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60 Conclusions and Future Research The objective of this research was to develop suggestions for bicycle lane widths for various roadway and traffic char- acteristics. The focus was on developing design guidance for bicycle lane widths for roadways in urban and suburban areas. An observational field study was conducted to evaluate the allocation roadway width on both bicyclistsâ and motoristsâ lateral positioning, taking into consideration various roadway and traffic characteristics. The general methodology of the field study involved installing temporary lane line markings to delineate bicycle lanes of varying widths at midblock loca- tions and observing the behavior of bicyclists and motorists. The final database from the observational field study included data on 4,965 bicyclists, 3,163 passing vehicles, and 994 parked vehicles. The primary roadway and traffic characteristics that fac- tored most into selecting sites for inclusion in the observa- tional field study were: ⢠Bicycle volume, ⢠Traffic volume, ⢠Vehicle mix (i.e., percent trucks), ⢠Lane width or total roadway width, and ⢠Presence/absence of on-street parking. Given the site characteristics and the study scenarios, the ranges in the primary roadway and traffic characteristics analyzed in this research were: ⢠Bike lane width: 3.5 to 6 ft, ⢠Parking lane width: 7 to 9 ft, ⢠Travel lane width: 10 to 18 ft, ⢠Presence/absence of buffer space, ⢠Traffic volume: 14,800 to 29,000 vpd, and ⢠Percent trucks: 2% to 20%. Posted speed limit and grade were additional character- istics of interest identified for evaluation in this research; however, all of the sites included in the observational field study had a posted speed limit of 30 mph and were on a level grade. The effect of grade on bicyclist behavior was evaluated through a supplemental grade study. This section presents the conclusions from the study and suggestions for future research. Section 5 provided general design guidance related to bicycle lane widths taking into account the range of roadway and traffic characteristics eval- uated in this research. 6.1 Conclusions The conclusions here should be considered within the con- text of the research. In particular, the conclusions are most applicable to urban and suburban roadways with a level grade and a posted speed limit of 30 mph and should be used cau- tiously for the design of roadways with motor vehicle speeds outside of the range of 25 to 35 mph and, in particular, for higher-speed roadways. General Conclusions 1. A buffered bike lane provides distinct advantages over simply providing a wider bike lane. 2. Narrowing the width of a bicycle lane reduces the variabil- ity of the bicyclistsâ lateral positions; however, this impact is relatively minor, at least for the bicycle lane widths evaluated in this research. 3. As traffic volume increases, bicyclists move away from vehi- cles in the travel lane and position themselves closer to parked vehicles or the curb. 4. As truck percentage within the vehicle mix increases, bicy- clists move away from vehicles in the travel lane and posi- tion themselves closer to parked vehicles or the curb. 5. For streets with on-street parking and where the parking lane width is between 7 and 9 ft and the bike lane width is between 4 and 6 ft, the effective bike lane will likely be less S E C T I O N 6
61 than the physical width of a typical adult bicyclist, and the majority of bicyclists will position themselves outside of the effective bike lane. 6. For streets without on-street parking, as long as the adja- cent travel lanes is at least 10-ft wide and the bike lane is 4 to 5 ft in width, most bicyclists will position themselves in the effective bike lane, and the effective bike lane will be equivalent to the width of the marked bike lane. Design Guidance 1. Travel lanes between 10 and 12 ft in width are appropriate for streets with a bicycle lane. 2. At sites with travel lane widths of between 16 and 18 ft on streets without on-street parking, marking a bicycle lane provides no distinct advantages for the lateral position- ing of bicyclists and motorists. While this statement is true with respect to the issues addressed in this particular study, there are other reasons why bike lanes on streets with 16- to 18-ft lanes would be desirable. These include using the bike lane to narrow the travel lane to provide a traffic calming measure, encouraging bicyclists to travel in the correct direction on the street, getting bicyclists off of adjacent sidewalks where they are generally less safe (Wachtel and Lewiston, 1994), and using the bike lane as a link to a larger bikeway network. 3. In most situations where a bicycle lane is adjacent to on-street parking, the suggested width for the parking lane is 8 ft. An 8-ft parking lane provides sufficient space for a large percentage of vehicles to park within the limits of the parking lane, and it is narrow enough that it allows more of the roadway cross section to be designated for bicyclists in the bicycle lane and motor vehicles in the travel lanes. This is consistent with current recommendations in the AASHTO Bike Guide and Green Book. 4. The AASHTO Bike Guide states that under most circum- stances, the recommended width for bike lanes is 5 ft. The guide also states that under certain conditions, wider bicy- cle lanes may be desirable. In particular, the guide states that when adjacent to a narrow parking lane (7 ft) with high turnover, a wider bicycle lane (6â7 ft) provides more operating space for bicyclists to ride outside of the door zone of parked vehicles. Based on the data collected in this study, a 6-ft bicycle lane does not provide additional benefits to bicyclists compared to a 5-ft bicycle lane. Most bicyclists will still position themselves within the open door zone of parked vehicles whether in a 6-ft bicycle lane or a 5-ft bicycle lane. A 7-ft bicycle lane may offer distinct advantages for bicyclists compared to bicycle lane widths of 5 and 6 ft; however, data for 7-ft bike lanes were not investigated in this research. Where space permits, the data suggest that installing a narrower bicycle lane with a parking-side buffer provides distinct advantages over a wider bike lane with no buffer. 5. For parking lanes 7- to 9-ft wide, assuming the 95th- percentile parked vehicle displacement and an open door width of 45 in., the open door zone width of parked vehi- cles extends approximately 11 ft from the curb. Therefore, the design of the bike lane should encourage bicyclists to ride outside of this door zone area and should account for the width of the bicyclist. 6. Taking into consideration the percentage of bicyclists riding within the effective bike lane and the estimated central positioning of bicyclists, which accounts for traffic volume, truck percentages, and the presence/absence of a buffer, Table 19 provides suggested lane widths for total roadway widths measuring 44 to 54 ft curb to curb. Where bicycle lanes are designed according to the guidance from Table 19, it should be recognized that bicyclists will still likely position themselves within the door zone of parked vehicles. 6.2 Future Research Suggestions for future research topics related to bicycle lane widths are as follows: 1. The primary roadway and traffic characteristics evaluated in this research to develop guidelines for bicycle lane widths were parking lane width, travel lane width, traffic volume, and vehicle mix (i.e., percent trucks). Future research could be conducted to develop recommended bicycle lane widths based on vehicle speeds (or posted speed limits) and grade (which was addressed in this research on a limited basis). 2. This research found a relationship between bicyclist posi- tion and traffic volume and vehicle mix (i.e., percent trucks). Both traffic volume and vehicle mix were dichotomized into high and low categories. It would be desirable to more fully evaluate the impact of a wider range of traffic volume and vehicle mix on bicyclist lateral position. Some value may also be added by analyzing bicyclist lateral position rela- tive to individual vehicle types (e.g., passenger cars, trucks, buses). 3. This research found that including a buffer space provides distinct advantages over simply providing a wider bike lane; however, only a limited number of buffered bike lane designs were evaluated. Additional research could inves- tigate a wider range of buffered bicycle lane designs to develop better design guidance for such lanes and, in par- ticular, bicycle lanes with buffers on both sides that poten- tially balance the threat of passing vehicles and the open doors of parked vehicles. 4. In this research, for streets with on-street parking, an effec- tive bike lane was defined based on the behavior of parked
62 vehicles and passing vehicles, and for streets where on- street parking was prohibited, an effective bike lane was defined based on the behavior of passing vehicles. Future research could be conducted to determine the relationship between effective bike lane widths, the physical and opera- tional widths of bicyclists, and bicycle crashes, including bicycle crashes in the presence of passing vehicles and parked vehicles (where applicable). 5. The frequency and severity of bicyclists colliding with open doors of parked vehicles should be assessed in future research. A safety analysis should be conducted to quantify the proportion of bicycle crashes that involve an open door of a parked vehicle compared to bicycle crashes that involve passing vehicles (i.e., moving vehicles in the travel lanes). In addition, the injury severity of such crashes should be assessed. This would help to better assess the magnitude of the problem associated with bicycle crashes involving an open door of a parked vehicle relative to bicycle crashes involving passing vehicles. 6. This research focused on developing design guidance for bicycle lane widths for roadways in urban and suburban areas, taking into consideration the roadway and traffic characteristics in those areas. A similar research effort should be conducted to develop design guidance for bicycle lane widths in rural areas, taking into consideration their roadway and traffic characteristics. 7. Future research should investigate the impacts of travel lane widths and bicycle lane widths on encroachment into adjacent travel lanes. This research did not collect data for vehicle classification and width for passing vehicles but rather assumed two vehicle widths to estimate a range of encroachment of passing vehicles into adjacent (motor vehicle) travel lanes to the left. It is important to determine how often vehicles encroach into adjacent travel lanes from 10-, 11-, and 12-ft travel lanes when adjacent to a bicycle lane. The number of lanes in the direction of travel should be considered in this research. 8. This research focused on analyzing the lateral position of bicyclists, passing vehicles, and parked vehicles where bike lanes were installed along midblock locations of two-lane and four-lane roadways. Future research should investigate the applicability of the results and guidelines for one-way streets, contra-flow lanes, and bike lanes at intersections and for cross sections with two-way, left-turn lanes.
