Prepared for Friends of
Lafferty Park by
Joern W. Kroll, PhD
June 17, 2003
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.
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).
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).
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.
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
2.1 2.2 2.3 2.4
See Segment 3