This has already previously been invented at subway stations. Take this. http://assets.nydailynews.com/polopoly_fs/1.474936.1314635607!/img/httpImage/image.jpg

This has already previously been invented at subway stations. Take this. http://assets.nydailynews.com/polopoly_fs/1.474936.1314635607!/img/httpImage/image.jpg_gen/derivatives/landscape_635/alg-subway-turnstiles-jpg.jpg http://assets.nydailynews.com/polopoly_fs/1.474936.131463560... and make it bigger. Boom, problem solved. Side benefit: This can be an entirely mechanical design = energy efficient.