1 Appendix C White Paper to Support Bicycle Facility Design Toolkit on Lane Width Design Modiications
2 MEMORANDUM Date: December 31, 2012 Project #: To: Washington County From: Hermanus Steyn, Pr. Eng., PE; Casey Bergh, PE; and, Jack Freeman, PTOE Subject: White Paper to Support Bicycle Facility Design Toolkit on Lane Width Design Modiications This paper supports the Washington County Bicycle Facility Design Toolkit prepared by Kittelson & Associates, Inc. (KAI) to supplement the current Washington County Road Design and Construction Standards. The Toolkit provides engineers and planners more options to address saety concerns and accommodate a wider range o bicyclists. Reducing travel lane width 1 to accommodate bicycle acilities on existing roadways or within existing right-o-way o uture roads is oten desired. However, conventional wisdom o most highway engineers is that use o narrower lanes will have negative impacts on saety and operations. Agency policies require that whenever the lane width standard is not met, a design modiication must be obtained. In recent years, deinitive research has been conducted to evaluate the validity o this conventional wisdom. This white paper provides summaries o literature that document the trade-os o reducing lane width and provides empirical support or small variations rom standard widths based on decisions that balance mobility targets, saety goals, and design user needs o each roadway. Many actors inluence the indings o the literature, including: acility type (e.g., intersection or segment), unctional classiication (principal arterial, collector, local road, etc.), number o lanes, travel speed, percent o truck traic, area type (e.g., urban, rural, etc.), and on-street parking. In many cases, the literature provides indings that inorm design decisions by allowing or calculation o the incremental impacts o reducing lane width on saety and operations. With this inormation, the overall eect o a cross-section change can be evaluated. On the basis o this type o quantitative evaluation, travel lane width reductions should be considered a viable solution or accommodating all motorized and non-motorized users. 1 Typically measured rom center o stripe to center o stripe, or rom center o stripe to ace o curb/edge o pavement shoulder). (i no striped FILENAME: C:\USERS\HSTEYN\DOCUMENTS\KAI-WORK\WASHINGTONCOUNTY_LANEWIDTHS_FINAL_ DOCX
3 December 31, 2012 Page 2 LITERATURE REVIEW The ollowing sections summarize literature that has documented how travel lane width impacts traic operations and saety. Findings rom the literature have been grouped by roadway characteristic (i.e., acility type, unctional classiication, or area type) where the literature includes ocused studies with that level o detail. The literature reviewed or this study includes the most-recent and widely-accepted national research available. The methods applied by the researchers to establish quantitative trends to inorm design decisions represent the state-o-the-practice. However, the results relect data collected in a limited number o states and are based on statistical methods that inherently include some bias. Thereore, the literature should be applied with engineering judgment. Actual impacts on operations and saety associated with changes in lane width may vary rom the indings outlined here due to many actors that inluence operations and saety (e.g., driver characteristics, geography, climate, presence o onstreet parking, shoulder type, etc.). Operations While travel lane width inluences multiple elements o operations, the most signiicant impacts that have been documented in literature relate to signalized intersection capacity, multilane segment travel speed, and two-lane highway segment travel speed. Urban Street Intersection Capacity Lane width is one o 11 actors relected in the calculation o saturation low. Saturation low is a key parameter o intersection capacity per the Highway Capacity Manual (HCM) 2010 methodology (1). Saturation low at intersections is also actored into Urban Street 2 Level o Service as it impacts average through-vehicle travel speed. Relevant research on the impact o lane width on saturation low or through lanes on signalized intersection approaches is in general agreement. The measured saturation low rates are similar or lane widths between 10 eet and 12 eet. For lane widths below 10 eet, there is a measurable decrease in saturation low rate. Thus, so long as all other geometric and traic signalization conditions remain constant, there is no measurable decrease in urban street capacity when through lane widths are narrowed rom 12 eet to 10 eet. 2 The 2010 HCM deines Urban Street Facilities as portions o roadways that typically have two or more lanes, serve multiple travel modes, and have intersections spaced less than two miles apart on average.
4 December 31, 2012 Page 3 Chapter 18 o the HCM 2010 describes the adjustment to signalized intersection saturation low associated with lane width. The 2010 HCM includes updated adjustment actors compared to those provided in the 2000 HCM. Per the HCM 2010, The lane width adjustment actor accounts or the negative impact o narrow lanes on saturation low rate and allows or an increased low rate on wide lanes. Values o this actor are listed in Exhibit Exhibit 18-13: Lane Width Adjustment Factors or Urban Signalized Intersections (1) Average Lane Width (t) Adjustment Factor < As indicated by the adjustment actors in Exhibit o the HCM 2010, the reduction o lane widths below ten eet is associated with a our-percent reduction in saturation low at signalized intersections on urban streets. Wide lanes, over 12.9 eet, are associated with the opposite eect, an increase in saturation low o 4 percent. Research on the impacts o lane widths on saturation low conducted in 2007 by Potts, I.B., et.al. inluenced changes to the HCM 2000 methodology that are relected in the HCM 2010 lane width adjustment actors (1, 2). Potts research is summarized, with other relevant research on the impact o lane width on signalized intersection saturation low, in the technical memorandum provided in Appendix A. Travel Speed Travel speed on multilane and two-lane highway segments is impacted by lane width. When determining segment capacity and level-o-service (LOS) on multilane and two-lane highways, the HCM 2010 suggests adjustments to ree-low speed (FFS) based on lane width. The FFS calculations and lane width adjustments apply to highways with directional traic in general terrain (level or rolling). The adjustments apply to acilities in rural, suburban, and limited urban areas, but do not apply on acilities deined by the HCM as Urban Street Segments that have signalized intersections spaced less than two miles apart. Table 1 presents FFS adjustments or lane widths o less than 12 eet on multilane highways.
5 December 31, 2012 Page 4 Table 1: Adjustment to Free-Flow Speed or Average Lane Width or Multilane Highways (1) Lane Width (t) Reduction in FFS (mi/hr) As shown in Table 1, on multilane highways ree-low speed is reduced by 1.9 miles per hour when a 12-oot lane is reduced by one oot or less. Free-low speeds are reduced by more than three times that amount when a 12-oot lane is reduced to between 10 and 11 eet. Table 2 summarizes the reduction in ree-low speed associated with various combinations o shoulder width and lane width on paved two-lane highways. The HCM generally describes two-lane highways as directional segments that may or may not include passing or truck climbing lanes. Table 2: Reduction to Free-Flow Speed or Lane and Shoulder Width on Two-Lane Highways (mi/hr) (1) Lane Width (t) Shoulder Width (t) 0 < 2 2 < 4 4 < 6 6 9< < < As shown in Table 2, regardless o shoulder width, i lane width on two-lane highways is reduced rom 12 to 11 eet, the dierence in ree-low speed reduction does not exceed 0.5 miles per hour. It is also notable that i shoulder width is at least 6 eet and lane width is reduced, speeds on two-lane highways will not drop more than 2.2 miles per hour. However, i shoulder width is limited to 2 eet and lane width is reduced, speeds on two-lane highways may drop as much as 4.8 miles per hour, relative to a 12-oot lane with 6-oot shoulder. The FFS reduction actors indicate the need to consider shoulder width and lane width together when evaluating the reallocation o right-o-way or bicycle lanes. It should also be noted that highway shoulders can be used as bicycle lanes, although posted speed and traic volume will inluence the percentage o the population that use the shoulder or bicycling. Saety Many highway engineers associate narrow travel lanes with a potential increase in crash requency. However, this exercise o judgment has historically not been based on empirical evidence and the degree, to which crashes may increase, relative to lane width, is not deinitive. In recent years several studies have been conducted to develop quantitative estimates o the impacts o lane width reductions on crash requency. The research provides quantitative estimates that can be used to inorm design decisions on rural roads and/or suburban segments.
6 December 31, 2012 Page 5 Rural Road Segments The Highway Saety Manual (HSM), published by the American Association o State Highway and Transportation Oicials (AASHTO), is the most-recently published reerence or evaluating the saety impacts o design decisions (3). The HSM provides Crash Modiication Factors (CMFs) that indicate the expected change in crashes associated with various roadway design eatures. The CMFs are developed through empirical studies conducted throughout the United States. The HSM provides a combination o equations and values that relect the change in crash requency associated with reducing lane widths. The HSM does not indicate how crash requency varies with respect to various combinations o lane and shoulder width. CMFs or lane width modiications are provided in Part C o the HSM. The CMFs were developed or Rural Two-Lane Roads and Rural Multilane Highways (4). The CMFs are a unction o acility type, Average Annual Daily Traic (AADT), and lane width. Table 3 includes a summary o the CMFs or each acility type. The base condition assumed is 12-oot lane width. The crash requency predicted or reducing a 12-oot lane to a narrower lane is calculated by multiplying the number o crashes o the speciic crash types (see table note) by the CMF or a narrower lane (considering acility type and AADT). Alternatively, the percentage change in crashes associated with a change o travel lane width can be calculated by dividing the CMF or the new width by the CMF or the existing width. Table 3: CMFs or Lane Width on Road Segments by Facility Type (4) Facility Type Rural Two- Lane Road Segments Rural Multilane, Undivided Road Segments Rural Multilane, Divided Road Segments Lane Width Average Annual Daily Traic (AADT) (vehicles/day) < to 2,000 > 2,000 9 t or less x10-4 (AADT-400) t x10-4 (AADT-400) t x10-5 (AADT-400) t or more t or less x10-4(AADT-400) t x10-4(AADT-400) t x10-5(AADT-400) t or more t or less x10-4(AADT-400) t x10-5(AADT-400) t x10-5(AADT-400) t or more NOTE: The collision types related to lane width to which these CMFs apply are single-vehicle run-o the-road and multiple-vehicle head-on, opposite-direction sideswipe, and same-direction sideswipe crashes. Standard error o the CMF is unknown. To determine the CMF or changing lane width and/or AADT, divide the new condition CMF by the existing condition CMF.
7 December 31, 2012 Page 6 The CMFs in Table 3 indicate a reduction in lane width rom 12 eet to 11 eet, on a rural two-lane road segment, would increase crashes by one percent i the AADT is less than 400 vehicles per day, or by approximately ive percent i the AADT is greater than 2,000 vehicles per day. Additional guidance on the use o CMFs is provided in Part D o the HSM. Urban and Suburban Arterial Saety The Florida Department o Transportation (FDOT) conducted statistical analysis to evaluate the impacts o restriping multilane roadways to increase the outside lane width by borrowing width rom inner lanes (5). The crash severity and requency o the imbalanced lanes (i.e., asymmetrical) was assessed on our-lane roadways with either a lush, two-way let-turn lane (TWLTL) or a raised median. The study did not dierentiate between the width o the center TWLTL or raised median. The study identiied two trends that were statistically signiicant, as ollows: given a 24-oot lane width or inside and outside through lanes, restriping the outside through lane to provide 13-oot lane width and leaving the inside lane with 11-oot lane width would result in a slight reduction o crashes or our-lane sections with raised median. i an extra 0.5 oot is added to the outside asymmetric lane to make it 13.5 eet wide while keeping the inside lane at 11 eet, a decrease in crashes is ound or our-lane sections with raised or two-way-let-turn-lane (TWLTL) medians. (5) Potts, I.B. et.al. conducted research on the relationship between lane width and saety or roadway segments and intersection approaches on urban and suburban arterials as part o NCHRP Project 3-72: Lane Widths, Channelized Right Turns, and Right-turn Deceleration Lanes in Urban and Suburban Areas (6). The research included data rom various roadway and intersection types in Minnesota and Michigan ranging rom 2-lane undivided to our-lane divided roadways. They also considered how Average Annual Daily Traic (AADT) and lane width together inluence saety. Several trends were observed in the data rom Minnesota and Michigan, but inconsistencies exist between states. The researchers noted that crash requency in [Minnesota] was higher or 10-oot lanes than or 11- and 12-oot lanes on our-lane undivided arterials, but the same was not true in Michigan. They also ound that crash requency in Michigan was higher or 9-oot lanes than or 10- oot lanes on our-lane divided arterials. The same was not true is Minnesota. The researchers concluded that there was no indication o an increase in crash requencies as lane width decreased or arterial roadway segments or arterial intersection approaches. They noted the inconsistencies between states related to our-lane undivided and divided arterials and that in these cases the inconsistencies should not iner the use o narrower lanes must be avoided. But, rather the inconsistencies indicate that narrower lanes be used cautiously in these situations unless local experience otherwise. indicates
8 December 31, 2012 Page 7 Impacts to Trucks and Buses Design decisions regarding travel lane width (measured rom center o stripe to center o stripe) require special considerations or trucks and buses with dimensions that exceed those o a passenger car. General guidance provided in A Policy on Geometric Design o Highways and Streets (i.e., The Green Book), published by AASHTO, indicates that I substantial truck traic is anticipated, additional lane width may be desirable (7). This guidance is ideal, but not oten realistic. When right-o-way is limited, the desire or additional travel lane width must be balanced with capacity needs and multimodal considerations, such as pedestrian crossing distance and presence o bicycle lanes. Increasing lane width to accommodate trucks and buses should take into account whether the travel lane is adjacent to a shoulder, center two-way let-turn lane, or buered pavement area. When a truck or bus travels in a lane adjacent to these acilities it minimizes the potential or sideswipe crashes and eectively allows more useable lane width or large vehicles, particularly those that have less than one oot o horizontal clearance on either side o their vehicle in a 12-oot travel lane. Inormation rom national design guides and applicable research conducted by the Florida Department o Transportation (FDOT) Transit Oice is summarized below in order to understand whether a minimum travel lane width is necessary and what dimension is reasonable. FDOT Transit Oice Research The FDOT Transit Oice documented an evaluation o lane width needs or transit vehicles in a report titled Integrating Transit into Traditional Neighborhood Design Policies - The Inluence o Lane Width on Bus Saety (8). The report summarizes their investigation into whether there is a signiicant relationship between travel lane widths and bus vehicle saety. Through surveys, crash analysis, incident report analysis, ield observations, and physical constraints analysis the agency identiied a strong correlation between lane width and bus vehicle saety. Speciically, the statewide crash database analysis identiied 10 segments with the highest requency o sideswipe crashes. Nine o the ten segments had average travel lane widths rom 9 to 11 eet and seven o the ten had average travel lane widths o 10 eet or less. The FDOT Transit Oice report suggests a minimum o 12-oot outside lane width or roadways that serve as bus routes to minimize sideswipe crashes, and to meet Florida bicycle clearance standards (Florida Statute ). A minimum inside lane width o 11 eet is suggested based on the 10.5 eet mirror-to-mirror width or buses. Lane Widening on Horizontal Curves The turn radii o trucks and buses generally results in the rear wheels tracking outside ront wheels (o-tracking) when negotiating horizontal curves. To accommodate or these design vehicles, wider lanes are generally provided on horizontal curves. In some urban and rural areas widening pavement on curves is not easible; in these cases the County has set length limits on vehicles. The amount o
9 December 31, 2012 Page 8 widening on the curve can be calculated using equations provided in Section o the AASHTO Green Book. The calculations are based on the design vehicle dimensions, which are also provided in the Green Book, or a range o vehicles, including multiple types o buses and trucks. A common method or evaluating whether adequate lane width is provided is to use a turn template program, such as AutoTurn in AutoCAD/Microstation, to project the vehicle s horizontal dimensions over the scaled roadway design. In situations where a lane width reduction is proposed on a horizontal curve the County could require the design be checked using AutoTurn. The FDOT Transit Oice research and turning width constraints indicate that lane width should be increased on acilities that serve as transit or reight routes. Given that there is no national empirical evidence to support a consistent minimum or all transit and reight routes, the design width should relect design vehicle dimensions and demonstrate the design vehicle can navigate the study roadway without encroaching on adjacent lanes or sidewalk acilities. DESIGN CONSIDERATIONS This section outlines local, state, and national guidance or establishing lane width. The review identiies design guidance that indicates how cross-sections can be varied to accommodate bicycles. The primary ocus o the review o design guidance is related to reducing lane width within a constrained corridor to reallocate right-o-way or bike lanes on acilities that do not provide dedicated right-o-way or bicyclists. An alternative approach is to apply a road diet in which a 4-lane section is reduced to a 3-lane section (two directional travel lanes and one center TWLTL) with bicycle lanes. An example o a beore and ater condition o a road diet is shown in Figure 1. Guidance on lane width rom the Oregon Department o Transportation (ODOT), Washington County, and AASHTO is summarized below. Figure 1: Example o Providing Bicycle Lanes through Implementation o a Road Diet (image credit:
10 December 31, 2012 Page 9 ODOT ODOT s Bicycle and Pedestrian Design Guide includes dimensions that serve as a best practice or application on state and local roadways (9). The document suggests that restriping the roadway to add bike lanes is a practical approach because it is oten impractical to alter the curb-to-curb width o an existing roadway that does not have bicycle lanes. The guide suggests narrowing travel lane widths to accommodate bicycle lanes should take into account the width o the center let-turn lane and posted speeds, as shown in Table 4. Table 4: Minimum Travel Lane Width by Travel Speed (9) Posted Speed (mph) Suggested Minimum Travel Lane Width (eet) Suggested Minimum Center Let- Turn Lane Width (eet) * * 14 eet width is suggested when truck volumes are high The Oregon Department o Transportation s Bicycle and Pedestrian Design Guide provides an example o how an existing 5-lane roadway with a 68-oot cross-section could be modiied to include bicycle lanes by narrowing travel lanes, as illustrated in Figure 2. Figure 2: Example o Reallocating Cross-Section to Accommodate Bicycle Lanes (9) Dimensions or ODOT highways must meet standards in ODOT s Highway Design Manual (10). On local agency projects, when unds are administered through ODOT, AASHTO standards must be met. On local projects, unded locally, the Bicycle and Pedestrian Design Guide provides dimensions that can be adopted by local agencies or they can adopt their own. AASHTO and Washington County Standards The AASHTO Green Book oers guidance on the selection o appropriate lane widths by acility type and roadway user based on established practices and national research (7). A comparison between AASHTO lane width guidance and the Washington County Road Design Standards or lane width is
11 December 31, 2012 Page 10 provided in Table 2 (11). This comparison is intended to serve as a reerence to identiy where County standards exceed AASHTO standards and could be modiied while satisying AASHTO standards. Table 2: Comparison o Washington County Road Design Standards with AASHTO Standards (7, 11) Functional Classiication Travel Lane Width (eet) Washington County Road Design and Construction Standards AASHTO Green Book 3 Arterial Collector , 5 Neighborhood Route , 2 See local street Local Street , 6 Commercial and Industrial (urban) Special Area N/A 1 Dependent on Washington County Designation determined by the county s transportation plan and land use decisions. 2 Dependent on whether parking is provided on one, both, or neither side o street. 3 Additional inormation regarding provisions or bicyclists are provided in the AASHTO Guide or the Development o Bicycle Facilities (12) 4 Dependent on average daily traic volumes. 5 Minimum lane widths o 9 or 10 eet is only suggested in highly restricted areas with little to no truck traic. 6 Up to 12-oot width at intersections. As shown in Table 2, the County s Road Design Standards generally exceed the AASHTO standards. Thereore, i the County were to allow reduced lane widths, in many cases they would still be consistent with AASHTO minimum lane widths. The FHWA publication, Flexibility in Highway Design, provides guidance that indicates the need to take into account right-o-way constrictions when evaluating lane width decisions (13). Generally, as the design speed o a highway increases, so must the lane width to allow or the lateral movement o vehicles within the lane. However, constricted right-o-way and other design restrictions can have an impact on this decision. Chapter IV o the Green Book recognizes the need or lexibility in these cases: Although lane widths o 12 eet are desirable on both rural and urban acilities, there are circumstances that necessitate the use o lanes less than 12 eet wide. In urban areas where right-o-way and existing development become stringent controls, the use o 11 eet lanes is acceptable. The AASHTO Green Book also identiies 11 eet as adequate width or center two-way let-turn lanes. Little to no discussion is provided regarding trade-os o TWLTL widths.
12 December 31, 2012 Page 11 FINDINGS The literature highlights the trade-os o reducing lane width and provides empirical support or small variations rom standard widths based on decisions that balance mobility targets, saety goals, and design user needs o the roadway. The ollowing trends and quantitative indings were identiied: Urban Signalized Intersections There is no measurable decrease in urban street capacity when through lane widths are narrowed rom 12 eet to 10 eet, assuming all other geometric and traic signalization conditions remain constant. Rural Road Segments On rural road segments, reducing lane width rom 12 eet to 11 eet may increase the requency o speciic crash types by less than ive percent. Multilane Highways On rural and suburban multilane highways, ree-low speed is reduced by 1.9 miles per hour between 11 and 12-oot lane width. Free-low speeds are reduced by more than 3 times that amount when the lane width drops below 11 eet. On rural and suburban multilane highways, given a 24-oot lane width or inside and outside through lanes, restriping the outside through lane to provide 13-oot lane width and leaving the inside lane with 11-oot lane width would result in a slight reduction o crashes or our-lane sections with raised median. For our-lane sections with raised or two-way-let-turn-lane (TWLTL) medians, i an extra 0.5 oot is added to the outside asymmetric lane to make it 13.5 eet wide while keeping the inside lane at 11 eet, a decrease in crashes is ound. Two-Lane Highways On two-lane highways (with more than two miles between signalized intersections), i lane width is reduced rom 12 to 11 eet, the dierence in ree-low speed reduction is less than 0.5 miles per hour. RECOMMENDATIONS While the literature provides support or reducing lane width in limited situations, overall trends indicate it should be considered in other situations when right-o-way constraints exist and there are ew other options or accommodating bicycle traic. An example o reallocating lane width to accommodate dedicated bicycle lanes is provided in Figure 2.
13 December 31, 2012 Page 12 The research provides or more deinitive assessments o design decisions regarding lane width reductions. Thereore, when right-o-way constrains a roadway cross-section, we recommended that travel lane width reductions be considered a viable solution or accommodating all motorized and nonmotorized users. Speciically, when right-o-way constraints exist we recommended that Washington County allow width reductions rom 12 eet to 11 eet in travel lanes that do not carry high volumes o transit and/or reight vehicles. For example, on multi-lane acilities the inside lane width may be reduced to 11 eet, but the outside lane may remain 12 eet to accommodate wider design vehicles.
14 December 31, 2012 Page 13 REFERENCES 1) TRB. Highway Capacity Manual ) Potts, I.B., et.al., Relationship o Lane Width to Saety on Urban and Suburban Arterials. Transportation Research Record: Journal o the Transportation Research Board, No Washington, DC ) American Association o State Highway and Transportation Oicials. Highway Saety Manual. Washington, DC ) Harwood, D. W., F. M. Council, E. Hauer, W. E. Hughes, and A. Vogt, Prediction o the Expected Saety Perormance o Rural Two-Lane Highways. FHWA-RD , Federal Highway Administration, U.S. Department o Transportation, McLean, VA ) Sando, T., and R. Moses, Operational and Saety Impacts o Restriping Inside Lanes o Urban Multilane Curbed Roadways to 11 Feet or Less to Create Wider Outside Curb Lanes or Bicyclists BDK Florida Department o Transportation. Tallahassee, FL ) Potts, I.B., et.al. NCHRP Project 3-72: Lane Widths, Channelized Right Turns, and Right-turn Deceleration Lanes in Urban and Suburban Areas. TRB. Washington, DC ) American Association o State Highway and Transportation Oicials. A Policy on Geometric Design o Highways and Streets, Sixth Edition. Washington, DC ) Florida Department o Transportation Public Transportation Transit Oice. Integrating Transit into Traditional Neighborhood Design Policies - The Inluence o Lane Width on Bus Saety. 9) Oregon Department o Transportation. Oregon Bicycle and Pedestrian Design Guide. Salem, OR ) Oregon Department o Transportation. Highway Design Manual. Salem, OR ) Washington County, Oregon. Road Design and Construction Standards ) American Association o State Highway and Transportation Oicials. Guide or the Development o Bicycle Facilities Fourth Edition. Washington, DC ) FHWA. Flexibility in Highway Design. Accessed on at: APPENDIX A. Technical Memorandum on The Eect o Lane Width on Urban Street Capacity
15 Technical Memorandum Date: March 22, 2007 Project #: To: From: Copy to: Subject: Sprinkle Consulting Engineers John Zegeer Patrick McMahon and Paul Ryus The Eect o Lane Width on Urban Street Capacity FDOT Conserve by Bicycle Project One o the goals o the FDOT Conserve by Bicycle project is to determine how the provision o bicycling acilities can enhance opportunities or recreational travel. One potential treatment that is being considered or accommodating additional bicycle travel along urban streets is the narrowing o street lane widths in order to provide a striped bicycle lane on the paved roadway surace adjacent to these narrower lanes. In considering this treatment, a concern has been raised regarding the reduction in roadway capacity (or motorized vehicles) that could occur due to the lane width reduction. The purpose o this memorandum is to provide a summary o relevant research that describes the relationship between lane width and urban street capacity. The next section o this memorandum summarizes the method by which urban street capacity is determined. Then, a summary o relevant research is provided. Finally, conclusions are drawn as to the impact o narrowing lanes on urban street capacity. How is Urban Street Capacity Determined? Chapter 15 o HCM2000 provides the methodology or analyzing urban streets. (Highway Capacity Manual Fourth Edition. Transportation Research Board., Washington, D.C ) Urban street level o service is based on average through-vehicle travel speed or the segment or or the entire street under consideration. The average travel speed is computed rom the running times on the urban street (between signalized intersections) and the control delay experienced by through movements at signalized intersections. (page 15-2) The capacity o an urban street is deined or a single direction o travel as the capacity o the through movement at its lowest point (usually at a signalized intersection). The capacity is determined by the number o lanes, the saturation low rate per lane (inluenced by geometric FILENAME: C:\Users\cbergh\Documents\KAI\projects\ WaCo White Paper Lane Width\ResearchSummary_LaneWidthEecton SatFlow.doc.doc
16 The Eect o Lane Width on Urban Street Capacity October 29, 2008 Page 2 design and demand actors), and the green time per cycle or the through movement at the intersection. (page 15-9) Chapter 16 o HCM2000 provides the methodology or analyzing signalized intersections. This methodology includes the determination o the saturation low rate or each lane group. The saturation low rate is the low in vehicles per hour that can be accommodated by the lane group assuming that the green phase were displayed 100 percent o the time (i.e., g/c = 1.0). (page 16-9) The equation or this calculation is shown below: where s s s o N w HV g p bb a LU LT RT Lpb Rpb (Equation 16-4) saturation low rate or subject lane group, expressed as a total or all lanes in lane group (veh/h); base saturation low rate per lane (pc/h/ln); s o N number o lanes in lane group; adjustment actor or lane width; w adjustment actor or heavy vehicles in traic stream; HV adjustment actor or approach grade; g adjustment actor or existence o a parking lane and parking activity adjacent to p lane group; adjustment actor or blocking eect o local buses that stop within intersection bb area; adjustment actor or area type; a adjustment actor or lane utilization; LU LT adjustment actor or let turns in lane group; RT adjustment actor or right turns in lane group; pedestrian adjustment actor or let-turn movements; and Lpb pedestrian-bicycle adjustment actor or right-turn movements. Rpb As shown in the above equation, the adjustment actor or lane width is the irst o the eleven adjustment actors that is used in calculating the saturation low rate or the subject lane group. The lane width adjustment actor, W, accounts or the negative impact o narrow lanes on saturation low rate and allows or an increased low rate on wide lanes. (page 16-10) Summary o Relevant Research Four relevant research documents were ound that provide guidance on the relationship between lane width and saturation low rate: Fort Lauderdale, Florida
17 The Eect o Lane Width on Urban Street Capacity October 29, 2008 Page 3 1. Potts, I.B., et.al. Relationship o Lane Width to Saturation Flow Rate on Urban and Suburban Signalized Intersection Approaches. Presented at the 2007 Transportation Research Board Annual Meeting. Accepted or publication in a Transportation Research Record. 2. Zegeer, J.D. Field Validation o Intersection Capacity Factors. Transportation Research Record 1091, Transportation Research Board Agent, K.R. and J.D. Crabtree. Analysis o Saturation Flow at Signalized Intersections. Kentucky Transportation Research Program University o Kentucky. February, Bonneson, J.A. Modeling Queued Driver Behavior at Signalized Junctions. Transportation Research Record 1365, Transportation Research Board The irst paper cited above provides an overview o the other three research documents. So, the remainder o this section contains direct quotes rom that paper. Zegeer (2) evaluated the saturation low rates on approaches with lane widths varying between 2.6 and 4.7 m (8.5 and 15.5 t). Saturation low data was collected rom 2,733 vehicles on eleven approaches with lane widths varying between 2.6 and 2.9 m (8.5 and 9.5 t). Four approaches with lane widths varying between 3.9 and 4.7 m (13.0 and 15.5 t) were also surveyed, with a sample size o 1,568 saturation low vehicles. All baseline conditions except or lane width were held constant at these locations. The survey results were then compared with those o the baseline condition surveys (with a sample size o 6,687 saturation low vehicles). The narrower lane widths demonstrated saturation low rates between 2 and 5 percent less than did those in the baseline surveys, while the wider lane widths demonstrated saturation low rates 5 percent greater than did those in the baseline surveys. Zegeer proposed the ollowing lane width adjustment actors: Saturation low Lane width category (t) adjustment actor A 1983 study by Agent (3) o the eects o lane width on saturation low indicated that lane width did not have an eect on saturation low or lane widths o 3.0 m (10 t) or more. For lane widths between 2.7 and 3.0 m (9 and 10 t), a 5 percent reduction in saturation low was ound compared to lane widths o 3.0 m (10 t) or more. No lane widths below 2.7 m (9 t) were observed. There was a slight unexplained reduction in saturation low or lane widths greater than 4.5 m (15 t). A similar analysis was perormed with the limited data available or commercial vehicles, and no eect was ound even or lane widths below 3.0 m (10 t). Table 1 illustrates the eect o lane width on saturation low ound by Agent. Fort Lauderdale, Florida
18 The Eect o Lane Width on Urban Street Capacity October 29, 2008 Page 4 TABLE 1. Eect o lane width on saturation low Lane width(t) Total headway (sec) Average headway (sec) Saturation low (vphg) a a , , , , , , , or more , , , or more 17, ,653 vphg vehicles per hour o green time. In a 1992 study by Bonneson (4), it was determined that discharge headway is a unction o a vehicle s position in the queue and, thereore, measurements taken between the ourth and eighth vehicles will have longer headways than measurements taken between the eighth and eleventh vehicles. Using empirical data rom two study sites, Bonneson developed a model to estimate the impact o queue position on saturation low rate. Bonneson ound that the minimum discharge headway using queue positions our through ten is about 0.02 s/veh shorter than that ound when using queue positions our through eight. This dierence translates into a base saturation low rate ratio o 1.3 percent. The ollowing text summarizes the research results rom the study conducted by Potts, et.al. (1): Field studies were conducted at signalized intersections to determine the dierence in saturation low rates o exclusive through lanes with 2.7-, 3.0-, 3.3-, 3.6-, and 4.0-m (9-, 10-, 11-, 12-, and 13-t) lane widths. Let- and right-turn vehicles were not surveyed. Data collection ocused on through travel lanes under the most ideal conditions possible to minimize the inluence o site-speciic actors. At those intersection approaches where exclusive let- or rightturn lanes were present, vehicles turning rom the exclusive turn lanes were observed or a minimum period o time to ensure that they did not inluence surveyed vehicles in the adjacent through lanes. To eliminate any inluence o turning vehicles at sites with shared through-right or through-let lanes, data were not collected or signal cycles during which any turning movement took place. Saturation low headways were measured beginning when the ront axle o the ourth vehicle in queue crossed the stop bar. The cumulative elapsed time was then measured when the ront axle o the last vehicle in queue (stopped at the onset o the green signal phase) crossed the stop bar. Any impedance (driveway movements, bus stop activity, pedestrian or bicycle activity) that could inluence the saturation low rate during a surveyed signal green phase was noted. The number o heavy vehicles per cycle was documented. For analysis purposes, the study sites were grouped into lane width categories as ollows: 2.9 m (9.5 t) (9 study sites) 3.3 to 3.6 m (11 to 12 t) (12 study sites) Fort Lauderdale, Florida
19 The Eect o Lane Width on Urban Street Capacity October 29, 2008 Page m (13 t) and greater (4 study sites) From the average headway o each headway sample, an average saturation low rate was calculated. Table 2 presents basic average saturation low statistics (sample size, mean, median, minimum, maximum, standard deviation, and relative standard deviation) or each lane width category. TABLE 2. Average saturation low statistics (pc/h/ln) or each lane width category Lane width category (t) N Mean Median Minimum Maximum Standard deviation CV (%) ,752 1, , to ,830 1, , ,913 1, , Total 1,196 1 Coeicient o variation = 100% x standard deviation/mean. The results o this research indicate that using narrow lanes (i.e., 2.9 m [9.5 t]) on signalized intersection approaches on urban and suburban arterials resulted in an average saturation low rate that is approximately 78 to 79 pc/h/ln, or 4.3 percent, lower than i 3.3- to 3.6-m (11- to 12-t) lanes are used. Similarly, using lane widths o 4.0 m (13 t) or greater resulted in an average saturation low rate that is approximately 82 to 84 pc/h/ln, or 4.3 to 4.4 percent, higher than i 3.3- to 3.6-m (11- to 12-t) lanes are used. Both relationships were negligibly aected by whether average saturation low was adjusted or the position o the vehicle in the queue. The HCM provides saturation low rate adjustment actors or lane widths that are greater than or less than 3.6 m (12 t). Table 3 compares the saturation low rate estimates based on HCM procedures to those measured in the current research. The table shows that the measured saturation low rate values are generally lower than those obtained rom HCM procedures. Furthermore, the percent dierence in saturation low rate between sites with 2.9- to 3.6-m (9.5-to 12-t) lanes was ound to be about hal the value used in the HCM. These indings should be considered as a basis or revisions to the HCM. In particular, there appears to be justiication or revising the HCM lane width adjustment actors or lane widths less than 3.6 m (12 t). TABLE 3. Comparison o saturation low rate values rom this research to HCM values HCM Current research Lane width (t) Adjusted saturation low rate a (pc/h/ln) Percent dierence rom value or 12-t lanes Adjusted saturation low rate b (pc/h/ln) Percent dierence rom values or 12-t lanes 9.5 1, , , ,815 c , ,815 c , ,815 c 0 Fort Lauderdale, Florida
20 The Eect o Lane Width on Urban Street Capacity October 29, 2008 Page 6 a b c d 13 1, ,898 d , ,898 d +4.5 The HCM saturation low rates have been adjusted or lane width. The saturation low rates rom the current research have been adjusted or queue position. This value was derived or sites with a range o lane widths rom 11 to 12 t. This value was derived or sites with a range o lane widths o 13 t or more. Conclusions Drawn rom the Research All o the relevant research is in general agreement as to the impact o narrowing lane width on saturation low or through lanes on signalized intersection approaches. The measured saturation low rates are similar or lane widths between 10 eet and 12 eet. For lane widths below 10 eet, there is a measurable decrease in saturation low rate. Thus, so long as all other geometric and traic signalization conditions remain constant, there is no measurable decrease in urban street capacity when through lane widths are narrowed rom 12 eet to 10 eet. Fort Lauderdale, Florida