Selective laser melting and building near the road (vibrations)
That depends...
Among the fact that can influence if vibrations could be the problem in this case, I see the following s is the most prominent:

Powder grain diameter
Powder composition
Powder shape
Aimed for resolution

The grain diameter and composition are the more influencing factors: fine powders of low-density metals will be much more affected by vibrations and almost behave like dust or fluid under resonating vibrations. On the other hand varicose powders of high-density metals will more easily settle down and have much higher thresholds for how much vibration affects them.
The shape mainly influences how particles coagulate: very coarse particles, think snowflakes, will interlock and bond to one another creating flakes of several powder particles. Vibration can influence the formation and compactness of such flakes and such influence the resolution.
If you want to print very coarse with rough, large powders, vibrational mitigation might be not needed. But should you want to print pretty much in aluminium dust... you need to take it into account.
Vibration mitigation
If the Vibrations influence your prospect machine placement, you'll have to inquire about at your machine manufacturer, best with a seismometer measurement of the places where you want to build. By placing special fasteners and vibration dampeners (springs or rubber feet) in between the machine and the floor, anchoring it to a separate foundation that is not connected to the surrounding building, or even by having the machine float, vibrations can be mitigated.
It's often much easier to install larger vibration mitigation equipment on lower floors and mounting it directly to the foundation of a building (ground floor and basement) is often preferred.
In case of special projects, the local municipality might allow banning (heavy) road traffic or even pedestrians from areas that affect the manufacturing in such a way - the Cologne Opera is partially under a public space right next to the cathedral, and during events, pedestrians may not use a chunk of it. In my city, the use of one road with trucks is banned due to vibrations they can cause in a nearby old building.