I have seen quite a few racetracks, but none of them are very large. So I decided to make a realistically sized race track that can be used with any vehicle mods. It's not a replica of any real life track, it's only realistic in size. Overall, the dimensions are 450x500 tiles and it features 5 different tracks. There is a countdown timer at the start/finish line which all 5 tracks share. A building in the middle of the grounds contains switches to allow you to select which track is active, there are also several smaller buildings scattered around which can act as 'reset' switches in case of an error (more on that below).

Special Considerations:
-When importing the schematic to a world with MCEdit, be sure to select 'Copy Air' otherwise the redstone connections underneath may get messed up if you sink it into the ground.
- If used on a multiplayer world, be sure to protect the track to prevent anybody from destroying pieces of it. You could also remove the doors that lead underground near the count down timer to prevent access to the internal workings.
- Due to the size of the track, it can easily hang outside the games chunk buffer, if you want to prevent this, use a chunk anchor mod.
- The track itself is made of black, grey, and yellow wool. If part of it overhangs a desert biome, lighting could strike and start a fire causing severe damage. any parts overhanging snow biomes will get covered in snow and racers wont be bale to see the track itself. A grassland or hills biome would be good locations, rain will put out any fire caused by lighting and thus damage will be minimal and easy to repair. Ocean would also be a good spot since it's not much use for anything else.

Troubleshooting and other questions:

Why is the track selection in the center of the track instead of at the starting area?
-It is in the center of the track to ensure the best chance that all the chunks the track is on will be loaded when you change tracks to prevent errors.

I flipped a track switch, but parts of the track didn't activate
-This is caused by parts of the track being on chunks not loaded. In single player, it can be solved by increasing your view distance, in multiplayer, I think the server can be set to load more chunks around players. In either case, a world anchor mod could be used as well. If you don't want to change view distance or chunk loading, there are small buildings scattered around the map near the intersections of the individual tracks. These small buildings have buttons on the sides of them which when hit will try and do a local redstone reset to fix any issues.

I'm trying to start the countdown timer but it's only reading 08
-You don't have it turned on. when it's off, the pistons deactivate and show the glow stones behind them. when turned on the display will be black and a green box will appear above the lever.

Can you separate Track # from the rest so I don't have to download the whole thing?
-I had thought about making different downloads for each track, but I doubt I will.

Future Plans:
-Lights will eventually be added around each track, toggleable via switches in the track selection building
-A bit of landscaping will be done as well so it's not so plain. Trees will likely be added on the eastern end of the track.
-Buildings might be changed from stone to a different material.