Reeker

A family is driving down a desert highway when they hit a deer. The dad gets out and comes back with half his face gone. The family is attacked by something.
In a lavish apartment, Trip meets with a dealer, Radford, who provides him with pills for a rave in the desert. Trip steals all of the pills. He meets with his friends Cookie, Nelson, Jack and Gretchen. The group notices an overturned car on the highway, which they dismiss as an abandoned accident.
Gretchen discovers that Trip is carrying drugs and stops the vehicle to kick him out. While parking at the side of the road, the group notices a short, but strong tremor. Gretchen agrees to take Trip back to a diner, which has been abandoned. The car runs out of gas and breaks down, so they decide to stay at the Halfway Motel. Learning from the radio that the highway has been closed, Trip decides to walk down the road for help.Jack and Gretchen pitch camp and sleep outside. After a haunting experience with the drug dealer Radford, Trip escapes and meets a man named Henry driving an RV, who is looking for his missing wife. He goes with Trip to the motel and sets up camp there. Trip doesn't tell his friends about his run-in with Radford, but warns them to be careful.
Looking for signal reception for his mobile phone, Trip releases from a rubbish skip the still-living head, torso and arms of a truck driver who then crawls away. Henry then meets Trip and discovers they both are seeing the dead people. While in his camper, Henry collapses, suffocating as a dark figure moves through the trailer. Still looking for reception, Trip is attacked on the roof by a hooded figure. Cookie is killed while sitting in an outhouse by being dragged into the hole.
Nelson is almost dragged under his bed, but pulls himself back up and jumps on top of the bed. He takes his sneaker off and drops it on the ground, watching as it is shredded. He then tries to escape by jumping through the window, but fatally cuts his throat on a shard of glass. Gretchen and Jack discover Nelson's and Henry's bodies, prompting Gretchen to look for Trip. Jack encounters the creature but escapes when Trip shoots at the figure with his gun. Trip is overpowered by the creature and loses his arm, then dies.
In reality an RV crashed into Gretchen's car after Trip stepped out to call for a ride, at the moment when they experienced the tremor. None of the group had noticed the RV, whose driver was Henry. His wife, Rose had explained that he was suffering a heart attack and lost control. The car that the group saw leaving the diner was in fact their own.
Each of the deaths at the hotel were reflected by their own deaths in the car: Cookie died from internal bleeding, Nelson cut his neck when he was thrown through the windshield, Trip's arm was severed gripping the cell phone and he died from blunt force trauma. Gretchen survived the crash because she was wearing her seat belt and Jack, despite receiving a massive head injury, also remained alive in the car. Radford, who had been stalking Trip, witnessed the accident and attempted to assist; hence the visions of him at the motel and highway.
In the final scene, Gretchen and Jack briefly discuss the fact that neither has any recollection of the accident, and no mention is made of the incidents at the motel. Jack comments that, for a moment in the crash, he thought he could see Gretchen, offhandedly mentioning the color of her eyes.

Question: What does the group notice while parked on the side of the road ?
Answer: A tremor.