Boone County Public Works Department

Traffic Safety Manual - Boone County, Missouri

 

3.0  ROAD SPEEDS

3.1 Design Speeds

The design speed for local, collector, and arterial roads is located in the Boone County, Missouri, Roadway Regulations, Chapter II, Road, Bridge & Right of Way Regulations.   The design speed is based upon the road type.   The County may complete an engineering study for different speed limits where conditions of safety dictate and costs can be supported.   In spot locations or segments where design speeds cannot be economically provided, appropriate warning and traffic control signs or devices will be installed.

3.2 Posting of Speed Limits

Speed limits on County roads will be posted adjacent to intersecting State routes or other major County roads and at other strategic locations where it becomes apparent that drivers should be reminded of the appropriate route speed.   A speed limit will be posted by default or by performing an engineering study. If it is determined that a study is to be performed, the following factors will be considered when establishing the speed limit (MUTCD, page 2B.11):

 

•  Road surface characteristics, shoulder condition, grade, alignment and sight distance,

•  The 85 th -percentile speed and pace speed,

•  Roadside development and culture and roadside friction,

•  Parking practices and pedestrian activity; and

•  Reported accident experience for a recent 12-month period.

 

The Reduced Speed Ahead (R2-5) should be used in rural areas to inform motorists of a reduced speed zone when an advance notice is needed to comply with the speed limit posted ahead.   A Speed Limit sign erected at the beginning of the zone where the altered speed limit applies shall always follow this sign.   The minimum standard size of Reduced Speed Ahead signs will be 24-inch x 30-inch.   Figure 4 displays examples of the Reduced Speed Ahead signs.

Figure 4 .   Reduced Speed Ahead Signs (R2-5 series)

Figure4

3.3 Advisory Speed Limits on Turns and Curves

Whenever it is practical, the curves along a road in Boone County will be constructed at the overall road design speed.   However, when this cannot be done on new roads or has not been done on existing roads, studies will be completed to determine if warning signs are needed.   Whether or not a curve should be provided with warning signs depends on the posted speed limit and the computed or measured safe travel speed.   If the radius and super elevation is known, the safe travel speed can be calculated.   On County roads where this information is not available, measurement of the safe speeds will be completed using an instrument called a ball bank indicator* mounted on a survey vehicle.   A series of test runs will be conducted on each curve along a route to determine the ball deflection readings for various speeds.   Readings of 10 degrees will be used to identify the safe speed of the curve.  

 

If the safe curve speed is less than the posted speed limit, then either turn or curve warning signs (i.e., W1-1, W1-2, W1-3, W1-4, or W1-5) will be installed as prescribed in the MUTCD, section 2C.06.   Examples of the turn and curve signs are shown in Figure 5.   Turn signs should be used for speeds of 30 miles per hour or less, and curve signs should be used for speeds of greater than 30 miles per hour.   Additional protection may be provided by use of advisory speed plates.   The minimum standard size for signs W1-1 through W1-5 will be 30-inch x 30-inch.

Winding road signs are applicable where three or more turns or curves, are separated by less than 600 feet of tangent.   For added emphasis on turns, a large arrow sign (W1-6) may be placed on the outside of a turn.   The minimum standard size for the large arrow sign will be 48-inch x 24-inch.   The large arrow sign is shown in Figure 6.

 

Table 5 sets forth guidelines to determine the appropriate warning signs based on the results of the ball bank indicator.   The appropriate placement of the turn or curve warning signs shall be determined based upon Table 6.

 

* The ball bank indicator consists of a steel ball sealed in a curved glass tube with a liquid.   The ball is free to move except for the dampening effect of the liquid.   The movement or deflection of the ball up either side of the curved glass tube is governed by the roadway superelevation (i.e. gravity) and the centrifugal force developed as the survey vehicle travels around a curve at a given speed.

Figure 5 .   Curve and Turn Signs

Figure5

Source:   Manual on Uniform Traffic Control Devices, Millennium Edition. (page 2C-9).

Figure 6 .   Large Arrow Sign

Figure6

Source:   Manual on Uniform Traffic Control Devices, Millennium Edition. (page 2C-9).

Table 5.   Signing for Curves and Turns

 

 

Advisory Speed Based on Ball Bank Indicator (MPH)

 

 

60

55

50

45

40

35

30

25

20 or less

 

Usual Operating Speed (MPH)

60

 

C

C

CA

CA

CA

TA

TA

TA

55

 

 

C

C

CA

CA

TA

TA

TA

50

 

 

 

C

C

CA

TA

TA

TA

45

 

 

 

 

C

C

TA

TA

TA

40

 

 

 

 

 

C

T

TA

TA

35

 

 

 

 

 

 

T

T

TA

30

 

 

 

 

 

 

 

T

T

25

 

 

 

 

 

 

 

 

T

 

20 or less

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

C =

Curve Sign, Reverse Curve Sign (or winding road sign if applicable)

 

T =

Turn Sign, Reverse Turn Sign (or winding road sign if applicable)

 

A =

Advisory Speed Plate

 

 

 

 

 

Source: Handbook of Traffic Control Practices for Low Volume Rural Roads,

Kansas Department of Transportation, 1991, modified to reflect practice of

Missouri Department of Transportation.

Table 6.   A Guide for Advance Warning Sign Placement Distance

Posted or 85th Percentile Speed (mph)

Condition A: High Judgment Required 2 (feet)

Advance Placement Distance 1

Condition B: Stop Condition 3 (feet)

  Condition C: Deceleration Condition to Listed Advisory Speed (mph) for the Condition 4   (feet)

10

20

30

40

50

20

175

N/A 5

N/A 5

 

 

 

 

25

250

N/A 5

100

N/A 5

 

 

 

30

325

100

150

100

 

 

 

35

400

150

200

175

N/A 5

 

 

40

475

225

275

250

175

 

 

45

550

300

350

300

250

N/A 5

 

50

625

375

425

400

325

225

 

55

700

450

500

475

400

300

N/A 5

60

775

550

575

550

500

400

300

65

850

650

650

625

575

500

375

  Notes:

 

 

 

 

 

 

 

1. The distances are adjusted for a sign legibility distance of 175 feet which is the appropriate legibility distance for the 5 inch Series D word legend.   The distances may be adjusted by deducting another 100 feet if symbol signs are used.   Adjustments may be made for grades if appropriate.

2. Typical conditions are locations where the road user must use extra time to adjust speed and change lanes in heavy traffic because of a complex driving situation.   Typical signs are Merge, Right Lane Ends, etc.   The distances are determined by providing the driver with PIEV time of 6.7 to 10.0 seconds plus 4.5 seconds for vehicle maneuvers minus the legibility distance of 175 feet for the appropriate sign.

3. Typical condition is the warning of a potential stop situation.   Typical signs are Stop Ahead, Yield Ahead, or Signal Ahead.   The distances are based on the 1990 AASHTO Policy for stopping sight distance providing a PIEV time of 2.5 seconds, friction factor of 0.30 to 0.40, minus the sign legibility distance of 175 feet.

4. Typical conditions are locations where the road user must decrease speed to maneuver through the warned condition.   Typical signs are Turn, Curve, or Cross Road .   The distance is determined by providing a 1.6 seconds PIEV time, a vehicle deceleration rate of 10 feet per second 2 , minus the sign legibility distance of 175 feet.

5. No suggested minimum distances are provided for these speeds, as placement location is dependent on site conditions and other signing to provide an adequate advance warning for the driver.

Source: Manual on Uniform Traffic Control Devices, Millennium Edition.   (Table 2C-4, page 2c-7)

3.4 Posting of Lower Speed Limits

The County does not endorse the posting of lower speed limits solely based on local requests to “improve safety.”   A common belief is that posting a speed limit will influence drivers to drive at that speed.   The facts indicate otherwise.

 

Many studies conducted over several decades in all parts of the country have shown that a driver’s speed is influenced more by the appearance of the roadway and the prevailing traffic conditions than it is by the posted speed limit.   Some drivers will obey the lower posted speed while others will feel it is unreasonable and simply ignore it.   This disrupts the uniform traffic flow and increases accident potential between the faster and the slower divers.   When traffic is traveling at different speeds, the number of gaps in traffic to permit safe crossing is reduced.   Pedestrians also have greater difficulty in judging the speed of approaching vehicles.

3.5   Changing of Posted Speed Limits

A posted speed limit shall be changed only by Commission Order.

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Last Updated: August 3, 2009