The gravity of the situation

Actually, I think there is a gravity to the situation of common collisions on the shoulder. Looking up the potential slope of roads, I see there's a cross slope to them:

"Cross slope, cross fall or camber is a geometric feature of pavement surfaces: the transverse slope with respect to the horizon. It is a very important safety factor. Cross slope is provided to provide a drainage gradient so that water will run off the surface to a drainage system such as a street gutter or ditch. Inadequate cross slope will contribute to aquaplaning. On straight sections of normal two-lane roads, the pavement cross section is usually highest in the center and drains to both sides. In horizontal curves, the cross slope is banked into superelevation to reduce steering effort and lateral force required to go around the curve. All water drains to the inside of the curve. If the cross slope magnitude oscillates within 1–25 metres (3–82 ft), the body and payload of high (heavy) vehicles will experience high roll vibration... Typical values range from 2 percent for straight segments to 10 percent for sharp superelevated curves. It may also be expressed as a fraction of an inch in rise over a one-foot run (e.g. 1/4 inch per foot)."

So it seems like parking on the shoulder at the inside of a corner would be the worst place (with the largest gravitational pull).

For instance, an investigation of fatal motor vehicle crashes on two-lane rural highways says "of the investigated crashes, 87.8% were superelevated, and 93.2% had shoulders . . ."

The Federal Highway Administration also lists Potential Adverse Impacts to Safety and Operations of both Cross Slope and Superelevation, which includes "run-off-road crashes on freeway, expressway, and rural 2-lane, etc. Another potential safety concern is present when a horizontal curve lies at the bottom of a steep grade. This combination of alignments increases the risk of severe run-off-road crashes."

Oh yeah, so parking along a curved shoulder is generally high risk, as the momentum of another vehicle running off road keeps it in line with that area (especially where it's sloped). I know this seems like common sense, but we can't always see the slope of the road, yet it's pretty much always there (just like gravity, because that's what it is).

 

There's always more to learn about gravity. It seems the main thing to consider about roads is that they involve trade-offs in their geometry, to correct for environmental conditions, and different sized vehicles.

Involving commercial motor vehicles . . .
"In some cases, crashes may occur or persist at certain roadway locations due to the geometrics of the road".

Likewise, cross-slope breaks are something to consider about the gravitational pull of road geometry. "The shoulder cross-slope may not follow the superelevation, depending on drainage and other design considerations. This break in the cross-slope can be an issue for vehicle stability, particularly for vehicles with a high center of gravity such as CMVs."

"The difference between the cross-slope of the shoulder and the cross-slope of the adjacent travel lane is called the roll-over."

Evaluation of Geometric Parameters that Affect Truck Maneuvering and Stability . . .

"The authors found that the height of the center of gravity of the vehicle and the load were significant factors in the roll behavior of trucks tested. Loads with high centers of gravity or with centers of gravity further forward than the center of the trailer may experience lower rollover speeds... The authors offered the opinion that the greatest reduction in rollover crashes may be reached by educating drivers that high speeds and sharp turns are the primary contributing factors to rollovers, and that rollovers can occur even at very low speeds within tight curves."

Some roads are not designed appropriately for trucks to follow the posted speed limit.

"Twice in just over a week, semi-trucks have rolled over on North Century Boulevard at Jackson Street in a curve town officials believed was ill-designed. And Wednesday, the Florida Department of Transportation agreed something needs to be done... FDOT traffic safety program engineer at the site of the accident. McCall said the slope and design of the roadway in the curve is such that trucks in the outer lane have a difficult time navigating the curve at the posted speed limit" . . .

Speed Concepts
"Negative superelevation is actually quite common. Think of a road with a normal crown (i.e., high point in center and sloping outward in both directions). If the normal crown is maintained through a curve, both sides have superelevation but with opposite signs. For a curve to the left, the right lane of a normal crown section has negative superelevation . . ."

Gravitation based on road geometry can also involve pedestrians.

Accessible Rights of Way

"Excessive cross slopes on curb ramps can make them unusable by most pedestrians with mobility impairments. The front casters of a wheelchair impart a downhill turning tendency when a cross slope is encountered. Counteracting this unwanted cross force requires manual or battery energy that is then not available for forward travel. Under slippery surface conditions, a loss of control or overturning can result. Such incidents usually end in the street. If a cross slope is extreme, scooters and power chairs with high centers of gravity risk tipping; sideways tips and falls are more common in power chairs and scooters, which are more frequently the aid of choice for outdoor travel. Ambulatory pedestrians who have gait impairments and those who use walking aids will have difficulty maintaining balance and continuing forward travel on curb ramps with excessive cross slope. Curb ramp cross slope in new construction should not exceed 1:48 (2%). Where street crossings are planned at midblock or at T-intersections on sloping roadways, careful engineering is required to blend curb ramp, landing, and crosswalk at the vehicular way. In alterations, it may be necessary to accept a steeper cross slope because of the difficulty and cost of correction."

 

Pedestrian Falling Accidents in Transit Terminals . . .
"The larger ratio of male head injuries in falling accidents may be associated with the males’ higher center of gravity and greater body mass. These two factors would tend to increase momentum of the male fall, making it more difficult to arrest before a head impact."

Oh yeah, so the bigger they are, the harder they fall.

Comprehensive Truck Size and Weight Study . . .
"Rollovers account for 8 to 12 percent of all combination truck crashes, but are involved in approximately 60 percent of crashes fatal to heavy truck occupants. They greatly disrupt traffic when they occur in urban environments."