We recently had the chance to participate in the 7th annual Walk to Whittier. It's a 2.5 mile one lane tunnel through a mountain. Officials shut down all the traffic for a specified period of time so walkers can go through. We started the day by heading out to the Portage Glacier visitor center - a nice little drive on it's own.
Please return to your vehicle... There are signs to evacuate to safe houses only when strobes are flashing. No flashing strobes and an open door apparently leads to canned warnings. The pullouts at the safe houses are electronically monitored - when the line is crossed an alert goes to the camera station so they can send out whatever assistance is needed. This includes towing for broken down vehicles, and even help for fixing flats. Since the tunnel is only one lane, they need to get vehicles out so they can switch direction, or clear everything to let the trains through.