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6Summary of Literature Review and Design Guidelines This section provides a summary of available literature related to the design of bicycle lanes. It is divided into two parts: Section 2.1 summarizes safety and design research related to bicycle lanes and shared use lanes, and Section 2.2 summarizes guidance from the AASHTO Bike Guide related to the design of bicycle lanes and presents a summary table of recommended bicycle lane widths from other domestic and international guidance documents. 2.1 Safety and Design Research Related to Bicycle Lanes and Shared Use Lanes The following discussion provides a summary of relevant research related to the design of bicycle lanes. 2.1.1 Safety Evaluations Traditionally, the safety effectiveness of roadway design ele- ments is evaluated in one of two ways. The first is by comparing the crash frequency at a site with a design element of interest against the crash frequency at a similar site without the design element. The second is by comparing crash frequencies before and after a particular design element has been implemented. However, an evaluation of the safety impact of design elements on bicycle crashes is difficult to ascertain for the following reasons: ⢠Bike crashes are rare. ⢠Bike crashes that do not involve a motor vehicle are not recorded in highway crash databases. ⢠Information on how a particular type of bike facility may have contributed to a bike crash is generally not included in crash reports. As a result, few authors have been able to directly link crash frequency or the likelihood of a crash to specific bicycle facil- ity designs. Since direct measures of safety are difficult to obtain for bicycle facilities, surrogate measures often are used to evalu- ate bicycle facility characteristics (e.g., lane width, markings). Surrogate safety measures include: ⢠Lateral positioning of the motor vehicle and bicycle traffic (namely the separation distance between the two modes), ⢠Lateral positioning of the parked vehicle and bicycle traffic (namely the separation distance between the two modes), ⢠Changes in motor vehicle speed, ⢠Encroachment of motor vehicle traffic into the oncoming lane when encountering cyclists, and ⢠Cyclist comfort level. The separation distances between cyclists and moving vehi- cles and cyclists and parked vehicles are typically used to assess the likelihood of bicycle/vehicle collisions. Motor vehicle speed is used to assess the severity of potential bicycle/vehicle colli- sions. Encroachment of a moving vehicle into the oncoming lane is used to assess the likelihood of vehicle/vehicle crashes. Cyclist comfort level is used to assess the likelihood of bicycle/ vehicle collisions; however, a strong relationship between cyclist comfort and safety has not been demonstrated. Generally, in the absence of sufficient crash data, these measures can be used to investigate the safety effects of street width allocations and markings for bicycle treatments, including bike lanes and wide curb or shoulder lanes. 2.1.2 Comparing Bike Lanes and Wide Curb Lanes Bike lanes and wide curb lanes (also referred to as shared lanes) are commonly used to promote bicycling and to create safe roads. Each facility type affects cyclist and driver behav- ior in different ways. The following paragraphs summarize behavioral differences and similarities resulting from the use of these two facility types. Bike lanes have a positive impact on safety when compared with unmarked roadways. Bahar et al. (2008) found that the S E C T I O N 2
7 presence of a bike lane reduces bicycle crashes by 36%. This finding is supported by other research. Reynolds et al. (2009) examined the relationship between bicycle infrastructure and cyclist safety through a review of 23 papers from 1975 through 2009. When examining the studies related to road- way segments (rather than intersections), marked bike lanes and bike routes were found to reduce crash rates and injuries by about half when compared to unmodified roadways. The safety effectiveness of specific bicycle facility designs was not described by Reynolds et al. Hunter and Feaganes (2003) examined the operational effects of converting 14-ft-wide curb lanes to 11-ft travel lanes with 3-ft undesignated lanes. The 3-ft lane was referred to as an âundesignated laneâ because it did not meet current bike lane standards in terms of lane width, signing, and marking; however, the lane was intended primarily for bicycle usage. The main findings and conclusions from this study were: 1. The lateral spacing of cyclists from the gutter pan seam was greater with the stripe as compared to with the wide curb lane. The combination of an 11-ft travel lane and 3-ft undesignated lane affected lateral spacing differently for various sites. On average, bicycles rode 7 to 9 in. farther away from the gutter pan seam at three sites where the stripe was added. This would provide a greater margin of safety for cyclists. 2. The lateral spacing of motor vehicles from the gutter pan seam was greater with the stripe than without the stripe. This would be expected with the shift of the travel lane by 3 ft with the addition of the stripe. 3. Overall, the lateral spacing between bicycles and motor vehicles was greater with the stripe than without the stripe; however, the effect was not as clear as for the previous two measures. The addition of the stripe affected lateral spac- ing differently for various sites. On average, passing motor vehicles were driven 3 to 5 in. closer to bicycles at three of the newly striped sites. This could possibly be indicative of increased comfort level for both road users, where motorists believe cyclists will ride within the striped area, and cyclists believe motorists will not cross into their space in the undes- ignated lane. Conversely, passing motor vehicles were 4 to 6 in. farther away from bicycles at the comparison sites where the stripe had already been in place for some time. 4. The addition of the stripe reduced the number of motor vehicle encroachments into the adjacent lane on these multilane roads. The effect varied by site. On average, encroachments were reduced by between 15% and 40% at sites where a stripe was newly added. Based on this information, even 3-ft bike lanes provide benefits over wide curb lanes. Hunter, Stewart, and Stutts (1999) found that under compa- rable speed and traffic conditions, the distance from the bicycle to the passing motor vehicle was a direct function of total width (defined as the bike lane width plus the width of the adjacent traffic lane, or simply the width of the wide curb lane when no bike lane was present), regardless of whether the primary bicycle facility was a bike lane or a wide curb lane. Harkey, Stewart, and Rodgman (1996) evaluated the impact of bike lanes, wide curb lanes, and paved shoulders on motor vehicle and bicycle traffic. Key findings and conclusions from this evaluation include: ⢠The separation distance between cyclists and motorists does not vary significantly by facility type (i.e., wide curb lane, shared lane, bike lane, paved shoulder). On average, motorists positioned their vehicles approximately 6.4 ft from a cyclist in a wide curb lane; 6.2 ft from a cyclist on a paved shoulder; and approximately 5.9 ft from a cyclist in a bike lane. ⢠The distance between the cyclist and the edge of the road- way was considerably less along wide curb lanes (1.4 ft) compared to that along facilities with paved shoulders or bike lanes (2.4 ft). ⢠Motor vehicles moved to the left about 1.4 ft further when passing a cyclist in a wide curb lane than when passing a cyclist riding on a paved shoulder or bike lane facility. ⢠Encroachment into the adjacent lane to the left by motor vehicles when passing a bicycle was greater on wide curb lanes (22.3%) than along bike lanes or paved shoulders (8.9%). ⢠Taking into consideration the change in lateral position of the motorist and the number of encroachments, bike lane widths as narrow as 3 ft can provide sufficient space for motorists and cyclists to interact safely; however, 4-ft-wide bike lanes or paved shoulders will optimize operating con- ditions for motorists and cyclists while minimizing the paved shoulder and right-of-way required. McHenry and Wallace (1985) analyzed the effectiveness of different wide curb lane widths ranging from 12 ft to 17.6 ft. Their study also compared wide curb lanes to a 4-ft bike lane adjacent to a 10.5-ft travel lane. They found that the optimal width for wide curb lanes was 15 ft, and that bike lanes had advantages over wide curb lanes such as less vehicle encroach- ment, lower vehicle displacement when passing a bicycle, and less variation in the lateral position of the vehicle and the bicy- cle. A 12-ft-wide curb lane does not provide enough room to allow vehicle traffic to pass comfortably, and cyclists tend to obstruct vehicle traffic as a result. A 13.8-ft-wide curb lane was more effective than a 12-ft lane, especially when the vol- ume of truck traffic was low; however, a 13.8-ft lane was still perceived as too narrow by both motorists and cyclists. In addition, both the 12-ft and 13.8-ft lane effectively reduced capacity of the travel lane as a result of the difficulty vehicles had in passing cyclists. Expanding the wide curb lane to 17.6 ft caused different problems. Here the motor vehicles had a greater degree of lateral placement, and the 17.6-ft lane width encouraged use by two motor vehicles at intersections when
8one vehicle was turning right. In contrast, a 15-ft-wide curb lane was found to be optimum since it provided a safe degree of space between motor vehicles and cyclists while not provid- ing enough space for motorists to attempt to use the additional space as a travel lane. Kroll and Ramey (1977) investigated the extent to which motorist and cyclist behaviors were affected by the presence of a bike lane. Observations were made in the field to exam- ine bike and vehicle displacement as functions of speed, lane width, presence of other vehicles, and the presence or absence of a bike lane. Based on their findings, Kroll and Ramey sug- gested that bike lanes are desirable on streets where the avail- able travel space, defined as the distance between cyclist and roadway centerline, is less than 15 ft. Although the mean sepa- ration distance between cyclist and motorist is the same for roadways with and without bike lanes, the variability in separa- tion distance decreases with the presence of bike lanes. There- fore, providing a bike lane appears to lower the likelihood of conflict between the two modes because the presence of a bike lane leads to fewer centerline violations, while the absence of bike lanes leads to more wide swerves and close passes. Table 1 summarizes the findings of Jilla (1974) and Hunter and Stewart (2009) on the various travel lane widths adjacent to bike lanes. The authors concluded the following: ⢠Travel lanes of 14 ft and under cause vehicles to slow while passing, creating a safer condition. ⢠Separation between bikes and passing vehicles increases with overall travel lane width. ⢠Lane sharing does not reduce roadway vehicle capacity if the travel lane is at least 15-ft wide. It is also important to note that research conducted by Potts et al. (2006) found that the use of travel lanes narrower than 12 ft on urban and suburban arterials does not increase the expected crash frequency. This finding suggests that geometric design policies should provide substantial flexibility for use of lane widths narrower than 12 ft. However, a few exceptions were present where the data were not clear. This research con- cluded that no indication is present to suggest that expected crash frequencies increase as lane width decreases for arterial roadway segments or arterial intersection approaches. 2.1.3 Bike Lanes and Parking Furth et al. (2010) conducted an examination of the lateral positioning of parked vehicles from the curb for a variety of parking lane widths. The distance between parked cars and the curb is an important consideration when bikes are riding adjacent to the parked cars because car doors typically open into the bike lane. A bicyclist colliding into an open door is a common crash type for bicycle riders, and a better under- standing of the relationship between parking lane width and the location of parked cars can help control the open door zone and design safety measures. Table 2 summarizes the findings by Furth et al. Furth et al. (2010) found that where there is no bike lane, the width of the travel lane adjacent to the parking lane has no significant effect on the distance of the parked car tire to the curb. The authors further reasoned that most drivers use the pavement marking, rather than the curb, as guidance when completing a parking maneuver because it is more readily vis- ible in a rearview mirror. They also generalized that a 6.5- to 7.5-ft parking lane is the most appropriate width for U.S. cities. Duthie et al. (2010) found that a wide curb lane causes significantly more cyclists to travel in the door zone, as com- pared to a bike lane site. This is likely due to the fact that a bike lane clearly shows cyclists and motorists where to posi- tion themselves on the roadway. Duthie et al. also found that a bike lane buffer was very successful in keeping cyclists out of Travel Lane Width Supporting Study 12.5 ft or Less 12.5 to 14 ft Greater than 14 ft Vehicle speed while passing Slows Slows Minor/no reduction Jilla (1974) Vehicle/bike separation Narrowest separation Greatest separation Hunter and Stewart (2009) Table 1. Effects of travel lane width. Lateral Position of Parked Vehicle Parking Lane Width 6 ft 7 ft 8 ft 95th-percentile distance from curb 0.8 ft 1.24 ft 1.68 ft Percent of cars over 1 ft from curb 1% 13% 44% Table 2. Effect of parking lane width on lateral position of parked vehicles.
9 the door zone. From their analysis, several important conclu- sions were drawn. First, bike lanes are operationally superior to wide curb lanes since they increase the safety and comfort of cyclists and motorists. Second, providing a buffer space between parked cars and bike lanes is very effective. Third, the utilization of on-street parking (either continuous or inter- mittent) has a significant effect on cyclist lateral position. Torrence et al. (2009) observed that when the lane adjacent to the motorist was a two-way left-turn lane, as opposed to a through lane for opposing traffic, drivers were 70% more likely to encroach on it when passing a cyclist, since the risk of collision with another vehicle was much less. Motorists were observed to move an average of 1.4 ft away from opposing traffic when not passing a cyclist, and 0.4 ft away when pass- ing. As the motorist moves closer to the cyclist, the cyclist moves closer to parked cars, making the likelihood of being within the door zone greater. The authors also noted that in residential areas, both cyclists and motorists moved farther away from on-street parking. Van Houten and Seiderman (2005) examined the effects of various pavement markings on the locations of cyclists and parked cars along a section of roadway in Cambridge, Massachusetts. Three pavement marking conditions were eval- uated in comparison to the baseline condition: a single lane line marking located 10 ft from the centerline, a lane line plus bike lane symbols with direction arrows, and a bike lane with symbols. The results showed that the first treatment, which was just the lane line, moved the bicycles the farthest from the curb but that parked cars were also farther from the curb, so the distance between the bicycle and the cars remained nearly unchanged. The addition of markings in the second and third scenarios resulted in bicycles and parked vehicles moving back toward the curb, so that the final treatment was not much different than the baseline. However, the additional treatments did result in a decrease in the variation of bicycle location, so that a larger percent of bicycles were traveling at least 9 ft or 10 ft away from the curb. At 9 ft, there is very little overlap in the door zone area and the cyclistâs profile, and at 10 ft, the cyclist should be clear of the door zone. This study shows that the presence of a bike lane helps to keep bicycles outside the door zone when compared to a shared lane. 2.1.4 Shared-Lane Marking The authors of a study of shared-lane markings used in San Francisco compared the effect of adding a bike and chevron symbol (i.e., shared-lane marking) to a bike-in-house symbol on bike routes with no marked bike lane and on-street park- ing (Alta Planning + Design, 2004). At each site, the shared- lane marking was used for one direction of travel, while the bike-in-house symbol was used in the other direction. The pavement markings were placed 11 ft from the curb. The distances between the bicycle tire and parked car tire, along with the passing vehicle tire and the bicycle tire, were measured during the before and after conditions. The results showed that the average distance from the bicycle to the parked car increased by 8 in. for both types of pavement markings when no passing vehicle was present. When a passing vehicle was present, the distance from bicycle to parked car increased by 3 and 4 in. for the shared-lane marking and the bike-in- house symbol, respectively. In addition, the distance of the passing car from the bike increased by 2.25 ft for the shared- lane marking and by 2 ft for the bike-in-house symbol. These increases were all statistically significant. The authors also found that the shared-lane marking signif- icantly reduced the number of cyclists riding on the shoulder and the number of cyclists traveling in the wrong direction. In conclusion, the authors suggested that the proper positioning of the shared-lane marking can help encourage proper lateral positioning of cyclists within the roadway. 2.1.5 Summary of Safety and Design Research Related to Bike Lanes Table 3 summarizes the behavioral differences and similari- ties between bike lanes and wide curb lanes. 2.2 Domestic and International Guidelines This section summarizes design guidance for bicycle lane widths provided in the 2012 AASHTO Bike Guide, followed by a summary of other relevant domestic and international guidelines on bicycle lane widths. 2.2.1 AASHTO Bike Guide (2012) The guidance provided in the AASHTO Bike Guide that is most relevant to this research is in the area of bicycle facility selection and design criteria for shared roadways and bicycle lanes. By definition, a shared roadway is a roadway open to both bicycle and motor vehicle travel. This may be an existing roadway, a street with wide curb lanes, or a road with paved shoulders. A bike lane is defined as a portion of a roadway designated by striping, signage, and pavement markings for the preferential or exclusive use of cyclists. The AASHTO Bike Guide lists several factors to be considered in determining the appropriate facility type, location, and priority for imple- mentation. These factors include: ⢠Skill level of users, ⢠Motor vehicle parking, ⢠Barriers, ⢠Crash reduction, ⢠Directness,
10 ⢠Accessibility, ⢠Personal safety/security, ⢠Stops, ⢠Conflicts, ⢠Maintenance, ⢠Pavement surface quality, ⢠Truck and bus traffic, ⢠Traffic volumes and speed, ⢠Bridges, ⢠Intersection conditions, ⢠Costs/funding, and ⢠State and local laws and ordinances. With respect to the design of bike lanes, the AASHTO Bike Guide indicates that bike lanes can be incorporated into a roadway when it is desirable or where there is a high potential for bicycle use to delineate available road space for preferential use by cyclists and motorists, which provides for more pre- dictable movements by both. Bike lanes should typically be one-way facilities and carry bicycle traffic in the same direc- tion as the adjacent motor vehicle traffic. On one-way streets, bike lanes should normally be placed on the right side of the street. The AASHTO Bike Guide provides the following guid- ance on bike lane widths: ⢠If parking is permitted, the recommended bike lane width is between 5 to 7 ft, and the bike lane is to be placed between the parking area and the travel lane. ⢠Where parking is permitted, the shared area consisting of the bike lane and parking lane should be a minimum of 12-ft wide, and desirably up to 15-ft wide. ⢠On high-speed and high-volume roadways or where there is a substantial volume of heavy vehicles, wider bike lanes are recommended. ⢠When the bike lane is along an urban curbed street where parking is prohibited, the recommended bike lane width is 5 ft from the face of the curb or guide rail to the bike lane stripe, given that there is a usable width of 4 ft. ⢠For roadways without curb and gutter, the minimum bike lane width should be 4 ft. The bicycle level-of-service model may be used to deter- mine appropriate shoulder width. This model includes fac- tors such as roadway lane width, lane use, traffic speed and volume, on-street parking, and surface condition. The AASHTO Bike Guide provides more design guidance concerning bike lane lines, markings, and signs, as well as bike lanes at intersections and in relation to turn lanes; however, this research focuses on bike lanes on basic roadway segments, away from the influence of intersections. Thus, the additional details are not covered in this report. 2.2.2 Other Domestic and International Guidelines on Bike Lanes In addition to the 2012 AASHTO Bike Guide, several other domestic and international guidance documents that addressed the design of bicycle lanes were reviewed. These guidelines tend to be very similar to the AASHTO guidance. Table 4 summarizes the findings and details where the other guidelines vary from the AASHTO guidance. In general, most agencies specify 5 ft as the minimum width for a bike lane; however, several agencies permit bike lanes as narrow as 3 ft. Several agencies also specify minimum or recommended widths for parking lanes in their guidelines, while at least one country (the Netherlands) recommends against bike lanes on roadways with parking. Behavior Findings Safer Facility Supporting Studies Separation between bikes and motor vehicles Bike lanes and wide curb lanes produce similar results. â Harkey, Stewart, and Rodgman (1996) Kroll and Ramey (1977) Bike distance from edge of roadway Compared to wide curb lanes, bike lanes provide greater distance between cyclist and curb. Bike lane Harkey, Stewart, and Rodgman (1996) Vehicle encroachment into adjacent lane when passing Compared to wide curb lanes, bike lanes result in less encroachment into adjacent lanes. Bike lane Harkey, Stewart, and Rodgman (1996) Hunter, Stewart, and Stutts (1999) Hunter and Feaganes (2003) Driver variability Compared to wide curb lanes, bike lanes result in less driver variability. Bike lane Kroll and Ramey (1977) Torrence et al. (2009) Bikes in door zone Compared to wide curb lanes, bike lanes result in fewer cyclists riding in the door zone. Bike lane Duthie et al. (2010) Torrence et al. (2009) Table 3. Behavioral impact of bike lanes and wide curb lanes.
11 Guide (see References) Vehicle Lane Width (ft) Bike Lane Width (ft) Parking Lane Width (ft) Minimum Recommended Minimum Recommended Minimum Recommended Domestic Guidelines AASHTO (2012) 4 (no parking); 5 (w/ parking) 5 7 8 Caltrans* (2005) 5 5 7 7 to 9 Chicago DOT (2002) 5 7 District of Columbia DOT (2005) 10 10 to 12 5 City and County of Durham (2006) 5 5 to 6 City of Minneapolis (2009) 5 5 to 6 8 8 to 10 City of Portland (2010) 6.5** 6.5 to 8.2 City of San Francisco (2003) 5 5 to 6 7 7 to 9 South Carolina DOT (2003) 4 4 to 6*** City of Syracuse (1996) 3**** Virginia DOT (2005) 5 7 (residential); 8 (community) Wisconsin DOT (2009) 5 5 8 8 to 10 International Guidelines Transportation Association of Canada (1999) 4.5 4.5 to 9 Netherlands (CROW 2007) 5 5 to 8.2 0***** 0***** Denmark (Vejdirektoratet, 2006) 3 5 Haliburton Highlands Cycling Coalition (2008) 3 3 to 5.25 City of Langley (2004) 5 5 to 6 Transport for London (2010) 8.2 8.2 to 9.5 4 4 to 5 Velo Quebec (1992) 3 3 to 7.5 * Caltrans provides the following additional guidance based on total available width: Recommended Bike Lane and Parking Lane Widths (Caltrans, 2005) Total Available Width Parking Lane Width Bike Lane Width 12 ft 7 ft 5 ft 13 ft 8 ft 5 ft 14 ft 9 ft 5 ft ** On low-volume streets with no center line, 5-ft âadvisoryâ bike lanes (dotted white lines) are permitted. *** When speeds exceed 50 mph, 8- to 10-ft lanes should be considered. **** Cites ITE Transportation Planning Handbook: even 3 ft of shoulder space to the right of the edge line can be beneficial to a cyclist, provided that there are no rumble strips. ***** CROW recommends against bike lanes on roadways with parking. An off-road bike path should be considered instead. Table 4. Summary of bike lane width recommendations.
