ANSWER:
15840 People
Let's first find the area, given:
3 miles by 4 feet
We know 1 mile = 5280 feet
3 miles = 3 * 5280 = 15840 feet
Area of the parade will span:
15840 * 4 = 63360 ft^2
If 5 people can fit in a 2.5ft^2 area, the number of people that could potentially be on the parade will be:

There will be a total of 31680 people on the parade.
The number of people that could potentially be on one side of the parade route will be:

Therefore, the number of people that could be on one side of the parade route is 15840 people.
ANSWER:
15840 People