South Dakota Road Signs: Fines Double Sign, Bus Rules & DMV Test

What is the "Fines Double" sign in South Dakota and where does it appear? How many attempts do you get on the South Dakota knowledge test? What are the work zone rules on I-90 and I-29? The SD Driver Licensing Division, DPS at dps.sd.gov/driver-licensing allows three opportunities to test for each fee paid within a six-month period, and the knowledge test requires a score of 80% or higher. In South Dakota limited work-zone speed zones, signs warning that fines will be doubled are posted in advance of the regulatory speed limit signs, and speeding fines are doubled while workers are present. When a stop is required for a school bus, stop at least 15 feet away and remain stopped until the flashing red signal lights are extinguished.

1. South Dakota "Fines Double" Sign, 3-Attempt Knowledge Test and What Driving the Great Plains Demands

South Dakota roads stretch from Sioux Falls I-29/I-90 interchange and Rapid City Black Hills gateway to the wide-open US-14 and US-83 prairie corridors and the Missouri River crossings near Pierre. The SD Driver Licensing Division, DPS at dps.sd.gov/driver-licensing requires vision, knowledge, and road skills tests. Application fee is $28. You are allowed three opportunities to test for each fee paid within a six-month period, and a failed test may not be retaken before the next working day. The application form may be completed before visiting a Driver Licensing location. In limited work-zone speed zones, South Dakota law requires advance signs warning that speeding fines will be doubled while workers are present.

2. South Dakota Sign Shapes: The "Fines Double" Regulatory Sign and What the Knowledge Test Covers

South Dakota law requires signs warning that a speeding fine will be doubled to be erected in advance of regulatory speed limit signs in limited work-zone speed zones when workers are present. On the vast open highways of the Great Plains, where a sign may be the only object visible for miles, shape and color register before text. Know every sign type in this table before your SD knowledge test.

Shape	Diagram	Meaning	Example Sign
Octagon (8-sided)		STOP. come to a complete stop, wheels fully stopped, check all directions, then proceed when clear
Triangle (inverted)		Yield slow down, check traffic; if clear proceed, if not stop and wait until safe to go
Diamond		Hazard ahead; slow down, stay alert, and be ready to stop or steer
Rectangle (vertical)		Regulatory information or instruction; obey the posted requirement
Rectangle (horizontal)		Informational sign; read it and use it to navigate or plan your route
Pentagon (5-sided)		School zone ahead; reduce speed and watch for children crossing
Round (circle)		Railroad crossing ahead; slow down, look both ways, and prepare to stop
Pennant		No passing zone; do not attempt to overtake any vehicle ahead
Orange (any shape)		Construction or work zone ahead; slow down, stay alert, and obey all posted signs
Blue (any shape)		Motorist services nearby; exit if you need fuel, food, or medical care
Brown (any shape)		Park or scenic area ahead; follow directional signs to reach it

3. South Dakota Regulatory Signs: Point System, Work-Zone Fines Double Signs and SDCL Violations

Regulatory signs carry legal force under South Dakota Codified Law (SDCL). Some sign-related violations add points to your SD driving record: failure to yield and improper passing are four points, and a stop sign/light violation is three points. South Dakota law expressly excludes speeding from the "other moving offenses" point category. In limited work-zone speed zones, advance signs warn that speeding fines are doubled while workers are present.

STOP Sign

Come to a full stop and yield before proceeding

YIELD Sign

Slow down and give right-of-way; stop if needed before proceeding

Speed Limit Sign

Do not exceed this speed; adjust for road and weather conditions

Do Not Enter Sign

Entry prohibited; turn around or find an alternate route

Wrong Way Sign

You have entered a one-way road against traffic

No U-Turn Sign

U-turns are prohibited at this location

No Left Turn Sign

Do not turn left here; continue straight or turn right

No Right Turn Sign

Do not turn right here; continue straight or turn left

One Way Sign

All traffic flows one direction; do not enter going the wrong way

Keep Right Sign

Move to the right side of the island or obstruction

No Passing Zone Sign

Do not pass here; wait until the no-passing zone ends

Do Not Pass Sign

Do not overtake any vehicle ahead until passing is permitted again

No Turn on Red Sign

Wait for a green signal before turning; do not turn on red

No Left Turn on Red Sign

Wait for green before turning left; no turn on red permitted

No Right Turn on Red Sign

Wait for green before turning right; no turn on red permitted

Turn Left Only Sign

You must turn left from this lane; going straight is not allowed

Turn Right Only Sign

You must turn right from this lane; going straight is not allowed

Straight or Turn Right Sign

From this lane you may only go straight or turn right

Minimum Speed Limit Sign

You must drive at least this speed; move right if driving slower

Slower Traffic Keep Right Sign

Move to the right lane if you are driving slower than traffic

No Parking Sign

Parking prohibited here; find a legal parking area nearby

No Trucks Sign

Trucks prohibited on this road; use an alternate truck route

No Bicycles Sign

Cyclists must not enter; use an alternate route

No Pedestrian Crossing Sign

Pedestrians must not cross here; use the nearest crosswalk

Reserved Parking Sign

Park here only if you qualify; others must find a different spot

Four Way Stop Sign

Stop completely, then take turns in order of arrival

Stop Except Right Turn Sign

Traffic must stop except for the permitted right-turn movement; right-turning traffic must still yield as required

Left Turn Yield on Green Sign

Yield to all oncoming traffic before completing your left turn

Lane Use Control Sign

Follow the sign; only the shown movement is permitted from this lane

Restricted Lane Ahead Sign

Check eligibility before entering; unauthorized use is a violation

4. South Dakota Warning Signs: Black Hills Curves, Prairie Railroad Crossings and the Slow-Moving Vehicle Triangle

Diamond-shaped yellow warning signs flag hazards before you reach them. on South Dakota roads that means sharp curve warnings on Black Hills and Badlands approaches, multi-track railroad crossing warnings across the state extensive grain-rail network, school zone fluorescent signs preceding reduced speed limits, and the reflective slow-moving vehicle emblem on farm machinery, animal-drawn vehicles, and highway construction or maintenance vehicles designed for or operated at 25 mph or less. Orange marks work-zone warnings; in limited work-zone speed zones, speeding fines are doubled while workers are present.

Left Curve

A curve to the left is ahead; reduce speed

Right Curve

A curve to the right is ahead; reduce speed

Sharp Left Turn

A very sharp left turn is ahead; slow down

Sharp Right Turn

A very sharp right turn is ahead; slow down

Winding Road

Multiple curves ahead; reduce speed and stay in your lane throughout

Set of Curves (Left-Right)

Multiple curves ahead curving left then right; reduce speed

Set of Curves (Right-Left)

Multiple curves ahead curving right then left; reduce speed

Sharp Turns (Left-Right)

Successive sharp turns ahead; slow down significantly

Slippery When Wet

Wet road is slippery; reduce speed and increase following distance

Deer Crossing

Animals frequently cross here; slow down and scan both sides of the road

Pedestrian Crossing

Pedestrians may cross ahead; slow down and yield as South Dakota law requires

Bicycle Crossing

Cyclists may cross here; slow down, watch for bicycles, and yield when required

School Zone / Crossing

Children may be present; slow to posted speed

School Crossing Ahead

School crosswalk ahead; yield to crossing guard and children

Divided Highway Begins

Road splits ahead; keep right and stay in your lane

Divided Highway Ends

Two-way traffic resumes ahead; stay right and watch oncoming traffic

Merging Traffic

Traffic is merging from another lane; adjust speed to allow safe entry

Added Lane

An additional lane begins ahead; entering traffic has its own lane

Lane Ends Ahead

Your lane ends soon; check mirrors, signal, and merge now

Lane Ends Merge Left

Your lane ends; signal and move left before the lane disappears

Left Lane Ends

Left lane ends ahead; signal and move right before it closes

Road Narrows

Road gets narrower ahead; stay centered and reduce speed

Narrow Bridge Ahead

Bridge is narrower than the road; slow down and stay centered

Low Clearance

Overhead clearance is restricted; do not proceed if your vehicle is too tall

Railroad Crossing (RR)

Tracks ahead; slow down, look both ways, and stop if a train is coming

Hill / Steep Grade (Downgrade)

Steep hill ahead; reduce speed now and use a lower gear if needed

Bump

Speed bump ahead; slow down before reaching it

Dip

Dip in the road ahead; reduce speed to avoid bottoming out

Loose Gravel

Loose gravel on road; reduce speed and increase following distance

Pavement Ends

Paved surface ends ahead; slow down before the surface changes

Shoulder Drop Off

Edge of road drops steeply; stay away from the shoulder

Watch for Ice on Bridges

Bridge may be icy; reduce speed and avoid sudden braking or steering

Two Way Traffic Ahead

Divided road ends; move to the right and watch for oncoming traffic

Cross Road

Intersection ahead; slow down and scan left and right before crossing

Cross Traffic

Traffic crosses from both sides; slow down and be ready to yield or stop

T Intersection

Your road ends ahead; slow down and prepare to turn left or right

Side Road

A side road joins ahead; watch for vehicles entering from the right or left

Highway Intersection Ahead

A major intersection is approaching; reduce speed and be ready to yield

Road Entering Curve

A road merges on a curve ahead; watch for vehicles entering

Roundabout

Yield to traffic already in the roundabout; then enter and keep right

Truck Rollover

Trucks must slow significantly; high rollover risk on this curve

Weave Area

Lanes are entering and exiting; stay alert, check mirrors, and adjust speed

Advisory Speed

Recommended advisory speed for the condition ahead; reduce speed appropriately

Traffic Signal Ahead

A traffic light is ahead; reduce speed and be ready to stop

5. South Dakota Guide Signs: I-90, I-29, US-14 and the Routes From Sioux Falls to the Badlands

Guide signs do not regulate driving. they show destinations, distances, and services. Green = highway navigation. Blue = motorist services. Brown = parks and recreation.

Interstate Route Marker

You are on this interstate highway; note the number for navigation

US Route Marker

You are on this US highway; note the number for navigation

State Route Marker

You are on this state route; note the number for navigation

Exit Sign (Exit Only)

This lane becomes exit-only; move out of it if you are not exiting

Mile Marker

Marks your location on the route; note it for emergency or navigation use

Services Sign

Services available at next exit; move right if you need to stop

Hospital Sign (H)

Hospital is nearby; follow signs if you need emergency care

Diesel

Diesel fuel available ahead; exit if your vehicle needs it

Park / Recreation Sign

Park or recreation area nearby; exit and follow directional signs

6. South Dakota Traffic Signals: Flashing Yellow, Turn on Red and Inoperative Signals

What does a flashing yellow light mean in South Dakota? Flashing red = stop. Flashing yellow = proceed with caution. A right turn on a steady red signal is allowed after a complete stop and yielding unless a local ordinance prohibits the turn and a sign gives notice. A left turn on red from a one-way roadway onto a one-way roadway moving left is allowed only where a municipality has authorized it by ordinance. If a South Dakota traffic signal is completely unlighted, stop as at a stop sign. Lane-control signals permit travel in a lane under green and prohibit entering or traveling in a lane under red. South Dakota work zones may also use temporary traffic signals; obey the displayed signal and any stop line.

Standard Traffic Lights

Steady Red

Stop before the crosswalk or intersection; after stopping, a right turn may be allowed unless prohibited by local ordinance and posted sign

Steady Yellow

Slow down and prepare to stop; do not speed up to beat the light

Steady Green

Proceed straight or turn unless prohibited; yield to vehicles and pedestrians lawfully within the intersection or adjacent crosswalk

Green Arrow (left)

Proceed only in the arrow direction; yield to pedestrians

Steady Yellow Arrow

The signal is changing toward red for the arrow movement; do not enter after the red signal appears

Steady Red Arrow

Stop; a right turn after stopping may be permitted unless prohibited, while a one-way-to-one-way left on red requires local authorization

Circular Green (no arrow)

Proceed straight or turn right or left unless prohibited; yield to lawful traffic and pedestrians

Opposing Traffic Extended Green

Oncoming traffic still has green; wait and do not turn left yet

Flashing Signals

Flashing Red

Treat as a stop sign; stop fully, then proceed when safe and clear

Flashing Yellow

Slow down, check for hazards, and proceed with caution

Flashing Red Beacon

Treat the overhead beacon as a stop sign

Pedestrian Signals

WALK / White Walking Figure & DONT WALK / Orange Hand

White figure = cross now; orange hand = do not start crossing

Flashing Orange Hand + Countdown

Crossing phase ending; finish if started, do not begin

Key test point: Flashing red requires a stop; flashing yellow allows you to proceed only with caution. At a completely unlighted traffic signal, stop in the same manner as at a stop sign.

7. South Dakota Pavement Markings: Lane Lines, Railroad Crossing RR Symbols and Bicycle Lane Rules

Yellow center lines generally separate opposing traffic, while white lane lines separate traffic moving in the same direction. Edge lines are different: yellow may mark the left edge of a divided roadway and white may mark the right edge. South Dakota roadway markings also include lane arrows, stop lines, crosswalks, railroad X and RR markings, and bicycle-lane symbols.

Line Colors

Yellow lines

Yellow center lines separate opposing traffic; a solid yellow center line restricts passing

White lines

White lane lines separate same-direction traffic; white is also used for roadway-edge and other pavement markings

Line Types

Broken yellow line (Single)

You may pass on your side if it is safe and clear ahead

Solid yellow + broken yellow

Pass only from the broken-line side when it is safe; do not pass from the solid side

Double solid yellow lines

Passing is not allowed in either direction; stay in your lane

Broken white line

You may change lanes if it is safe; always check mirrors and signal first

Solid white line

A solid white line may mark a roadway edge or separate same-direction lanes where crossing is restricted or discouraged; follow signs and markings

Double solid white lines

Do not change lanes here; stay in your current lane

Center Turn Lane

For left turns only; signal, enter the lane, and turn; do not use it to travel

Center Turning Lane (sign)

For left turns only; signal, enter, and turn; do not drive through it

Pavement Symbols & Special Markings

Arrows in lane

You must follow the arrow direction from this lane

Stop line (wide white bar)

Stop completely behind this line; do not roll past it

Stop Here on Red

Stop at this painted line; do not inch forward past it

Crosswalk lines

Pedestrian crossing zone; yield to pedestrians when South Dakota law requires

Broken white line (pavement)

You may change lanes if it is safe; check mirrors and signal first

Solid white line (pavement)

A solid white line may mark a roadway edge or separate same-direction lanes where crossing is restricted or discouraged; follow signs and markings

Double solid white lines (pavement)

Do not change lanes here; stay in your current lane

Railroad crossing markings (X + RR)

Tracks ahead; slow down, look both ways, and be ready to stop

Diagonal yellow lines (gore area)

Diagonal stripes mark an area not intended for normal travel; follow the channelizing markings around it

Bicycle lane marking

Marks a designated bicycle lane; watch for cyclists and do not use it as a normal travel lane

HOV / Carpool lane diamond

Use this lane only if you have the required number of occupants

South Dakota statewide parking law relies on official signs and traffic-control devices; local authorities may impose additional parking restrictions, so follow the posted local rule rather than assuming one statewide meaning for every curb color.

Key test point: Yellow center lines generally separate opposing traffic and white lane lines separate same-direction traffic; edge lines and special markings can use those colors differently.

8. South Dakota Work Zone Signs: When Speeding Fines Double and What the Fines Double Sign Means

Orange signs with black letters or symbols mark construction and maintenance warnings in South Dakota. Under SDCL 32-25-19.1, speeding fines in a limited work-zone speed zone are doubled while workers are present, and advance signs must warn that the fine will be double before the regulatory speed-limit signs. Give construction workers a "brake", reduce speed, be prepared to stop suddenly, and do not tailgate in work zones.

Road Work Ahead

Construction zone begins ahead; reduce speed and stay alert

Flagger Ahead

A person is directing traffic; obey their signals

Workers Ahead

Road workers are present; slow down and be ready to stop

Active Work Zone

You are in an active work zone; drive with extra caution

End of Active Work Zone

Work zone ends; resume normal speed and check posted limit

Lane Closed Ahead

A travel lane ends ahead; merge early

Road Closed

Road is completely closed; follow detour signs

Orange barrels / cones

Active work zone boundaries; stay within marked lanes

Channelizing devices (barricades)

Follow the barricades; do not drive through or around them

• You must follow flagger instructions in South Dakota work zones. they direct traffic by sign or flag.
• In a limited work-zone speed zone, a speeding fine is double the usual fine while workers are present, subject to the statutory Class 2 misdemeanor maximum.
• South Dakota requires advance signs warning that a speeding fine will be doubled before the regulatory speed-limit signs where the doubled-fine rule applies.
• Work zone temporary traffic signals may be used. Obey the displayed signal and stop at the marked stop line when required.

Key test point: Orange always means construction or work zone. If you see orange, reduce speed and increase following distance immediately.

9. South Dakota Railroad Crossings: 15-50 Foot Stop Rule, Multiple Tracks and ENS Signs

How far from railroad tracks must you stop in South Dakota? When a law enforcement officer or a clearly visible or audible signal warns of the immediate approach of a train, rail car, or other on-track equipment, stop within 15-50 feet of the nearest rail and do not proceed until it is safe. SDDOT says crossbuck signs should be treated like yield signs, and a sign below the crossbuck identifies the number of tracks when more than one track is present. The blue-and-white Emergency Notification System (ENS) sign lists the railroad emergency number and the USDOT National Crossing Inventory Number for that crossing.

Stop Line

Stop here completely when required before proceeding

RR Crossbuck Sign (X-shaped)

Located at the crossing; yield to trains

Railroad Crossing (at crossing)

You are at the crossing; stop if lights flash or gate lowers

Crossing Gates

Lower when a train is coming; never drive around a lowered gate

Pavement markings (X + RR)

Tracks ahead marked on pavement; slow down and prepare to stop

Flashing Red Lights

A train is approaching; stop and wait until fully clear

• The round yellow warning sign with "X" and "RR" warns that an SD railroad grade crossing is ahead. slow down, look, and listen.
• The white crossbuck X sign is treated like a yield sign. Slow down, look and listen, and stop when a warning signal or approaching train requires it.
• When alternating lights flash, you must stop. At crossings with gates, never drive around them.
• SD crossings with multiple tracks may post the number of tracks. but not all do. Always check for a second train before crossing after the first passes.
• Vehicles carrying passengers for hire, school buses carrying passengers, certain nonprofit passenger vehicles, and vehicles carrying explosive or combustible/flammable cargo generally must stop at railroad grade crossings, subject to statutory exceptions.
• Use the blue-and-white ENS sign to report a crossing emergency or safety problem; it provides the railroad emergency number and the USDOT crossing inventory number.

Key test point: When an immediate train or on-track-equipment warning requires a stop, South Dakota law requires stopping 15-50 feet from the nearest rail. Check every track before proceeding, and use the ENS sign information to report crossing emergencies.

10. South Dakota School Zones and School Buses: 15 mph, 15-Foot Stop Distance and Four-Lane Rule

Pentagon-shaped, fluorescent yellow-green signs mark SD school zones. South Dakota law sets a maximum of 15 mph when passing a school during recess or while children are going to or leaving school during opening or closing hours, and posted local school-zone traffic-control devices must also be obeyed. For school buses, the rules depend on the roadway: when a stop is required for flashing red lights, stop at least 15 feet from the bus and remain stopped until the red lights are extinguished.

School Zone

School ahead; reduce speed and watch for children

School Speed Limit

Obey posted school-zone controls; state law sets 15 mph when passing a school during the specified child-arrival, dismissal, or recess conditions

Speed Limit Lights Flashing

When a posted school-zone speed sign is activated by flashing lights, obey the displayed restriction

School Crossing Ahead

School crosswalk ahead; yield to crossing guard and children

• South Dakota law sets a 15 mph maximum when passing a school during recess or while children are going to or leaving school during opening or closing hours. Also obey posted school-zone traffic-control devices.
• At a school crossing with a traffic patrol officer, stop and yield if signaled to do so.
• On a highway with less than two lanes of travel in each direction, or on a private road, amber school-bus lights require slowing to 15 mph or less; flashing red lights require a complete stop at least 15 feet from the bus until the red lights are extinguished.
• On a highway with two or more lanes of travel in each direction, opposite-direction traffic does not have to slow for the bus amber lights or stop for its red lights. Same-direction traffic must slow to 15 mph or less for amber lights and stop for red lights. A turn-only lane does not count as a travel lane.
• On highways with separate roadways, you need not stop for a school bus on a different roadway. South Dakota also has a limited/controlled-access loading-zone exception where pedestrians are not permitted to cross.
• A violation of the school-bus stopping rule is a Class 1 misdemeanor.
• Children have been known to run in front of inter-city buses and other large vehicles too. apply the same caution near any stopped bus.

Key test point: The pentagon shape is used exclusively for school signs. No other sign category uses this shape. making it one of the easiest shapes to identify on the exam.

11. South Dakota Pedestrian and Bike Signs: Crosswalk Right-of-Way and 3-Foot / 6-Foot Bicycle Passing Rule

South Dakota crosswalk right-of-way depends on the traffic-control situation. In a business or residence district, drivers must yield to a pedestrian in a clearly marked crosswalk or a regular unmarked crossing at the end of a block unless traffic is being regulated by a signal or officer; at controlled intersections, drivers must yield to pedestrians crossing or already started across on a green signal. When overtaking a bicycle, South Dakota requires at least 3 feet of separation where the posted limit is 35 mph or less and at least 6 feet where it is above 35 mph.

Pedestrian Crossing Sign

Pedestrians may cross ahead; slow down and yield as South Dakota law requires

Bicycle Crossing Sign

Cyclists may cross here; slow down, watch for bicycles, and yield when required

No Bicycles

Cyclists must not enter; use an alternate route

No Pedestrian Crossing

Pedestrians must not cross here; use the nearest crosswalk

• In business or residence districts, yield to pedestrians in clearly marked crosswalks and regular unmarked crossings at the end of a block, except where traffic is controlled by a signal or officer; at controlled intersections, yield to pedestrians crossing or already started across on green.
• When passing a bicyclist traveling in the same direction, leave at least 3 feet if the posted speed limit is 35 mph or less and at least 6 feet if it is above 35 mph, and maintain that separation until safely past.
• Give extra space to bicyclists, especially children. they can move into your path quickly and without warning.
• South Dakota allows a motor vehicle passing a bicycle to partially cross the centerline or a same-direction lane line when the maneuver can be completed safely.
• Be alert for unskillful cyclists who may swerve unpredictably. if you see one, be ready for sudden movements.
• Fluorescent yellow-green may be used for school, pedestrian, and bicycle warning signs; use the symbol and shape to identify the exact warning.

Key test point: South Dakota does not use a one-size-fits-all crosswalk rule; follow the applicable signal and crosswalk right-of-way law. For bicycles, remember the 3-foot rule at 35 mph or less and the 6-foot rule above 35 mph.

12. How to Pass South Dakota Knowledge Test: 3 Attempts in 6 Months, the "Fines Double" Sign and Prairie-Specific Rules

• 3 opportunities per fee within 6 months, $28 application fee. South Dakota allows three opportunities to test for each fee paid within a six-month period. If you fail, you may not retest before the next working day. The knowledge test requires 80% or higher.
• Know when the Fines Double warning applies. In a limited work-zone speed zone, South Dakota requires advance signs warning that a speeding fine will be doubled before the regulatory speed-limit signs.
• Work-zone speeding fines double while workers are present. This is the statutory trigger in a South Dakota limited work-zone speed zone; obey posted speed limits and flagger directions.
• School bus: know the lane-count rule. On roads with less than two travel lanes in each direction, flashing red lights require stopping at least 15 feet from the bus. On roads with two or more travel lanes in each direction, opposite-direction traffic does not have to stop; same-direction traffic still must.
• Railroad crossing stop distance: 15-50 feet. When an immediate train or on-track-equipment warning requires a stop, stop within 15-50 feet of the nearest rail and proceed only when safe. Check every track before crossing.
• Slow-moving vehicle emblem = 25 mph or less. Farm machinery, animal-drawn vehicles, and highway construction or maintenance vehicles designed for or operated at 25 mph or less must display the emblem on public highways.
• South Dakota bicycle passing distance: 3 feet / 6 feet. Leave at least 3 feet at posted limits of 35 mph or less and at least 6 feet above 35 mph, maintaining the separation until safely past.
• School-zone speed: 15 mph in the statutory school conditions. The maximum is 15 mph when passing a school during recess or while children are going to or leaving school during opening or closing hours; also obey posted school-zone devices.
• Flashing signals. Flashing red = full stop. Flashing yellow = proceed with caution. A completely unlighted traffic signal is treated as a stop sign.
• Stopped-vehicle warning lights. For a stopped authorized emergency vehicle using red signals, come to a complete stop before reaching it and proceed cautiously when safe. For a vehicle stopped or on the shoulder using amber, yellow, or blue warning lights, move to the farthest lane at least 300 feet before it on a same-direction multilane highway, or on a two-lane highway slow at least 20 mph below the posted limit (or to 5 mph if the limit is 20 mph or less) at least 300 feet before it.