Rules
What each vehicle is told on a segment, and why. Every rule that applies is given as a reason; the action is the most severe.
The actions
- Proceed
- Nothing on the segment calls for a change.
- Lengthen the gap
- Keep a longer following distance: the road is wet.
- Slow
- Reduce speed for the surface, the visibility or the wind.
- Route around
- Leave this stretch out of the route: water may be over the road.
- Exit
- Leave the road at the next safe stop and wait.
- Not known
- No instrument reads the segment closely enough to say. Never shown as clear.
The rules
The defaults, for a truck with a high trailer, empty or loaded, and a robotaxi able to drive in snow. A fleet sets its own.
| On the segment | Truck, empty | Truck, loaded | Robotaxi |
|---|---|---|---|
| Ice likely (a chance over 0.3) | Exit | Exit | Slow |
| Visibility under 0.1 mile | Exit | Exit | Slow |
| Visibility under 1/4 mile | Slow | Slow | Slow |
| A gust of 58 mph or more | Exit | Exit | Exit |
| Gusts over 40 mph across the road | Exit | Exit | Slow |
| Gusts over 30 mph across the road | Exit | Proceed | Proceed |
| Snow Squall, Dust Storm, Tornado, Extreme Wind or High Wind Warning on the segment | Exit | Exit | Exit |
| Flash Flood Warning on the segment | Route around | Route around | Route around |
| Snow or slush on the road | Slow | Slow | Slow |
| A wet road | Lengthen the gap | Lengthen the gap | Lengthen the gap |
| Not known, and cold enough to freeze | Slow | Slow | Slow |
| None of these | Proceed | Proceed | Proceed |
Wind across the road is the gust times the sine of the angle between the wind and the road’s heading on that segment: a 35 mph gust square across the road is 35 mph across it, and along the road it is none. An empty high trailer slides on a slick road at about 29 mph of crosswind and can overturn at about 40; a loaded one stands far more.
Method
A corridor is cut into quarter mile segments from the national highway file; each bridge is a segment of its own, because a deck freezes before the road beside it. Each segment is read against every hourly instrument within 15 miles that reported in the last 90 minutes, and the next two hours against Storm Station’s own forecast, read every 20 miles along the road.
| Evidence | What it says |
|---|---|
| A road station within 10 miles, reported within the hour | The surface it reports, and its pavement temperature. |
| Freezing rain, freezing drizzle or ice pellets | Ice. |
| Snow | Snow at or below 34 F; slush above it. |
| Rain or drizzle | Wet; ice where the pavement, or with no pavement sensor the air, is at or below 32 F. |
| No weather reported at an airport | Dry. |
| A temperature and nothing else | Neither dry nor wet: not known. |
| Pavement at or below 32 F and at or below the dew point | Frost or black ice likely. |
| A bridge at or below 37 F | Icier than the road beside it. |
| An ice, freezing rain, snow squall, blizzard or winter storm alert on the segment | What it warns of is more likely. |
Every answer carries a confidence from 0 to 1. It is highest where a road station near the segment reported minutes ago, and falls with distance, with age and with each hour ahead. Where it is 37 F or colder and the confidence is under 0.5, the surface is not known and every vehicle is told to slow. Each answer names the instruments that gave it, and is to be scored the next day against what the road stations later report, as the forecasts are.