For a very quick dirty fix, select (lasso) all terrain anchors immediately surrounding the pitlane. in topview or in 3D, then press "4" to get the full view. Then zoom to any of those selected anchors in either of the sideviews. Zoom very, very close and move the selected terrain anchors couple of centimeters down. 2-5cm will do, that way your car can get back to the track, more than 10cm and they get stuck. But... it's not the right way of doing this, unfortunately.. Reasons for this are complex and important enough to sometime soon learn how to do it properly. Check Simtrakcipedia for tutorials.
For proper way of doing this: you need to create terrain for both the pit-lane and the track separately and then join them. So you pretty much have to delete all terrain between and immediately surrounding the pit-lane, everywhere that the terrain goes beneath the pit-lane (rule of thumb is: no terrain under tracks, track and terrain should join seamlessly!! This is the true for 99% of the cases)
Then select the main track first (activate it in the edit&switch active track menu, not in the terrain tool) and create a new terrain with drag&click tool using settings: one panel, right side (or left, not sure which way this part of your track is going) and try to make the panel width under half of the distance between those two tracks, less is better here, you wan't to have some sort of a gap (it's visually easier to do the next steps). Don't create a new terrain on the crossroad of the pit lane and main track. Then select the pit-lane and create a new terrain to that using drag&click and both sides, one panel.
Then you need to make sure "Anchor auto merge" is on and "Top view only" is off in the terrain tool, Shape tab. Then drag one of the anchors and drag it in top of the closest terrain anchor of the second track and let go. Those two anchors should now merge. Repeat that and patch up/weld the rest of the terrain. You can create new triangles with "Pull the Edges..." tool, just select that and click any terrain edge to get a new triangle. Most likely you need to arrange and create new tris since those two tracks will not have the same panel lengths... Just remember to switch that "Top view only" back on before you start shaping the terrain more, the option is only useful when doing this kind job.