There is something about a moving car that just works magic on crying babies. Even the crankiest is bound to fall asleep once you do a few laps around the block, while maintaining your car at a steady pace. Unless your baby is crying because they are sick or something is seriously off, this always works.
What is it about moving cars that puts babies to sleep?
It appears that there is actually a scientific evidence to explain why this happens.
The Car Vibrations Mimic The Internal Womb Conditions
Apparently, the vibrations in the car lull the baby to sleep because they are similar to the vibration in the mother’s womb.  The baby quickly falls to sleep because they are comforted by the familiar vibrations. The low level rocking is similar to the movement of the mother as she used to go about her daily activities. Science has shown that the gentle rocking regulates the sensorimotor system and calms the baby. It also has an effect on the parasympathetic system through the cerebellum.  This delays the panic response and promotes relaxation and sleep.
In spite of any commotion in the environment the baby will still feel secure and protected.
Additionally, the inside of your car is likely to be warm, cozy, and very unstimulating. Colors are uniform and most likely dull and uniform. The baby can feel your presence close by as you are also tuned-out from the external environment. This kind of bonding environment mimics the conditions of the womb.
What About White Noise
White noise is a lull kind of noise that has an even frequency and intensity. This kind of noise is what the baby hears when they are in the womb. It the sound of everything going on the mother’s body such as the heart pumping blood and the process of digestion.
The noise of your car’s engine is humdrum and consequently a kind of white noise. White noise has the ability to mask other noises in the surrounding, even when they are loud. That is why white noise puts babies to sleep even in a stimulating environment. 
One study showed that white noise increases the chances of babies falling asleep in five minutes by 25% to 80%.
Cars are not the only producers of white noise. White noise is equally produced by rain, ocean waves, hair dryers, fans, and vacuums. These could also be used to lull your baby to sleep.
Next time your cranky baby falls asleep in the car, think of the gentle vibrations and white noise that reminds them of being in the womb.
1- Esposito, G., Yoshida, S., et al. (2013). Infant calming responses during maternal carrying in humans and mice. Current biology : CB, 23(9), 739–745.
2- Gianluca Esposito, et al. (2013). Infant Calming Responses during Maternal Carrying in Humans and Mice. Current Biology. Volume 23, Issue 9, Pages 739-745.
3- Spencer, J. A., Moran, D. J., Lee, A., & Talbert, D. (1990). White noise and sleep induction. Archives of disease in childhood, 65(1), 135–137.