A traffic warning sign is a type of traffic sign that indicates a hazard ahead on the road.

In most countries outside of North America, they often take the shape of an equilateral triangle with a thick red border and a white background.

In the People's Republic of China, however, they appear with a black border and a yellow background. In Sweden, Serbia and Montenegro, Croatia, Finland, Slovenia and Macedonia they have a red border with a yellow background, this is due to the weather as easier to see a red/yellow sign in the snowy weather then a red/white sign.

In the United States and Canada, warning signs are black on yellow and usually diamond-shaped, and construction signs are black on orange. Ireland also adopted these standards, diverging from UK standards. Some other countries also use these standards for some signage.

Warning signs

Warning signs can indicate any potential hazard, such as:

General Caution

Non-specific warning to pay attention, the word "Caution," often for ambiguous reasons.

Obstacles

Diamond-shaped with reflectors are placed at point of curbs, dividers, or other lane obstacles. Rectangular signs with diagonal stripes indicate solid objects such as barricades, bridge abutments, utility poles or natural obstacles near the roadway. Left side obstacles are marked with stripes running high to low, left to right; right side obstacle signs use stripes running high to low, right to left; in a sense akin to International symbol of arrow pointing down toward side toward roadway.

Things Crossing the Roadway

Warns of wild animals (moose, bear, elk, deer, wallabies, kangaroos, aligators, etc) or farm animals (cows, horses, ducks, sheep) that can stray onto the road. Also equipment (tractors, forklift, snowmobiles, golf carts, bicycles, handicapped) crossing the road.

Road works or construction

Indicates road works (construction), poor roads, or temporary conditions ahead on the road including: flagmen, survey crew, single-lane, detour, bridge out, utility crew ahead, blasting area, bump, dip, frost heaves, flooding (or "High water"), soft shoulder, uneven pavement, freshly oiled road, loose gravel, smoke on road, trucks entering, etc.

Bends and Turns

Used to indicate unexpected bends in the road, some being continuous, others being sharp-ended (right-angle turns). Left curve, right curve, series of curves. May also indicate direction of intersections along the upcoming curve. Also used to indicate "merge" with other traffic, as for an on-ramp of a limite-access highway.

Tunnels

Used to indicate tunnels, where lights are usually required, and a general change in the light level. May also indicate low ceiling clearance. Truck drivers should also watch for prohibited cargo signs (e.g., Hazmat, propane, explosives) upon approach to tunnels.

Bridges

Used where traffic may be constricted to a narrow bridge, or where the bridge may have a movable span closed to vehicles while boats pass (e.g., drawbridge or floating bridge). Also used for underpass to indicate low overhead clearance.

Traffic Lights

Used to indicate traffic lights, present when it is difficult to see that a traffic light may already be showing red, to warn a driver to prepare to slow down. May be supplemented with flashing light or lighted sign when light is red or turning red.

Warning Signs for Regulatory Signs

As for traffic signals, above, some "stop" or "yield" signs may require additional warning or reminder, especially in dense areas or where the sign has been added recently.

Level Crossings and Intersections

Warns drivers of road crossings at even level (crossroads, T-intersection, forks (Y-intersection), rotary/roundabout). May also indicate "hidden driveway" intersecting the road ahead. (Compare with bridges/overpasses/viaducts).

Lane Starts/ends

Indicates when a multilane highway is being narrowed, a passing lane is ending, or where the road is widening or a passing lane starting. Another type is used to indicate central "two-way" left turning lane in center of roadway. Warning signs may also warn of "Highway ends", where the road changes class or type. Also used for "dead end" or "no outlet" roadways.

No Passing Zone

Vertical yellow sign in triangular shape (in U.S.) to supplement solid yellow lane stripe where it is dangerous to overtake another vehicle, especially where roadway may be obscured by snow or other precipitation.

Pedestrians

Used to warn drivers of people walking in the street. Also used to warn of children playing, playgrounds, bicycle area, deaf child, blind pedestrians, and thickly settled zones where pedestrians may enter the road.

In parts of the United States near the southern border with Mexico, there are warning signs showing a running family. This is to warn motorists to look out for illegal immigrants who try to escape authorities by running through freeway traffic.

Schools

For school zones (slow down), student crossings (pointed at top), crossing guards or signals ahead.

Fire stations

Warning upon approach to where firefighters may be entering the road with fire engines or other emergency apparatus, where other drivers will have to stop and wait until they pass.

Oncoming Traffic

Used to warn people of oncoming traffic; shown when a motorway becomes a dual carriageway or a normal road without a central reservation or median.

Railway Crossings

Used to warn people of railway crossings ahead. The actual crossing is marked with crossed "Railroad Crossing" sawbuck signs (Stop, look, listen) and possibly lights, bells, and barriers.

Falling Rocks

Used to indicate the hazards of fallen or falling rocks on the road ahead. May be works or pictographs. May be "Fallen rock", "falling rock", or "rock slide" areas posted as such.

The Unexpected

For example, a warning sign with the image of an aircraft in the middle of it indicates an airport or airfield, where drivers should be prepared for low-flying aircraft.

General dangers are signalled by placing a black exclamation mark in the middle of the red triangle.

Road conditions

"Slippery when wet", warnings for motorcyclists of "grooved pavement", "Open joints on bridge", "Icy Road", "Bridge freezes before roadway," also "bump" or "dip" ahead (not related to construction). Truck drivers will need to pay attention to "Steep grade" warnings (or "Down grade, use lower gear"), sometimes posted with the percent grade (e.g., 5 percent), but they may be relieved to see "Runaway truck escape" near the bottom of the hill! The U.K. has a sign warning of "Adverse camber" on a curve. Also "Loose gravel", "Soft shoulder", "Speed hump", and "Watch for Ice."

Side Wind

Flying socks, as indicated by a windsock in a red triangle, indicate locations where a strong side wind may cause the trajectory of the moving vehicle to change drastically, perhaps even "flying" across lanes, causing an accident.

Slow Down

Used at least in U.S. and China, this sign advises drivers to slow down.

Merge To Stay With Through Traffic

In the United States, there is special signage for lanes that are about to exit, so that drivers who wish to remain on the main road have adequate time to merge.  All such lanes are sometimes indicated by special striping ("alligator stripes") and the sign, "Thru Traffic Merge Left" (or right).  On freeways, the green directions sign for the exit ramp may have the additional notation, "Exit Only," and should have black letters on a yellow background for emphasis.