Anyone can download Fairfax Co GIS data. Wanting to know how many people live in my area, went to the Address Points data layer under Property & Land. I zoomed to my area and used the area selection tool on the right, then used Filter to this area which selected points within the area. Those were downloaded.
I entered the points data into QGIS, in a csv file that contained x, y data in the first two columns. I did this by selecting Open Data Source Manager in the upper left. I then selected Delimited Text. The first row contained feature names.
Using QGIS Select Features by Freehand I selected the addresses in my area. The Select Features by Polygon didn’t work on the MacBook because I couldn’t close the polygon. I then used Open Attribute Table (Selected Features) and there were 691 addresses selected. According to the Census demographic for Virginia, there is an average of 2.4 people per household in this area (ESRI data for 3 mile radius). My area then is estimated to have 2.4 x 691 = 1,658 people. According to the Census data for Virginia, there are 128 towns smaller than my immediate neighborhood in Reston. Everyone driving a vehicle must enter and exit the neighborhood using one road. Bicyclists have a few options other than the road.