I'm obliged to Wirecutter for posting this image as #6 in his daily GIF blog post. It shows an airbag going off in front of the passenger seat of a car, with a crash test dummy's legs positioned as I've often seen some passengers ride - with their feet propped up on the dashboard. I routinely see people - particularly adolescent children - doing this when I drive on Interstate and regional highways.
That shows graphically why it's not a good idea to ride like that. As a pastor, there have been two occasions when I had to counsel a family whose daughter put her feet up on the dashboard, only to have them severely injured when an accident occurred and the airbag went off. In one case, the injuries were so severe that she (at 15 years of age) had to have both knees replaced, with who knows what complications that's likely to cause as she grows older.
NOT a good idea! Keep your feet down where they belong!