Bike/Pedestrian Path Design Considerations
The San Francisco-Oakland
Bay Bridge
East Span
Prepared for
San Francisco Bicycle Coalition
Bicycle-Friendly Berkeley Coalition
May 24, 1998
Berkeley, California
CONTENTS
3.1 Pathway Slope and Resting Spots
3.2 Pathway Location, Noise, and Headlight Glare
3.3 Width, User Separation and Number of Pathways
3.4 Wind, Air Pollution … which side is best?
4.4 Gratings and Drainage Scuppers
4.6 Emergency Telephones, Security Cameras
Introduction
This study has been prepared at the request of several bicycle advocacy groups to assist these groups in formulating clear design recommendations to the Bay Bridge East Span design team.
Recommendations to a design team which only identify issues for study, or which appear technically, economically, or politically unfeasible are likely to be ignored, while clear, specific and well-thought-out recommendations are more likely to be taken seriously. Well prepared design recommendations can often also make something appear feasible which at first appeared unfeasible to the people considering them.
Any design process invariably involves trade-offs, however a fast-paced design process such as is currently underway for the Bay Bridge also limits the number of design options which can be studied. It is therefore especially important that the bicycle and pedestrian community formulate clear, specific and feasible recommendations as early in the process as possible.
To the extent possible Caltrans Class I bikeway design standards should be obtained for the Bay Bridge bikepath. It should be noted however that these standards are generally considered guidelines only, not required codes, and are likely to be compromised unless specific attention is called to features bicycle and pedestrian interests are particularly concerned about. As an example, blindspots and tight turns are specific areas where Class I standards are likely to be relaxed. With a double portal design both of these hazards are likely to occur where the path goes around portal tower legs. Yet with early specific from the bicycle/pedestrian advisory group early on potentially hazardous compromises in the final design might be avoided.
Another general consideration is that amenities need not be continuous to be valuable. For example one or two large resting/viewing platforms could be worth more than an extra six inches of width along the entire path. Similarly, noise sheltering spots may be more valuable than continuous noise protection.
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As noted in the group’s previous recommendations a 1:20 maximum slope is preferred. Where a slope is longer than 400’ resting spots should be provided for wheelchair users.
Along with fencing and railing, pathway location will be one of the most important determinants of the quality of the experience for pathway users. The greater the separation of the pathway from the roadway, the greater the sense of autonomy, and the cleaner and quieter the pathway experience will be. Maximum separation would be afforded by supporting the pathway on a partially or entirely separate structural system. Unfortunately, the cost of providing a separate structural system for the pathway has been shown to be cost-prohibitive in initial design studies and is not currently being seriously pursued.
As shown in Figure 1, if the user’s line-of-sight of automobile traffic can be intercepted, there will be very significant noise reduction and no headlight glare issues. A pathway raised by 10-15 feet (drivers would view under it) would act to some extent as its own noise shield but has been eliminated from consideration because it would require a separate structural system.
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A depressed pathway is still under consideration by the design team. For a depressed pathway to be truly effective against noise and headlight glare (50% noise reduction, and no headlight glare) it must be either:
Unfortunately, depressing a pathway at the outside edge of a trapezoidal girder is awkward structurally, even for the 2 foot depression currently being considered. Although we do not yet have figures, awkward structural systems are generally expensive structural systems, and pushing much beyond 2 or 3 feet is not likely to be feasible without significant re-thinking of overall structural system for the bridge.
Option 2 above may be more feasible. At southbound I-80 in Berkeley, a precedent exists for a higher than standard concrete safety barrier (3’-6" instead of 2’-8") adjacent to an important viewshed. BCDC has apparently not ruled this out from consideration for the new east span of the Bay Bridge. Such a scheme is pictured in Figure 2 below from both a pathway user’s and a driver’s points of view.
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Benefits associated with a significantly depressed pathway include:
Issues associated with a significantly depressed pathway include:
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Should line-of-sight obstruction of noise and headlights prove infeasible, the costs associated with depressing the pathway only 2 feet (see Figure 3) might yield equal or greater benefits if they were instead spent on other amenities such as better lighting, a wider path, better railings or viewing spots, etc.
With a pathway depressed 2 feet or less, the headlight glare issue is at its worst (path users' eyes would be close to the roadway level which is where headlight beams are focused). This issue might be solved using fins that are oriented perpendicular to the direction of traffic. Unfortunately, existing installations of these fins (such as on I-80 south of University Avenue in Berkeley) have proved difficult to maintain.
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Benefits associated with a slightly depressed pathway include:
Issues associated with a slightly depressed pathway include:
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Caltrans’ preferred path location is at roadway level. This geometry is simulated in Figure 4 below.
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Benefits associated with an at-roadway level pathway include:
Issues associated with an at-roadway pathway include:
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Despite high noise levels, a raised pathway configuration has proven very successful on the Golden Gate and Brooklyn bridges. On the Golden Gate Bridge (see Figure 5) this success may be largely due to the psychological value of being higher than the roadway (a traditional road-sidewalk relationship) and the festive, human activated atmosphere which results from pedestrians and cyclists being completely visible to motorists.
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An arrangement similar to that of the Golden Gate Bridge is simulated in Figure 6 below. Note that unlike for the Golden Gate Bridge guardrails are shown on both sides of the pathway. It is not yet clear whether both guardrails shown would be required.
6. Raised Pathway (2’-0" above roadway, 12’-0" wide, with 2’-8" Concrete Safety Barrier)
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Benefits associated with an above-roadway level pathway include:
Issues associated with an above-roadway pathway include:
Long experience with pedestrian/bicycle pathways on the Golden Gate and Brooklyn bridges show that the greatest safety concerns are accident safety concerns due to conflicts between pathway users. In particular, conflicts between bikes and pedestrians are quite common. This is largely due to inadequate width for separation of fast and slow moving users. For this reason, a single 12’ wide pathway is perhaps least attractive. If a single pathway scheme is chosen, it should be at least 15 feet wide if possible so that pedestrians and cyclists can be more effectively separated.
For structural, construction detailing and architectural reasons, there will be a strong tendency on the part of the bridge design team to pursue a symmetrical arrangement. This means that two-path solutions might be favored for many reasons which do not necessarily relate to actual pathway use. It should be noted however that a two-path arrangement works well on the Golden Gate Bridge and that such an arrangement could eliminate headlight glare issues if managed properly.
For cost reasons, with two paths, the tendency will be to use quite narrow paths. For example the 30% Design Report states "eight feet is the minimum considered safe for bicycles riding in opposite directions"
Multi-use trail guidelines are generally consistent regarding appropriate two-way trail widths. For bikes, with 5 feet as the standard for a single lane, guidelines state that 10-12 feet are needed for overall width, and that 8 feet may suffice only when warranted by special circumstances such as:
Widths of up to 22 feet are recommended if:
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According to the criteria above, eight foot wide pathways may not be adequate for the Bay Bridge. This is especially true because the width criteria given above assume an open pathway with no guardrails and 2 to 3 feet clear graded area on each side. When guardrails are placed at the immediate edge of a pathway, the effective usable width is reduced by an amount called the "shy" distance.
While required minimum distances from intermittent obstructions is 2 feet, and 3-4 feet are recommended, the shy distance from continuous objects like fences or walls may be reduced to as little as one foot, according to some experts. The Caltrans Highway Design Manual appears to support this by saying, "If a wide path is paved contiguous with a continuous fixed object (e.g. a guardrail), a 100 mm (4") white edge stripe, 0.3 m (one foot) from the fixed object, is recommended to minimize the likelihood of a bicyclist hitting it."
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One Path
Benefits associated with having one path:
Issues associated with having one path:
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Two Paths
Benefits associated with having two paths:
Issues associated with having two paths:
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As shown in Figure 7 below wind blows predominately from due west. The amount of time that wind blows from the north or south sides of the bridge is historically speaking virtually equal. Warm weather winds however tend to be from the south and, strong winds tend to be from the north. For this reason a pathway on the south side might be more sheltered.
The virtually equal distribution of wind direction from northerly or southerly directions means that design considerations based on wind direction and air pollution are probably unwarranted. In any case air quality on San Francisco Bay bridges tends to be quite good relative to urban areas on land because exhaust fumes are carried away quickly.
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While skid resistant qualities are important, coarse broom or burlap drag finishes such as are used on concrete roadway surfaces can present a hazard to in-line skaters and other small-wheeled users. A highly troweled finish is equally unacceptable because it can become slippery under wet conditions.
Expansion and construction joints on the pathway way should be bicycle-safe and meet ADA requirements for maximum vertical allowances.
Speed bumps, bump strips, and other pavement modifications intended to warn, slow, or calm traffic should not be used as they can present safety hazards to cyclists.
The accepted minimum guardrail height for pedestrians and wheelchairs is 42 inches above the pathway or sidewalk surface. This is also the recommended handrail height along bikeways according to the Florida Bicycle Planning and Design Manual. Various experts, as well as design precedents in Europe and the United States, indicate that a vertically oriented "flatrail", also referred to as a "rubrail", is the preferred type of handrail next to a bikeway for safety reasons. This is because bicycle handlebars do not get caught up in flatrails as easily as in standard round handrails with exposed supporting brackets. Flatrails, as a consequence, have the additional advantage of reducing the shy distance required. On the other hand, flatrails have the disadvantage of obstructing views more than standard top rails.
On bikeways, in addition to a handrail, most guidelines specify that an additional rail, a higher guardrail, be provided to prevent toppling, since cyclists have a higher center of gravity than pedestrians. Generally, the recommended bicycle guardrail height is 4’-6". New Caltrans publications however require a minimum railing height of 1.3 meters (4’-7") alongside bikeways.
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Caltrans requires missile-proof fencing configuration on pedestrian structures directly above or adjacent to roadways. This enclosure mus t be at least 2.51 meters (8’-3") high and extend 0.915 meters (3 feet) inward at the top. It is unclear whether such an enclosure will be required for the Bay Bridge pathways. If required, this enclosure will have a very significant impact on aesthetics, cost and visibility.
Visibility is a key design consideration for both safety and aesthetics. The 30% Design Report recommends vertical pickets for bike/pedestrian path railings. While such railings have visibility advantages for motorists and are harder to climb than some other types of railings, vertical picket fences are not transparent when viewed at a small angle along a pathway. If tall railings or fences are required, this issue should be taken into consideration (see Table 1 below).
Angles of incidence at which view through fencing becomes obscured|
| Construction |
Beta (observed) | Beta (calculated) |
| 2" chain link (galvanized) |
1/8" SQ. @ 2" O.C. |
6.0 Ÿ |
- |
|
Omega |
0.192" diam. @ 2" O.C. | - |
6.8 Ÿ |
|
Typical steel picket fence | 3/4" SQ. @ 4 3/4" O.C. | 12.2 Ÿ |
10.9 Ÿ |
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Figure 8 offers two photographs of Omega fencing, demonstrating that this material offers high transparency.
8. Omega Fencing
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3. Signage and Striping
Some pavement marking materials are slippery when wet and can pose a hazard to cyclists even when they pose no hazard to automobiles. These materials should be avoided in favor of more skid resistant marking materials.
4. Gratings and Drainage ScuppersGratings should be avoided on the pathway. If unavoidable, gratings should be oriented with the long openings perpendicular to the path of travel.
Drainage scuppers which direct water and debris from the roadway onto the pathway should be avoided.
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5. LightingFor accident safety and night view reasons pathway lighting should illuminate the pathway primarily. Lights which are built-in to handrails are preferred over overhead lights because they do not cast large shadows or reduce visibility by creating a glare.
ADA requirements for clearance at light fixtures and other obstructions in the traveled way are not adequate for the safety of cyclists, skaters, and other faster moving users. Light fixtures should be placed outside the pathway (unlike on the Golden Gate Bridge pathways).
6. Emergency Telephones, Security CamerasBecause actual graffiti removal, even when facilitated by graffiti-proofing surface treatments, is often impractical or cannot be done quickly enough to deter additional graffiti and because accessible surfaces will be tagged with graffiti. For surfaces which may be accessible to graffiti artists, surface treatments which accept cover-up paint and choose colors which match available spray paint colors are preferred.