The mechanism, stated plainly
In a rear-end impact the struck vehicle is pushed forward. The occupant's torso accelerates with the seat, the head lags for a fraction of a second, then whips forward and back through extension and flexion. That sequence — acceleration, lag, rebound — is the whiplash mechanism, and it can strain the neck's muscles, ligaments, and joints without the head striking anything.
The clinical frame is the Quebec Task Force's WAD classification: whiplash-associated disorders graded 0 through IV. The grade is set by findings, not by vehicle damage, and not by how quickly symptoms appeared.
The grades
| Grade | Presentation |
|---|---|
| WAD 0 | No complaint about the neck; no physical signs. |
| WAD I | Neck pain, stiffness, or tenderness only; no physical signs. |
| WAD II | Neck complaint plus musculoskeletal signs, such as decreased range of motion or point tenderness. |
| WAD III | Neck complaint plus neurological signs, such as weakness or sensory changes. |
| WAD IV | Neck complaint plus fracture or dislocation. |
The San Diego frame
The mechanics are statewide — the same acceleration-lag-rebound sequence plays out wherever traffic collides. What is local is the geometry: San Diego County concentrates rear-end risk where freeway speed meets the merge queue. The I-5/I-805 merge pattern puts moving traffic beside slowing traffic; intersection impacts load the neck in rotation; and a rear-end on the 5 at speed is the same whiplash mechanism as anywhere in the state, delivered in San Diego traffic.
The statewide desk carries the full whiplash file — the grades in depth, the healing schedule, and the documentation frame: californiasofttissue.com — the statewide whiplash file (editorial cross-link).
The law around the merge
California requires drivers to stay within a marked lane (Vehicle Code §21658) and to move between lanes only when the movement can be made with reasonable safety (§22107). These rules exist because lane transitions are where the merge pattern becomes a collision pattern — and the record of who moved when is what the claim file reads later.
The same record rules apply. A whiplash injury from a San Diego merge follows the same documentation clock as any collision injury: the SR-1 report, the first evaluation, and the two-year filing horizon. The documentation file carries the dates.