New Entry Gate - Planning

The drive to my property gate is formed from a 50' easement taken equally from my land an my neighbors.  My neighbor states that when no one is around, local high school students park at the end of our drive and drink.  I've never seen any or beer bottles but he states he has pictures of them from game cameras he has setup in the area. 

Two weeks ago while mowing at our land, the neighbor stopped by to say there were some visitors on the drive the week prior.  We discussed adding a gate at the start of the drive and placing a shared combination lock on the gate.  I like the idea as it keeps people off the road but also keeps people from driving down the road and being able to see my cabin.

The following picture is looking from towards where the gate would be added.  I'd like to add on to the post which is closest in the image below.  I'll replicate the existing format with two new vertical posts in the ground with a 16' long top post.  

I measured the existing posts and the vertical posts appear to be 2 7/8" pipe.  

I could not measure the horizontal or top pipe so I measured the circumference.  A little math helped me to find this is 2.5" pipe.  

I need to work on the plan with the neighbor and get more details on his thoughts.  I think it is easiest if I do my side of the easement and the does each.  We can each have a 14' gate which provides a total of 28' opening.  If don'e properly, the center of the two gates is the property line and we each maintain our side.