A valuable model is one that is simple, yet describes the core phenomenon. An example of such a model, we claim, is the Chemical Process Quantitative Risk Analysis (CPQRA) published in 1989. Current approaches to tunnel safety include QRA which, to an outsider, lacks transparency and resembles a black box. Although complex and time consuming, uncertainty in the QRA output is believed to be several orders of magnitude. The purpose of this paper is to present a simple model that is relevant for tunnel fire safety. Fire hazards restricting a safe evacuation are modeled using simple empirical physical formulas. The formulas are validated against experimental data from one full-scale tunnel fire experiment. Hazard characteristics are visualized and analyzed in a time-distance diagram downstream a tunnel fire.