It should be the first thing that anyone contemplating building a motorized bicycle understands. 'They're not motorcycles'. Unless you plan to race it, there really isn't any need for a bicycle to be pushed into the 40s or heaven forbid,the 50s. They're bicycles and most of them were never designed or built to be riden at those speeds.
Yes, there are some heavy duty bikes out there built to take some abuse. Worksman comes to mind, but alas, all too often when a bike gets an engine attached to it, it's an inexpensive, department store bike meant for gentle pedaling under little stress.
The safety factor alone should dictate that when you motorize a bike you should understand the limitations of the bike's construction and parts. Bearings, brakes and frame integrity of many bikes available today are not up to the standards required to safely attain the speeds that they are often asked to deliver.
Getting the most in the way of performance is probably natural, human nature even, but one has to keep in mind that reaching unreasonable speeds on a bicycle has certain inherent limits when safety is concerned.
There is also the skill level of the builder that must be brought into the equation. A builder with only rudimentary mechanical ability who either buys a running bike or manages to install a kit and get it running might not have the assessment skills to comprehend what is required to make the bike safe under the conditions imposed by an engine. Sort of like the old maxim, "an accident waiting to happen".
Want to keep up with traffic? Do what the OP did and buy a motorcycle.
No animals were harmed during the typing of this opinion