The following was written by 3rd generation islander Joshua Putnam (email@example.com) to represent his personal views. The excellence of his analysis led to its wholehearted adoption as the Vashon Island cyclist's position (or white) paper at our 4/31 meeting.It will be expanded upon as it mainly refers to roadway engineering topics, less so the political and community outreach components that are also essential for a successful community endorsed project.
Community Planning Context
For more than thirty years, Vashon Island Highway has consistently been identified as a primary bicycle transportation route for on-Island and ferry traffic.
Vashon Community Plan
The 1983 Vashon Community Plan, adopted by the King County Council after thousands of hours of public input, called for paving the shoulders of Vashon Island Highway, six to eight feet wide, from the Heights Ferry Terminal to SW 240th, and their designation as bicycle lanes:
Prior to the Community Plan, the shoulders of Vashon Island Highway were a mix of gravel and narrow pavement. There was strong public support for cycling, coming out of the 1970s bicycle boom and the Island’s environmental sensitivities, but given the state of Island roads, bicycle commute mode share was a fraction of one percent. The paving of these shoulders was a major infrastructure investment for bicycle access on Vashon.
The Community Plan met with reality at many steep slopes on the Island, where 6 feet of pavement simply could not be provided. These areas were paved to the maximum practical width, though subsequent lack of maintenance has narrowed many of these locations.
While the 1983 Vashon Community Plan is no longer in effect, Vashon planning documents have continued to recognize Vashon Island Highway as a primary bicycle facility for commuting, shopping, and recreational use. Many on the Island continue to refer to these shoulders as bike lanes despite the lack of official designation, and much local transportation planning includes the assumption of six- to eight-foot wide paved shoulders as a baseline condition thanks to the 1983 Plan.
This context is critical to understanding the vehement public demand for wide, unobstructed shoulder pavement for cyclists on Vashon Island Highway. These shoulders should continue to be treated as primary bicycle transportation facilities.
Shoulder Width Expectations
WSDOT standards call for a bare minimum of 4 feet of clear pavement to the right of shoulder rumble strips. This standard is intended as a minimum for any road shoulders where bicycles are expected.
Vashon residents, however, continue to expect and demand the 6-8 foot shoulders called for in past Community Plan policies. Cyclists are accustomed to having these wide shoulders available for safe cycling in the presence of pedestrians, joggers, and domestic animals. Motorists are accustomed to passing cyclists, pedestrians, and animals with safe clearance, without needing to change lanes.
Consistent with three decades of public policy, the shoulders of Vashon Island Highway should not be targeted to the 4-foot WSDOT minimum, but rather the 6-foot minimum width previously included in the Vashon Community Plan.
FHWA research makes it clear that shoulder rumble strips are most effective on low-engagement roads – long, straight, boring roads that can lull a motorist to “unintentionally drift over the edge line.”1 Rumble strips are primarily effective against inattentive run-off-road accidents, not those caused by excessive speed, navigational errors, or loss of traction.
No part of Vashon Island Highway fits the description of a low-engagement roadway – it is a high-engagement road, with frequent curves, grade changes, intersections, and driveways. Relatively few single-vehicle crashes on Vashon should be of the “drift-off-road” variety; far more should be navigational errors – failure to follow a curve in the road – an accident type more effectively addressed with improved visual delineation of the road.
In the context of rural residential traffic, it is important to consider the many private intersections along Vashon Island Highway, including individual driveways, multi-family driveways, commercial driveways and parking strips, and private roads.
At all of these intersections, cyclists require the ability to safely merge off of the shoulder, into the traffic lane, to avoid the most common types of car/bicycle collisions. Cyclists on the right of the shoulder have minimal conspicuity to motorists emerging from intersections or turning off of the road, and are far more likely to suffer “left hook” and “right hook” accidents.
WSDOT requires shoulder rumble strips to stop a minimum of 40 feet from any right-turn radius intersection, at the beginning of any shoulder taper for an intersection, and at least 100 feet before any feature requiring a narrowing of the shoulder below 4-feet width clear of the rumble strip. The shoulder rumble strips already installed on Vashon Island Highway do not reliably terminate before all intersections as required by WSDOT standards.
The high intersection frequency of Vashon Island Highway, including both roads and driveways, also makes it more hazardous for cyclists to habitually ride to the right side of the shoulder. Proper lane positioning for cyclists on such a road is generally no more than one foot to the right of the lane edge stripe – exactly the portion of the shoulder make unavailable by standard shoulder rumble strips.
This is an important difference between rural residential roads, such as Vashon Island Highway, and the rural highways where shoulder rumble strips are more commonly used.
Any shoulder treatment used on Vashon Island Highway should allow cyclists to use a safe position on the road, near the lane edge where lanes are wide enough to allow cars to pass safely, and should allow cyclists to merge safely across the lane edge at every intersection or driveway.
FHWA guidance calls for rumble strips on roads with a posted or statutory speed limit of at least 50mph. This recognizes both the increased severity of crashes at high speeds, and the increased frequency of bicycle and pedestrian use of lower-speed roads.
WSDOT has set a minimum posted speed limit of 45mph for the use of shoulder rumble strips.
Both FHWA and WSDOT are well aware that posted speeds are generally lower than prevailing traffic speeds, and considered this when establishing limits based on posted speed limits.
King County should not reduce the minimum speed limit at which shoulder rumble strips are appropriate, but should abide by the minimum posted speed limit set by WSDOT, 45mph.
WSDOT prohibits the installation of shoulder rumble strips where downhill grades exceed 4% for more than 500 feet.
1T 5040.39, Revision 1
Centerline rumble strip standards assume that “bicyclists will rarely need to cross a centerline rumble strip.”2. This is true on rural highways where rumble strips are commonly used. But this premise does not hold on Vashon, where left-side intersections are as frequent as right-side intersections. On Vashon Island Highway, centerline rumble strips should also be discontinued where slopes over 4% will produce high-speed bicycle traffic.
Minimal Truck Traffic
Standard highway shoulder rumble strip patterns have been developed to produce noise and vibration inside the cab of a heavy truck. Vashon Island Highway has a very low exposure to fatigued long-haul truckers, so the more aggressive rumble strip patterns should be of little benefit here.
According to FHWA, “A six-inch length appears to be adequate and some States are now experimenting with four-inch edge line rumble stripes.”3
Average Trip Length
Inattentive-driving drift-off-road accidents are most common on long drives, where a fatigued or distracted driver can slowly drift out of the lane. Vashon is an island only 12 miles in length; few trips will be long enough to produce the sort of inattention that rumble strips are designed to counter. Rather, most run-off-road accidents on Vashon are more likely a result of excessive speed, alcohol impairment, or navigational error.
2T 5040.40, Revision 1 3Rumble Strip Types, http://safety.fhwa.dot.gov/roadway_dept/pavement/rumble_strips/rumble_types/
Rumble Strip Hazards for Cyclists
Cyclists Approaching Intersections
Intersection conflicts are far more frequent for cyclists than being hit from behind. Motorists generally look for conflicting traffic within traffic lanes, not on the periphery of the road. For safety, best practice for cyclists is to approach intersections in or at the edge of the travel lane. (It is important to note that, for car/bicycle accident purposes, “intersections” should include driveways and parking lot entrances, not just road crossings.)
Rumble strips continued too close to a road intersection or driveway can make it hazardous for cyclists to assume a proper lane position, and may give less experienced cyclists the impression that they are expected to remain at the far right edge of the shoulder – one of the most hazardous locations short of wrong-way travel.
A cyclist proceeding straight through a 4-way intersection should do so within the traffic lane, to provide maximum conspicuity to both cross traffic and opposing traffic that may be turning through the intersection. Shoulder rumble strips may deter this safety practice, especially if the rumble strips are continuous at minor intersections such as driveways.
Shoulder rumble strips that resume too soon after an intersection may also deter a cyclist from moving left before the intersection, or may make it harder for a cyclist to return to the shoulder after the intersection.
A cyclist riding on the right shoulder requires ample room to merge left from the shoulder into the lane, to establish proper positioning left of center within the lane, and to turn safely across the center line.
Turning left from the right shoulder is an exceptionally hazardous maneuver for cyclists, as overtaking motorists hit the cyclist fully broadside at speed. Such accidents have a fatality rate over 20%. Shoulder rumble strips discourage cyclists from merging to the left of the traffic lane before making a left turn.
For this reason, shoulder rumble strips must be stopped well in advance of intersections on the left side of the road, not merely intersections on the right side of the road.
FHWA’s guidance on centerline rumble strips assumes that “bicyclists will rarely need to cross a centerline rumble strip.”4 This is plainly not the case on road such as Vashon Island Highway, where driveways and road intersections are frequent. While not addressed by FHWA guidance, left-turning cyclists require center line rumble strips to be interrupted opposite every driveway or intersection so that the cyclist may safely cross the center line without hesitation or loss of control.
A cyclist approaching an intersection, preparing to turn right, should do so in or near the right edge of the traffic lane to ensure visibility to conflicting traffic. Rumble strips may prevent the cyclist from merging into this safe lane position. If continued through a minor intersection such as a driveway, rumble strips may impede the cyclist making a right turn from a safe position to the left of the rumbles.
Rumble Strip Alternatives
Where accidents are primarily navigational errors, improved visual delineation of the roadway can improve motorist safety5without reducing cyclist safety like rumble strips.
Edge Line Rumble Strips
When accommodating bicycles, shoulder rumble strips can be replaced with edge-line rumble
4T 5040.40, Revision 1 5Guidance Memorandum on Promoting the Implementation of Proven Safety Countermeasures
strips.6 Moving the rumble to the edge line, rather than offsetting it a foot to the right of the edge line, restores cyclist access to the safest portion of the road shoulder, and reduces the need for more aggressive road sweeping schedules to keep shoulders clear of debris where cyclists must ride.
Besides improved bicycle access, FHWA indicates that “edge line rumble stripes or shoulder rumble strips with a narrow offset (A) from the edge line have been shown to be most effective, because the driver is alerted sooner and it provides a slightly larger recovery area after being alerted.”7
FHWA further notes that locating a milled rumble strip under the lane edge stripe gives a vertical elevation to the retroreflective paint, improving visual delineation in bad weather.
Narrower Rumble Strips
FHWA guidance on accommodation for bicycles also suggests using narrower 6” or even 4” rumble strips.8 These are still sufficient to alert motorists that they are leaving the lane, while consuming less shoulder space and allowing easier passage for bicycles.
Narrower rumbles are often combined with edge line location of rumbles to produce a safer, more effective, more bicycle-friendly rumble stripe.
Rumble Strip Mitigation
Rumble Strips Ahead
Where conditions permit the safe installation of rumble strips, FHWA suggests further mitigating the risk to cyclists with the installation of “RUMBLE STRIPS AHEAD” warning signs.9 These warnings can alert cyclists to watch for the beginning of rumbles, to avoid accidentally riding
6Longitudinal Rumble Strips and Stripes on 2-Lane Roads 7T 5040.39, Revision 1 8Ibid. 9Low Cost Treatments for Horizontal Curve Safety, Ch. 5
through them, and can provide the cyclist time to determine a safe lane position.
Bicycles May Use Full Lane
Shoulder rumble strips, no matter how well planned and installed, increase the probability that cyclists will choose to ride in the traffic lane rather than on the shoulder. Motorists unaware of hazardous shoulder conditions often react with hostility to cyclists they see as willfully obstructing traffic.
R4-11 “Bicycles May Use Full Lane” signs may mitigate this hostility by reminding motorists of cyclists’ right to use the road, and by indicating that conditions local to the sign may make full-lane cycling more appropriate.
Suggestions for Vashon Island Highway
Rumble Strip Standards
- Minimum Shoulder Width: given the Community Plan history of Vashon Island Highway, rumble strips should not be used where they will leave less than six feet of smooth pavement for cycling.
- Speed Limits: rumble strips should be used only where posted speed limits are 45mph or higher, in compliance with WSDOT standards.
- Intersections: both shoulder and center line rumble strips should be discontinued well in advance of intersections and driveways on either side of the road.
- Grades: both shoulder and center line rumble strips should be discontinued where grades exceed 4% for 500 feet or more.
- Edge Line Striping: as recommended by FHWA, rumbles should be as close to the edge line as possible, ideally under the edge line, to leave as much shoulder as possible for cyclists while increasing the effectiveness of the rumbles for motorists.
- Narrower Rumbles: as recommended by FHWA, rumbles should be no wider than 6” to maximize room for cyclists.
Rumble Strip Mitigation
- Install “RUMBLE STRIPS AHEAD” warning signs before each installation of shoulder or center line rumble strips, and after any major intersection within an installation.
- Install R4-11 “Bicycles May Use Full Lane” signs at the beginning of each installation of shoulder rumble strips, and after any major intersection within an installation.
Restoration of Deficient Installations
- Wherever rumble strips have already been installed, field verify that the installations meet the standards above. Wherever existing installations fall short of these standards, restore the roadway to meet these standards.
Written by Joshua Putnam (firstname.lastname@example.org)
Endorsed without opposition by over 25 local Vashon cyclists at our 4/31/12 meeting.