 years old, and this was not an easy situation for me to be in!!