It is a certain spacial reality check. While 1000 feet may seem quite a bit, vectoring it out the line of site, (especially without ref at 39K feet), it may seem about level.
An Aircraft such as a 747, is 250 feet long, so the 1000 feet is only 4 aircraft lengths in spacial reference. That will appear very close whn looking out the windows.
Now, if you can see a shocked look on the drivers face,
then I would be concerned.