Tracking offline events is difficult. Be prepared that it will be impossible to track 100% of your offline branding efforts. For example imagine somebody sees your poster, remembers the name, googles your org and lands on your site. They will be considered organic traffic.

The old way was to use different phone numbers for each poster, and measure the number of calls that each poster generated using call tracking. This is the same way that infomercials test markets/times/channels/etc...

Nowadays we use websites but the process is still the same. You can use bitly codes, or multiple domain names to measure which poster drove which traffic, just keep in mind that rarely does a poster convert right away. People riding the subway or driving etc are in a completely different state of mind. You could try and attach a unique incentive to manipulate that (free t-shirt!).

Alternatively you can _infer_ the success of your posters based on what neighborhoods traffic is coming from, how much on mobile, and other analytical cross sections, but you should remain skeptical of the accuracy of this process.

Personally I wouldn't worry about which poster, I would just worry about posters total. If you see traffic increase and keep all other things equal then yay!

Good luck!