Source? Assuming you're talking about 18% of traffic and not percentage of websites, how do you define what counts as traffic in that case? Transfer between AS's? Does internal traffic within AS's count? Does traffic between entities within the same AS count (e.g traffic from one AWS customer to another, or traffic from a Netflix OCA to an ISP?) I'm skeptical of any entities ability to fully measure the throughput of the internet even remotely accurately. The closest estimate you'll likely get is if you're a transit provider able to measure data transfer, and even then you'll be lucky to extrapolate within the correct order of magnitude from that for total global inter-AS traffic.

The 18% likely comes from here (% of websites) https://w3techs.com/technologies/overview/proxy

also, what is considered internet Traffic? lots of private wan's also exist. which complicates this comparison even further.