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FHWA Home / Safety / Roadway Departure / Low Cost Treatments for Horizontal Curve Safety

Low Cost Treatments for Horizontal Curve Safety

CHAPTER 7. INNOVATIVE AND EXPERIMENTAL TREATMENTS

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The MUTCD considers the two treatments in this section as "experimental" and does not approve them for general use. Review the Introduction section of this publication (ABOUT THE MUTCD), which advises agencies to get FHWA approval before installing any experimental treatment. Agencies wishing to use either of these treatments, should visit http://mutcd.fhwa.dot.gov for information on how to submit a request for experimentation.

OPTICAL SPEED BARS

Description

As shown below, Optical Speed Bars are transverse stripes spaced at gradually decreasing distances. The rationale for using them is to increase drivers’ perception of speed and cause them to reduce speed. The Optical Speed Bar name comes from this intended visual effect on drivers’ speed as they react to the spacing of the painted lines. These white transverse stripes are 18 in long and 12 in wide. The preferred material is thermoplastic because of the exposure to traffic volume over time.

Application Guideline

Optical Speed Bars are applied almost exclusively to road segments where vehicles traveling at highway speeds are required to slow for curves or other instances where traffic speeds should be reduced. To date, the treatment has been restricted to known accident locations or situations requiring traffic to significantly reduce speed. Agencies should avoid applying Optical Speed Bars just to reduce traffic speed because overuse could jeopardize the visual effect of the treatment.

Photo.  This photo shows optical speed bars placed on a curve.  The speed bars are transverse lines which extend from the centerline and edge line into the travel lane at varying intervals.  They are placed closer together as the vehicle enters the curve, thus making a driver perceive a faster traveling speed

Photo: Courtesy of Virginia Department of Transportation

Optical Speed Bars used to reduce vehicle speed.

Design

Various Optical Speed Bars are designed and spaced to produce a gradual slowing from a vehicle’s initial approach speed to the reduced curve speed. As spacing between bars gradually narrows, drivers sense they have increased speed and will slow down to keep the 4-bar/sec spacing. For example, think about an applied layout designed to slow a vehicle from 55 mi/h (81 ft/sec) into a 35-mi/h (51 ft/sec) curve. The initial bar separation distance is 81÷4 or 20 ft, and the final separation distance is 51÷4 or 13 ft. The table below shows New York Department of Transportation applied spacing between successive bars designed to cause drivers to reduce vehicle speed from 65 mi/h to 30 mi/h.

Example Spacing Between Sequential Pairs of Optical Speed Bars.

Bars

Spacing (ft)

Bars

Spacing (ft)

Bars

Spacing (ft)

1-2

24

11-12

19

21-22

15

2-3

23

12-13

19

22-23

15

3-4

23

13-14

19

23-24

15

4-5

23

14-15

18

24-25

14

5-6

22

16-17

18

26-27

13

6-7

22

16-17

18

26-27

13

7-8

21

17-18

17

27-28

13

8-9

21

18-19

16

28-29

12

9-10

21

19-20

16

29-30

12

10-11

20

20-21

16

30-31

12

The total length of the paving-marking segment depends upon the speed difference (from the approach and to the lower curve speed) the application is designed to produce. The following table suggests approximate lengths. Use these lengths as guidelines. The basis for the numbers is the need to produce a comfortable speed reduction and provide drivers with a minimum 4 seconds of driving time within the painted marking segment length.

Guideline for Length (ft) of Speed Bar Segment in Advance of Curve.

 

 

Approach Speed, mi/h

 

 

45

50

55

60

65

70

Curve Speed, mi/h

15

300

385

470

565

670

785

20

275

350

440

535

640

755

25

235

315

405

500

600

720

30

 

270

360

450

560

670

35

 

 

300

400

500

620

40

 

 

 

335

440

555

45

 

 

 

 

370

480

50

 

 

 

 

 

405

Effectiveness

Studies by New York, Mississippi, and Texas show transverse pavement markings can effectively reduce mean speeds, 85th percentile speeds, and speed variance. Initial 85th percentile speed reductions varied from 0 to 5 mi/h. However, their long-term effectiveness is not known. The Virginia Department of Transportation (VDOT) was conducting a comprehensive study of Optical Speed Bars as this publication was being prepared; the results are expected early 2007.

Cost

Based on a VDOT 2006 estimate, cost to install the marking on two directions is approximately $2,000 (labor and materials).

Contacts and Further Information

Gene Arnold, Virginia Transportation Research Council, Virginia Department of Highways; Phone: (434) 293-1990, E-mail: gene.arnold@vdot.virginia.gov

PENNDOT CURVE ADVANCE MARKING

Description

PennDOT has experimented with an innovative pavement marking designed to alert motorists that they are approaching a curve and should slow down. Referred to as the "PennDOT Curve Advance Marking," this treatment involves two transverse bars, a SLOW legend, and an arrow indicating the direction of the upcoming curve. PennDOT developed the treatment to address driver behavior at locations identified as having a high number or rate of curve-related crashes. The device objective is that reducing the upper percentile speed will reduce the number of run-off-road crashes.

Photo shows a two lane road with an advance curve pavement marking in the right lane as it approaches the curve.  The pavement marking shows a curve arrow and the word slow bordered by transverse lines.

PennDOT curve advance marking.

Application Guideline

The PennDOT Advanced Curve Warning marking is designed for two-lane locations having a high number of curve-related crashes. Agencies should avoid any location where there is any potential for driver confusion because of intersecting roadways or driveways. Agencies applying advanced curve warning at a location should also bring all existing signs, delineation, and pavement markings to standard, and consider posting an advisory speed limit.

As with any treatment used with multiple or compound curves, treat the most hazardous curve first.

Design

See the detailed design layout below. The chart on the following page indicates the distance of the marking from the point of curvature based on posted speed and posted warning speed. Designers used MUTCD Table 2C-4, Column B to calculate distance, L, to the advance warning sign.

Agencies can adjust the curve marking location when it is not plainly visible to the driver because of vertical geometry or other similar sight distance problems.

Figure.  This is a diagram of the measurements and placement of the Penn DOT advance curve pavement marking.  From top to bottom: An 8ft transverse line measuring 2 ft in thickness, a 2 ft, 6 in space, an arrow 13 ft, 6 in in height and 5 ft 9 in in overall width (actual arrow is 12 in line thickness), a 5 ft gap, SLOW lettering is 8 ft tall by 5 ft 9 in overall width (entire word), a 2 ft 6 in gap, and another 8ft transverse line measuring 2 ft in thickness.  The entire marking should take up 35 ft 6 in of linear length.

Source: Pennsylvania Department of Transportation

Effectiveness

This device has been shown to reduce overall speeds by 6 to 7 percent with slight reductions in the proportion of high-speed traffic in curves. Its effect on crash reduction is not yet established.

Contacts and Further Information

William Crawford, Pennsylvania Department of Transportation, Phone: (717) 705-1437 wicrawford@state.pa.us

Figure.  This is a diagram of the distance from the point of curvature based on posted speed limit and posted warning speed.  L is the distance form the posted warning speed sign to the point of curvature and x is the distance from the end of the curve pavement marking to the point of curvature.  A table is shown as part of the diagram which gives values for x based on the posted speed and warning speed.  At a posted speed of 20 mph, the value of x at 10 mph warning speed is 46 feet.  At a posted speed of 25 mph, the value of x at 10 mph warning speed is 70 feet and at 20 mph warning speed is 39 feet.  At a posted speed 30 mph, the value of x at 10 mph warning speed is 99 feet, and at 20 mph warning speed is 67 feet.  At 35 mph posted speed, the value of x at 10 mph warning speed is 133 feet, at 20 mph warning speed is 101 feet, and at 30 mph warning speed is 49 feet.  At a posted speed limit of 40 mph, the value of x at warning speed limit of 10 mph is 172 feet, at warning speed limit of 20 mph is 140 feet, and at warning speed limit of 30 mph is 88 feet.  At a posted speed limit of 45, the value of x at 10 mph warning speed is 216 feet, at 20 mph warning speed is 185 feet, at 30 mph warning speed is 133 feet, and at 40 mph warning speed is 59 feet.  At a posted speed limit of 50, the value of x at 10 mph warning speed is 266 feet, at 20 mph warning speed is 234 feet, at 30 mph warning speed is 182 feet, and at 40 mph warning speed is 109 feet.  At a posted speed limit of 289 feet, at 30 mph warning speed is 237 feet, and at 40 mph warning speed is 164 feet.  At a posted speed limit of 60, the value of x at 10 mph warning speed is 381 feet, at 20 mph warning speed is 349 feet, at 30 mph warning speed is 297 feet, and at 40 mph warning speed is 224 feet.  The value of L is taken from Table 11-1, pg 2C-3 of the MUTCD.

Distance from the point of curvature based on posted speed and posted warning speed.

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Page last modified on June 20, 2011.
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