A car wash tunnel is not an obvious fire hazard, which is part of why it is worth assessing properly. Nothing in it is burning by design and there is water everywhere. The hazards are instead a combination of ordinary industrial exposures that happen to coexist in a difficult environment: substantial electrical load in a permanently wet space, a meaningful quantity of combustible plastic in the tunnel components themselves, and a chemical store holding acids, caustics, and solvents.
The electrical side is the most consistent problem. Conveyor drives, brush and wrap motors, high-pressure pumps, and particularly the dryer blowers represent a large connected load, and every one of those motors and its wiring sits in an environment of constant moisture, chemical mist, and washdown. Corrosion at terminations, degraded insulation, and moisture intrusion into enclosures produce the same tracking and arcing failure sequence as any other electrical installation, accelerated substantially by the conditions.
What surprises people is the fuel load. Modern tunnel components are largely plastic: closed-cell foam wraps and brushes, polyethylene and PVC piping and manifolds, plastic guards and splash curtains, and frequently plastic wall and ceiling liner panels. That is a continuous run of combustible material along the entire tunnel length, positioned directly above and around the electrical equipment most likely to start a fire. Once ignited, the tunnel provides both the fuel and the path.