Sonoma Mountain Road to Lafferty Park

 

Safety Analysis & Recommendations

 

 

 

 

 

 

 

Prepared for Friends of Lafferty Park by

Joern W. Kroll, PhD

 

June 17, 2003

 


Sonoma Mountain Road to Lafferty Park

Safety Analysis & Recommendations

 

 

Background

Since 1996, the City of Petaluma has been pursuing the creation of an open space park on their Lafferty Ranch property. Concern over the safety of the three-mile stretch of Sonoma Mountain Road leading to the Lafferty gate has arguably been the biggest single obstacle raised by opponents of the park. This report recommends a middle ground between taking no action to improve road safety and complete reconfiguration of the roadway to meet the AASHTO standard.

 

 

Methodology

The roadway was divided into 30 segments of a length of 0.1 mile each, numbered 1 to 30, starting from the Lafferty gate. The roadway, shoulder, lines of sight, signage, and other factors were assessed to determine recommendations that are presented in the following pages. The recommendations are discussed in table below (Specific Recommendations), and are summarized and presented graphically on topographic map segments of the road in Appendix A. The ID numbers of the table refer to the thirty 0.1-mile long segments (for instance, "6b" refers to the 0.6-0.7 mile segment, second recommendation).

 

 

Comments and Conclusions

So far, two strategies for minimizing the risk of accidents have been presented. The AASHTO strategy, ostensibly supported by the park opponents, is based on creating a roadway that can accommodate higher vehicle speeds by providing broader travel lanes, wider shoulders, and flatter horizontal and vertical curves. The Traffic Calming strategy, presented here, while addressing areas where vehicles are at the most significant risk of leaving the roadway, emphasizes maintenance of curves, narrower roadways, and uneven pavement, with adequate signage, to keep traffic speed at a safe level.

 

Given the prohibitive financial and environmental costs of the AASHTO strategy, I think it prudent for the County and State agencies involved in the creation of Lafferty Park and the Petaluma-Sonoma Trail to pursue the Traffic Calming strategy in a form similar to the one presented in this report.

 

The U.S. Department of Transportation has recently begun to recognize the need and desirability to design highways that incorporate community values, and now gives designers increased flexibility in deviating from the rigid AASHTO guidelines in order to incorporate such community values. The basic geometric design criteria are set forth in A Policy on Geometric Design of Highways and Streets (Green Book), published by the American Association of State Highway and Transportation Officials (AASHTO). The U.S. Department of Transportation gives the following rationale for the newly increased design flexibility with respect to the Green Book:

"If highway designers are not aware of opportunities to use their creative abilities, the standards or conservative use of the Green Book criteria and related State standards, along with a lack of full consideration of community values, can cause a road to be out of context with its surroundings. It may also preclude designers from avoiding impacts on important natural and human resources." (Source: U.S. Department of Transportation, Federal Highway Administration, Flexibility in Highway Design. Publication No. FHWA-PD-97-062. Washington, DC, page vi).

 

The recommendations contained in this report are consistent with Flexibility in Highway Design. They lead not only to design and maintenance of a road in its rural setting, but are also fiscally responsible. The modest but effective safety recommendations suggested in this report amount to about $100,000. This is a very low cost for increasing our community's access to the natural beauty of Sonoma County.

 

Sonoma Mountain Road, rebuilt according to AASHTO standards, would not only be prohibitively expensive, financially and environmentally, but would also increase vehicle speeds, thereby inadvertently offsetting the safety improvements intended by a road retrofit according to AASHTO standards.

 

This report pursues a Traffic Calming strategy to address safety, environmental, and cultural concerns. Specific recommendations are outlined on the following pages.

 

 

 


Specific Recommendations

 

ID #

Situation

Recommendation

Notes

1a

Horse fence on Pfendler property has been hit by uphill cars failing to complete turn

Repair fence (30 ft), install retroreflective delineators or paint fence white

To make the curved road alignment more obvious

1b

During reduced visibility (weather) the sharp curve may not be sufficiently visible

Install a rumble strip (50 ft; northeast of the 15 MPH sign) for uphill traffic

To alert drivers of the approaching curve

1c

The existing 15 MPH sign is not firmly secured in the  ground

Secure base of existing 15 MPH sign

 

2a

Embankment

Add 25 ft of guardrail on the northwest side, for downhill traffic

To prevent vehicles from driving off the roadway

4a

Roadway narrows

Install a "Road Narrows" sign

 

6a

Sharp curve

Install on east side a "Single Head Arrow" sign (W57, right) facing downhill traffic

 

6b

As 6a

Install a "Single Head Arrow" sign (W57, left) facing uphill traffic

 

8a

Visibility of road alignment needs enhancement

Install retroreflective delineators

To emphasize road alignment

10a

Vertical curve

Paint fence posts tips with white retroreflective paint

To illuminate vertical curve at night

12a

Drivers needing to check on their vehicles, etc.

Install a turnout on downhill side (just behind culvert)

To accommodate malfunctioning vehicles, etc.

13a

Drivers needing to check on their vehicles, etc.

Install a turnout on uphill side (slightly downhill from the turnout at 12a)

To accommodate malfunctioning vehicles, etc.

13b

Crest of vertical curve

Paint fence posts tips with white retroreflective paint

To enhance visibility of roadway alignment

14a

Road narrows north of  existing guardrail at retaining wall

Install a "Road Narrows" sign, a white edgeline, retroreflective delineators, and a 25 ft guardrail in front of oak tree

To keep vehicles from veering off the road into oak tree

15a

Road narrows, and no shoulder on southwest bound (downhill) side

Install a "Road Narrows" sign and a "No Shoulder" sign on downhill side

 

15b

3 ft deep ditch at edge of pavement

Install 120 ft of guardrail with retroreflective delineators

To keep vehicles from veering off the road into ditch

15c

Steep embankment

Install 150 ft of guardrail with retroreflective delineators from dirt driveway to phone pole, above steep embankment

To keep vehicles from veering off the road into steep embankment

15d

Road narrows approaching steep embankment on northeast bound side

Install a "Road Narrows" sign on uphill side

 

16a

Retroreflective delineators in need of replacement

Replace retroflective delineators along curve

 

17a

Tree branches blocking existing 20 MPH sign

Remove small tree branches that are blocking existing 20 MPH sign

 

17b

Sight line across curve needs improvement

Keep vegetation low and, if acceptable with property owner, shave grade down several feet

To improve sight line across curve

21a

Road embankment is slumping

Install 330 ft of guardrail with retroreflective delineators on eastside of roadway

To keep vehicles from veering off the road into embankment

21b

Curve and steep bank

Install a curve warning sign

 

24a

Patchwork bumpy road surface

Retain patchwork bumpy road surface (do not resurface)

To control speeding

25a

Opportunity for a turnout and advance signing

Install a "Turnout 300 ft" sign 300 ft in advance of existing wide section

 

25b

As 25a

Install a "Turnout 300 ft" sign 300 ft in advance of existing wide section

 

28a

Patchwork bumpy road surface

Retain patchwork bumpy road surface (do not resurface)

To control speeding

29a

Winding road ahead, requiring a maximum speed limit of 35 MPH and lower where posted on advisory speed plates

Install a "Winding Road" (W-14) sign supplemented by a "Next 3 Miles" plate; install a 35 MPH speed limit sign

To advise motorists unfamiliar with the road of the winding roadway and to set an overall speed limit consistent with road characteristics

 

 

 

 


About the Report Author

 

Joern Kroll earned the following degrees (all from the University of California, Berkeley):

 

Master of City and Regional Planning (1986)

Master of Transportation Engineering (1987)

Ph.D, Architecture, with a dissertation on streets and highways as public architecture (2001)

 

Since 1987, I have been working in transportation planning and traffic engineering, mostly for the City and County of San Francisco. I have presented several papers at local, national, and international transportation conferences.

 

Contact Information

 

Joern W. Kroll

Department of Parking and Traffic

City and County of San Francisco

25 Van Ness Avenue, Suite 345

San Francisco, CA 94102

Phone: (415) 554-2347

Email: joern.kroll@sf.gov.org

 

Residence:

906 Samuel Drive

Petaluma, CA 94952

Phone: (707) 765-2234





Text Box: Segment #4   
2.0 to 3.0 miles from Lafferty Gate

See Segment 3

 

2.1

 

2.2

 

2.3

 

2.4