There are three pennant shaped flags flying on poles in the far distance. The colours are red, blue, and yellow.
Flags of Hilldale Housing Community
Marty McFly and Jennifer Parker are in the black Toyota. Marty makes the right choice by avoiding a drag race with the red truck. This again alters their future in amazing ways. One really has to move past the paradox to understand the different realities that Marty McFly has jumped to.
DeLorean on Rail Road Track
next to Hilldale Housing Flags
The flags of Hilldale McMansion community use skyblue and yellow horizontal tri-bars in an alternating pattern. Some of the flags have a yellow-sky blue-yellow pattern while others have a sky blue-yellow-sky blue pattern.
link to blog post "Flags in Back to the Future Part I"