CAN ONLY FIND 15 OF THE 18 JUMPS, CAN ANYONE HELP. ARE THERE HIDDEN JUMPS

Just look for the hovering Authority drones. From there, you can find places to reach them. Not always easy to find the actual ramp, but thats part of the fun. Good luck!

Check youtube, there's some videos of the locations and how to get them.

They aren't hidden, it's just some of them are hard to get at.

The last two go along with the final area you can get to. You can only get to them if you're on the final mission.

I'd only worry if you've opened up the bridge already the Authority has on lock-down on your way to Capital Prime. This is a whole new driving area with a couple drones like Schnakers says.

If you've got that last area open and are still missing some it may be near the Northern Watchtower or Kvasir's lab. These were the most hidden jumps IMO, but most are out in the open. I found a few by just listening for the hum noise while driving or coming in & out of the sewers.