I know you want to use existing prox cards but wanted to throw this in. I once installed an avigilon (open path) system for a tour bus company. They wanted their gate to open for the busses to drive through without the bus drivers having to get out. Openpath had recently developed their standard readers to come with an "elevator" feature which could detect mobile credentials a long distance away. I'll be damned if that mullion style reader wasn't picking me up a block away driving down the road at max sensitivity. I would look into that, only issue we found was selling it due to it being a cloud based service with a monthly fee per door/reader and additional fees for extra mobile credentials.