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
- The road is read dry and 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
- Nothing reads the segment's surface closely enough to say. Never shown as clear, and never told to proceed.
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: packs.
| 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 |
| Not known, and warmer | Not known | Not known | Not known |
| None of these, on a dry road | 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.
Packs: a fleet's own limits
The rules above are two packs of data, one for a truck and one for a robotaxi. A fleet sends its own vehicle's limits as odd, JSON of the names below, to /api/corridor (with class), /fleet/exceptions.json, /wdms/rwm/download and a hook. Every answer says the pack it was judged by, what it took, what it refused and why, and the pack's hash. A limit may be stricter at will. It may never ask less than the floor: ice is never less than slow, a flood never less than route around, a warning that closes the road never less than exit, and a road not known is never proceed.
| Name | What it sets | Truck | Robotaxi | May be set |
|---|---|---|---|---|
iceP | Ice: the chance of ice above which the ice action applies | 0.3 | 0.3 | 0.05 to 0.5 |
ice | Ice: the action | Exit | Slow | never less than slow |
visExitMi | Visibility: the miles under which the first action applies | 0.1 | 0.1 | 0.05 to 0.5 |
visExit | Visibility: the first action | Exit | Slow | never less than slow |
visSlowMi | Visibility: the miles under which the second action applies | 0.25 | 0.25 | 0.1 to 2 |
visSlow | Visibility: the second action | Slow | Slow | any action |
gustMph | Gusts from any way, in mph, at or above which | 58 | 58 | 30 to 75 |
gust | Gusts: the action | Exit | Exit | never less than slow |
crossMph | Gusts across the road, in mph, above which | 40 | 40 | 20 to 60 |
cross | Gusts across the road: the action | Exit | Slow | any action |
crossEmptyMph | Gusts across the road, in mph, above which an empty trailer | 30 | - | 15 to 50 |
crossEmpty | Gusts across the road, empty trailer: the action | Exit | - | any action |
warning | A snow squall, dust storm, tornado, extreme wind or high wind warning | Exit | Exit | never less than exit |
flood | A flash flood warning or emergency | Route around | Route around | never less than route around |
snow | Snow or slush on the road | Slow | Slow | any action |
wet | A wet road | Lengthen the gap | Lengthen the gap | any action |
coldUnknown | Not known, 37 F or colder | Slow | Slow | never less than slow |
curl "https://stormstation247.com/fleet/exceptions.json?c=i45&vehicle=robotaxi&odd=%7B%22ice%22%3A%22exit%22%2C%22visSlowMi%22%3A0.5%7D"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. The only reading that makes a road dry now. |
| The forecast an hour or two ahead, sure of itself (confidence 0.5 or more) and above 37 F, with nothing falling | Dry. |
| 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, or no report | Not known: an airport that sees nothing falling says nothing of a road surface miles off. |
| 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, measured by a road station's pavement or an instrument's air | Icier than the road beside it. |
| A bridge with no measured temperature, forecast at or below 37 F or with none at all | Not known. |
| Two vehicles on the segment in the hour, each with wipers on and no traction lost, where the pavement was measured above 35 F, by a road station within 10 miles or by two vehicles' own road surface sensors | Wet, where the road was not known. One vehicle moves nothing, and an airfield's air temperature never stands in for the pavement (the road weather message). |
| One vehicle's traction or brake flag with the pavement at or below 32 F | An ice candidate, shown and never called. A vehicle's own air temperature never sets the pavement. |
| Two vehicles' own road surface sensors on the segment in the hour, within 3 F of each other, where no road station within 10 miles reads the pavement | The pavement, their mean, as measured. One vehicle's reading is not used. |
| An ice, freezing rain, snow squall, blizzard or winter storm alert on the segment | What it warns of is more likely. |
Why a road is not known
Every not known segment says why, in one clause, and carries the id of the rule that fired (in the API, rule and notKnown).
| Rule | The clause |
|---|---|
not-known-gap | gap, no instrument within 15 miles |
not-known-stale | report older than 90 minutes |
not-known-bridge | a bridge with no measured temperature |
not-known-nothing | nothing reported that reads the road |
not-known-temperature | temperature only |
not-known-no-surface | nothing falling, and no road surface reported |
not-known-unsure | the reading unsure, or the forecast unsure |
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. A road is dry only where a road station says so or a sure forecast does; anywhere else with nothing falling it is not known. 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.