We experience higher or lower g forces when we are rapidly changing speeds or directions. Normal humans can withstand no more than 9 g's, and even that for only a few seconds.
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Under 9 g, you're going to have a very hard time moving your head sideways or backwards, seeing as it feels nine times as heavy as it actually is. When a passenger aircraft takes off and we are suddenly stuck to our seat – that's a force equaling approximately 2 g. Rollercoasters usually only go as far as 4 g.
While commercial flights exert only very minimal positive and negative G-forces on passengers, several orders of magnitude greater are the G-forces experienced by astronauts, fighter pilots and stunt pilots. These types of pilots can experience brief periods of extreme forces of nine and 10 Gs.
100 mph is about 45 m/s, so the accelleration would be a little under 0.5g. 100 km/h is about 28 m/s, so the accelleration would be a little under 0.3g.