CONSIDERING AND APPLYING DRIVEWAY DESIGN …docs.trb.org/prp/13-2223.pdf · Gattis, Gluck, Barlow, Eck, Hecker, Levinson 13-2223 11 November 2012 2 2 ABSTRACT NCHRP Project 15-35,

Gattis, Gluck, Barlow, Eck, Hecker, Levinson 13-2223 11 November 2012 1

CONSIDERING AND APPLYING DRIVEWAY DESIGN FOR ALL USERS 2

by J. L. Gattis, J. S. Gluck, J. M. Barlow, R. W. Eck, W. F. Hecker, Jr., and H. S. Levinson 4

AUTHORS: J. L. Gattis 4190 Bell / Civil Engineering Dept. Univ. of Arkansas Fayetteville, AR 72701 voice: (479)575-3617 fax: (479)575-7168 [email protected]

J. S. Gluck AECOM 605 Third Avenue New York, NY 10158 voice: (212)973-2962 [email protected]

J. M. Barlow Accessible Design for the Blind 3 Manila Street Asheville, NC 28806 voice: (770)317-0611 [email protected]

R. W. Eck Dept. of Civil Engineering, West Virginia Univ. P.O. Box 6103 Morgantown, WV 26506-6103 voice: (304)293-9931 [email protected]

W. F. Hecker, Jr. Hecker Design, Ltd. - Accessible Design

Consultants 3568 Hampshire Drive Birmingham, AL 35223 voice: (205)298-1900 [email protected]

H. S. Levinson Transportation Consultant 5305 Ashlar Village Wallingford, CT 06492 voice: (203)949-9700 [email protected]

6 Word count: 8 3704 words + 15 exhibits x 250 = 3704 + 3750 = 7454

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TRB 2013 Annual Meeting Paper revised from original submittal.


ABSTRACT 2 NCHRP Project 15-35, Geometric Design of Driveways, was initiated to help address the lack of current comprehensive research and national design guidance for the design of driveway 4 connections to roadways. The research initiated with this project included an extensive literature review, a survey of state agencies and contacts with interest groups, and field work to measure 6 traffic attributes. The project produced two publications, a research report on the NCHRP website and the Guide for the Geometric Design of Driveways, NCHRP Report 659. This paper 8 considers the following topics. What design issues were identified? Current design practices may not adequately consider 10 the range of all driveway users – bicyclists, motorists, and pedestrians. This section will also report vulnerability of various users, based on historic crash data. 12 What user attributes were found? The research conducted produced information about the driveway grades at which the undersides of vehicles may drag, and the speeds at which vehicles 14 on urban arterials entered commercial driveways having radii ranging from 13 to 20 ft. What design practices were recommended? To address the needs of all users, the design 16 guide offered a number of design applications that differ from commonly-seen practices. This paper provides useful information for design consultants and local government 18 professionals.

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CONSIDERING AND APPLYING DRIVEWAY DESIGN FOR ALL USERS by 2

J. L. Gattis, J. S. Gluck, J. M. Barlow, R. W. Eck, W. F. Hecker, Jr., and H. S. Levinson 4 INTRODUCTION Driveways are private roads that provide access (both ingress and egress) between a public way 6 and abutting properties, and facilities on those properties. When roadway designers speak of “driveways”, they often actually mean the part of the driveway in the area near to where the 8 driveway connects to the public highway or street: this is the use employed in this paper. Since driveways are private roads, it can be easy to overlook and dismiss them. But in 10 many settings, driveway connections are the most common form of intersection found on public streets and highways. It is not uncommon for the driveway connections along a road lined with 12 commercial or industrial land uses to have higher volumes than many of the streets in the urban network. 14 Overall, relatively little comprehensive research has been conducted, and the amount of guidance for driveway connections is relatively small compared to that for other roadway 16 elements. In 1959, the American Association of State Highway Officials (AASHO) published An Informational Guide for Preparing Private Driveway Regulations for Major Highways (1), 18 and in 1987 the Institute of Transportation Engineers published Guidelines for Driveway Location and Design (2). In the 1970s, Texas Transportation Institute researchers conducted 20 driveway-related studies (3, 4, 5). Changes in roadway design, function, and volumes, along with a growing emphasis on recognizing and accommodating not just motorists but other 22 roadway users, influenced the decision to conduct National Cooperative Highway Research Project (NCHRP) 15-35, the Geometric Design of Driveways. This paper summarizes some of 24 the documented design issues, identified user attributes, and recommended design practices from this research. 26 PROJECT STRUCTURE 28 The research team had two underlying perspectives. One was that there are a variety of users in the area where driveways cross sidewalks and intersect with roadways. The user groups listed 30 were bicyclists, motorists, pedestrians, pedestrians with disabilities, and transit riders. Members of these groups may have different attributes (e.g., speed), as well as different and sometimes 32 conflicting needs. The second perspective was that the test of how well or how poorly a driveway 34 connection is designed is determined by how well or how poorly the connection operates after it is opened. When comparing two or more competing design alternatives, one that operates better 36 and with fewer problems is likely to be the preferred choice. Driveway connections create intersections, which in turn create conflicts with bicyclists, 38 pedestrians, and other motor vehicles. An objective of good design is to seek a balance that provides travel and access while minimizing the actual conflicts. The following driveway design 40 objectives guided the authors during the project. • Provide a safe environment for various users: bicyclists, motorists, and pedestrians 42

(including pedestrians with disabilities and transit passengers). • Provide geometry that accommodates the characteristics and limitations of the various 44

users, and avoid geometric conditions that create traffic operations problems. • Provide driveways that allow traffic to flow smoothly. 46



• Avoid driveway locations that create traffic operations problems. • Provide driveways that are conspicuous and clearly delineated for the various users. 2 It may not always be possible to perfectly achieve these objectives, but some designs come closer than others in achieving them. 4 Based on transportation agency responses to a survey, numerous transportation agency design standards, information gleaned from a review of approximately 100 reports, and input 6 from interest groups and individuals, the research team developed a list of 65 considerations or elements that are often within the control of the designer, and another 32 that are generally 8 outside of the control of the designer, but do create issues or a context that may affect design decisions. The Figure 1 graphic depicts a few of these, and Tables 1 and 2 list them. 10 The project produced a web only research report (6) and NCHRP Report 659, Guide for the Geometric Design of Driveways (7), both of which were reviewed and approved by a panel 12 comprised of 12 people from both consulting firms and public agencies. 14

FIGURE 1 Selected Driveway Design Elements 16

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TABLE 1 Driveway Considerations That Are Generally Outside the Control of the Designer 2

Shared Elements, Surroundings1 Land use2 User and vehicle mix and composition3 Temporal variation: season, day of week, time of day4 Weather and weather effects

Sidewalk-Driveway Intersection5 Sidewalk placement (adjacent to or offset from the curb or edge)

Roadway-Driveway Intersection 6 Elevation difference between roadway surface and abutting property

Roadway in vicinity of the Driveway 7 Width of roadway8 Lanes (number, width)9 Lane type (travel, HOV, bicycle, turn, parking)10 Cross slope (travel lanes, shoulders)11 Horizontal alignment of roadway12 Vertical profile of roadway13 Sight distance restrictions

User characteristics - Bicyclist14 Bicyclist perception-reaction process, time15 Speed16 Braking capability17 Sight distance need

User characteristics - Pedestrian 18 Pedestrian perception-reaction process, time19 Speed20 Sight distance need

Special needs groups21 General - children, elderly22 Disabled (e.g., mobility, visually)23 Legal mandates - disabled

User characteristics - Vehicle, Driver24 Driver perception-reaction process, time25 Speed26 Deceleration characteristics (typical)27 Braking capability (limiting)28 Sight distance need29 Vehicle width30 Vehicle length31 Vehicle turning radius32 Vehicle front overhang, wheelbase, rear overhang, and ground clearance dimensions

4

6



TABLE 2 Driveway Geometric Design Considerations That May Be Within the Control of the Designer 2

Shared Elements, Surroundings1 Illumination2 Conspicuity (to visually detect an element at a distance) 3 Sight obstructions

Driveway4 Width (maximum and minimum; sufficient for ped. refuge)5 Lanes (number, width)6 Median in driveway: (absence or presence)7 width8 type (raised, flush, depressed)9 nose-end recessed from edge of through-road10 Cross slope, cross slope transition runoff11 Horizontal alignment, curvature12 Connection depth (throat length)13 Traffic controls or other potential impediments to inbound traffic (inc'l entry gate)14 Paving length (applicable where have unpaved driveway)15 On-site turn-around capability (where backing into roadway is undesirable) 16 Driveway edge (edge drop off, barrier)17 Space for nonmotorized users (e.g., pedestrian movement parallel to driveway) 18 Driveway border treatments (sideclearance, sideslope)

Vertical profile19 grade (maximum and minimum)20 change of grade (grade breaks)21 vertical curve design criteria22 Vertical clearance (from overhead structures, utility lines)23 Drainage (separate from intersection drainage)24 Other special situations (e.g. railroad crossing, trail, bridle path, etc.)

Sidewalk-Driveway Intersection25 Sidewalk cross slope (i.e., driveway grade)26 Path definition (e.g., visual, tactile cues)27 Crossing length (i.e., driveway width)28 Angle of intersection with driveway:

flat-angle (turn angle < 90O); right-angle (turn angle ≈ 90O); sharp-angle (turn angle > 90O)29 Bearing of sidewalk relative to street:sidewalk diverging from, parallel to, or converging with the street30 Grade of sidewalk (i.e., driveway cross slope)31 Vertical profile of pedestrian route (abrupt elevation change: max. 1/4" )32 Sidewalk-driveway interface treatment:detectable warnings for visually impaired (e.g., truncated dome)

(only at certain locations, inc'l. at signalized crossing; refer to guidelines )

Roadway-Driveway Intersection 33 Angle of intersection with street:

flat-angle (turn angle < 90O); right-angle (turn angle ≈ 90O); sharp-angle (turn angle > 90O)34 Cross slope of street and shoulder, considered with driveway grade35 Curb threshold treatment (rolled, vertical lip, counterslope, continuous)36 Curb-termination treatment (abrupt end, drop-down, returned)37 Entry transition shape (e.g. radius, flare/taper, straight, etc.)38 Entry transition-shape dimensions (radius, flare dimensions)39 Channelization of right turn from street into driveway40 Channelization of right turn from driveway into street41 Channelization in the driveway: triangular island to prohibit in and out left-turns42 Channelization in street - street median prohibits all left-turns in/out of driveway43 Channelization in street - street median prohibits one but not both left-turns44 Drainage: confining the gutter flow45 Drainage: inlet type and location46 Clearance from fixed objects, appurtenances47 Pavement surface deformity (corrugation, potholes)



TABLE 2 - continued

Traffic Controls (for driveway vehicles)48 Driveway-roadway intersection control (none, yield, stop, signal)49 Turn restrictions50 One-way operation (one-way, do not enter)51 Markings (pavement, delineators)52 Other

Roadway in vicinity of the Driveway 53 Right-turn lane attributes: (absence or presence)54 right-turn lane width55 right-turn lane deceleration, storage length56 right-turn lane entry transition shape57 right-turn lane offset58 Left-turn lane attributes: (absence or presence)59 left-turn lane width60 left-turn lane deceleration, storage length61 left-turn lane entry transition shape62 left-turn lane offset63 Number of driveways per site64 Driveway spacing from upstream access connection65 Driveway spacing from downstream access connection 2

4 DESIGN ISSUES AND IMPLICATIONS Many design issues can be categorized as relating to safety, mobility, convenience, or a 6 combination of these for one or more user groups. The research team identified aspects of currently-observed design situations that are candidates for reexamination and revision. 8 Vulnerability of Various Users 10 Driveway design not only affects convenience and mobility, but also affects safety. Team members found a few studies that examined driveway safety. 12 Box examined relationships among land uses, volumes, and accidents related to driveways in Skokie, Illinois (8, 9). He first considered driveway collisions on 40 miles of the 14 major traffic streets. Left-turns were involved in 60% of all and 75% of the injury accidents. Driveways on these routes had an average of 0.13 crashes per year, but for the 569 residential 16 driveways on the major streets, the rate was 0.02 crashes per year. Routes with barrier medians had 0.02 accidents per driveway per year, as compared to other routes that had 0.17 – a ratio of 18 about 1 to 8. An expanded study, with five years of data, showed that 11% of all reported crashes 20 involved driveway movements, with driveways a factor in 12% of the crashes on major streets and 9% of those on residential streets. Driveways with extremely wide (100 to 120 ft) openings 22 had four times the accident frequency of those with narrower openings. At service stations, the greater number of driveways per station, the greater the number of accidents. Of the 407 24 pedestrian and bicycle rider accidents during the five years, 3% involved driveways, most often with a motor vehicle leaving the establishment. 26 Examining four years of crash data from 100 roadway sections on Indiana roadways, McGuirk and Satterly (10) found that 14% were driveway related. Of these driveway crashes, 28 left-turn in or out movements were involved in 65% of all and in 76% of injury crashes.



A review of driveway related accidents in Texas (11) found that 93% of all driveway-related accidents occurred in cities and towns. About two-thirds of the crashes involved a 2 vehicle leaving the driveway and less than one-third involved a vehicle entering the driveway. Studying a sample of 3000 bicycle-motor vehicle crashes in six states, 1.7% occurred at 4 alleys and driveways (12). From a sample of over 800 pedestrian crashes, Stutts et al. (13) found that 3% were at alleys and driveways. 6 Six years of Washington state data (14) produced 8,540 bicycle collision records. For all roads and for city streets, Collision Group C (a motorist entering or leaving the roadway at a 8 mid-block location, back from driveway) accounted for less than 1% of crashes. Group F (motorist turning, bicyclist not) included 1.1% on all roads and 1.4% on city streets. Less than 10 0.5% of the crashes on roads or on city streets fell into the “motorist drive out from park” subgroup within Group G. 12 Rawlings and Gattis (15) examined over 2,000 accident reports from Springdale, Arkansas, for one year to identify which crashes were driveway-related. Driveway-related was 14 defined as a collision that occurred either directly or indirectly due to the operation of a driveway. After the detailed review given to each crash report, it was determined that a number 16 of driveway-related crashes had not been coded to indicate the driveway relationship. They found that the single highest proportions of driveway crashes involved left-turn egress. Almost 18 1/6 of the crashes involved vehicles backing from a driveway. Over 1/6 of the crashes involved maneuvers in a two-way left-turn lane that possibly would not have occurred had a restrictive 20 (raised or depressed) median, with or without left-turn lanes, been in place. Table 3 compares their findings with those of previous studies. 22 TABLE 3 Comparing Driveway-Related Collision Studies 24 Attribute Percent of all crashes with attribute Skokie Indiana Texas Arkansas Springdale Urban that are driveway-related 11 14 15 13 19 Occurred at commercial sites 75 72 - - 73 Occurred at restaurants 16 - - - 17 Occurred at service stations 16 - - - 10 Involved left-turns 60 65 - - 63 Resulted in injury 31 14 11 38 unknown Involved pedestrians or bicyclists 4 - - 1 1 NOTE: “-” means no value for this Source: Rawlings and Gattis, TRB Paper 08-0710 26 Visual and Tactile Cues and Pedestrian Route Accessibility Pedestrians with low vision may find it challenging to stay on the sidewalk path as it crosses the 28 driveway, and not veer into the street or into the driveway. Visual and tactile cues can help people who are blind or have low vision identify and negotiate the driveway location, and 30 determine the sidewalk path across the driveway. In addition, pedestrians with a range of disabilities find their travel impeded when they encounter inadequate widths or abrupt surface 32 elevation changes. Figure 2, showing a sidewalk crossing a driveway, displays some possible treatments to 34 provide cues. The slope between the street edge and the sidewalk edge (i.e., the driveway grade) is much greater than the sidewalk cross slope. The difference between the slopes may help 36



pedestrians who are blind distinguish between the two areas, and avoid accidentally veering into the street area as they cross the driveway. There is also a color difference between the sidewalk 2 and the driveway throat area, and a slight texture difference between the sidewalk and driveway surface behind the sidewalk which can be detected by some pedestrians using a cane. 4

6 FIGURE 2 Sidewalk Crossing a Driveway Offering Pedestrian Cues 8 Space for Bicyclists and Pedestrians 10 As a result of the positions of some parking lots relative to nearby buildings, in conjunction with the layout of the parking lot, the desire lines (i.e., the desired path between an origin and a 12 destination) of pedestrians and bicyclists may coincide with the driveway location. This leads to the situation shown in Figure 3, where a pedestrian is walking in the same narrow space that 14 motor vehicles occupy. A driver preparing to turn into the driveway may be monitoring traffic ahead and not detect the pedestrian in her or his peripheral vision, and upon turning may not 16 have time to react to this sudden presence of a pedestrian in the vehicle path. At driveways used by bicyclists, a preferable design should either provide wider lanes or 18 a separate parallel path to accommodate bicyclists. This need is even more critical on a steep upgrade, on which bicyclist will struggle to maintain speed and a straight trajectory. 20 If pedestrian’s desire lines will place them parallel to the driveway, then a relatively parallel sidewalk nearly adjacent to the driveway may be in order. 22 Driveway Threshold and Connection Geometry 24 Observations of relatively new driveway connections, such as the driveway with the rutted edge in Figure 4, show that there is a need for improved driveway connection design. Elements that 26 should often be relatively simple to design well include the shape of the connection transition, dimensions of the connection transition, connection width, and gradient of the connection 28 transition.



FIGURE 3 Lack of Sidewalk Forces Pedestrian Into the Driveway 2 4

6 FIGURE 4 Misfit Driveway Connection 8 Connection Transition Shape 10 Figure 5 displays the types of connection transition shapes that were noted during the course of the study: perpendicular edge, rectangular apron, flare or taper, and curved radius. Each of these 12 shapes has advantages and disadvantages, some of which are affected by the particular setting in which the driveway is found. But upon evaluation, these alternative shapes do not all seem to be 14 equal. The AASHTO (American Association of State Highway and Transportation Officials) Green Book (Guide for the Geometric Design of Highways and Streets) states “Flared driveways 16 are preferred because they are distinct from intersection delineations …”(16); in other words,



because they do not look like roadway intersections, motorists can distinguish between driveways and side streets. While this may be a benefit in a few situations, in many situations 2 there is no benefit to be had from this distinction, and even if there were, other aspects of driveway design will provide a visual difference for motorists to rely on. 4

THESE ARE PLAN (TOP) VIEW DRAWINGS

Curved radius

drivewayback of curb

roadway

Flare/Taper

drivewayback of curb

roadway

back of curb

Rectangular apron

driveway

roadway

Perpendicular edge

driveway

roadway

back of curb

6 FIGURE 5 Driveway Connection Transition Shapes 8 10 In Table 4, each of the four alternative treatments is ranked according to how well it meets the listed design objectives. For this matrix, the flare/taper and curved radius are ranked 12 higher. When evaluating these two options based on how well they accommodate the users, the curved radius is the better one. 14 16 TABLE 4 Comparison of Connection Transition Shapes

18 Connection Transition Dimensions and Width 20 The dimensions of the radius or taper act in concert with the width to affect the ease or difficulty of turning into and out of a driveway connection. In environments with very low volumes and 22 speed, such as those found in some residential areas, vehicles can often utilize the middle of the road for driveway turning maneuvers. However, in many roadway environments, a vehicle 24 encroaching into adjacent lanes can create operational and safety problems. Also consider that outside of metropolitan centers, pedestrian volumes can be low. Figure 6 shows appropriate 26 design objectives to meet drivers’ needs for such situations. 28

Design Objectives Perpendicular Edge

Rectangular Apron

Flare/Taper Curved Radius

Conforms to path of turning vehicle worst (1) poor (2) better (3) best (4) Definition of edge for motorists poor (2) worst (1) better (3) best (4) Definition of edge for pedestrians best (4) worst (1) better (3 tie) better (3 tie) Ease of construction best (4) better (3 tie) better (3 tie) worst (1) Overall score 11 7 12 12 NOTE: Cannot compare scores directly, because the importance or weights of each objective are not equal.



Does a vehicle turning into the driveway encroach into the adjacent lane?

Does vehicle encroach into adjacent lane?

roadway

sidewalk

driveway

Does vehicle encroach upon curb or sidewalk?

Does vehicle encroach into adjacent lane?

Does vehicle encroach upon curb or sidewalk?

Does a vehicle turning out of the driveway encroach

into the adjacent lane?

FIGURE 6 Driveway Transition Connection Design Objectives 2 4 The research team recorded data for over 1500 vehicles on suburban multilane arterials, having either a raised median or a two-way left-turn lane, with speed limits of 40 or 45 miles per 6 hour, turning into 12 commercial driveways with an entry radius ranging from 13.0 to 19.5 ft and entry lane widths of about 13 ft. 8 Very few vehicles about to enter a driveway exceeded 20 mph at the locations a few feet in advance of the driveway entry. After crossing the driveway threshold, average speeds for 10 vehicles turning left into the driveway were around 10 mph. Vehicles that had turned right into the driveways were slightly slower, with average speeds around 7 mph. It was not uncommon to 12 observe drivers turning right into a driveway with these dimensions to exhibit driving that indicated feeling slightly constrained. Table 5 displays the recommendations based on 14 engineering judgment and approved by the project panel. 16 Driveway Throat Length A number of access management documents address the issue of driveway throat or connection 18 lengths for commercial sites. In addition to including commercial driveway throat length guidance, NCHRP Report 659 also addressed a situation that is likely to concern bicyclists and 20 pedestrians in residential and in dense urban settings. Figure 7-a shows vehicles parked in short driveways protruding into the sidewalk space; in addition, items in the bed of the pickup in the 22 foreground extend even further, and at an elevation that could be very problematic for a visually impaired pedestrian. 24 This is a scenario that calls for traffic engineers to interact with other municipal departments to require minimum dimensions graphically depicted in Figure 7-b, in order to 26 preserve a usable pedestrian access route. 28 Driveway Visibility Since the concept of sight distance has been documented and available for more than half of a 30 century, one might assume that designers would know to eschew impediments that interfere with users’ lines of sight. But as with driveway connection geometry, observations of relatively new 32 construction (see Figure 8) show that this is not the case. The Green Book explains stopping sight and intersection sight distances in detail. 34 Therefore, additional guidelines are not needed, but rather education and awareness on the part of practitioners and officials. 36



TABLE 5 Driveway Width and Radius Recommendations Category Description of Common

Applications Driveway Width Driveway Curb Radius

Higher speed road

Moderate speed road

Lower speed road

STANDARD DRIVEWAYS Very high intensity

Urban activity center, with almost constant driveway use during hours of operation.

Many justify two lanes in, two to three lanes out. Refer to street design guides.

R = 30’ to 50’

R = 25’ to 40’

na

Higher intensity

Moderately-sized office or retail, such as community shopping center, with frequent driveway use during hours of operation.

One entry lane, 12 to 13 ft wide. Two exit lanes, 11 to 13 ft wide.

R = 25’ to 40’

R = 20’ to 35’

na

Medium intensity

Smaller office/retail/apartment complex, with occasional driveway use during hours of operation. Seldom more than one exiting vehicle at any time.

Two lanes, 24 to 26 ft total width.

R = 20’ to 35’

R = 15’ to 30’

na

Low intensity

Single family or duplex residential, other types with very low use, on lower speed/volume roadways. May not apply to rural residential.

May be related to the width of the garage, or driveway parking. Single lane: 9 to 12 ft Double: 16 to 20 ft

R = 15’ to 25’

R = 10’ to 15’

R = 5’ to 10’

SPECIAL SITUATION DRIVEWAYS Central business district

Building faces are close to the street.

Varies greatly, depending on use. Often related to the width of the opening in a building face.

na R = 20’ to 25’

R = 10’ to 15’

Industrial Driveways used by large vehicles.

Min. 26 ft. R = 50’ to 75’

R = 40’ to 60’

R = 40’ to 60’

NOTES: These widths do not include space for a median, or a parallel bike lane or sidewalk. Additional width may be needed if the driveway has a curved horizontal alignment. For a flare/taper design, use the radius as the dimension of the triangular legs. For Industrial or other driveways frequented by heavy vehicles, consider either a simple curve with

a taper or a 3-centered curve design. For connection angles greatly different than 90O, check the radius design with turning templates. For connection corners at which a turn is prohibited, a very small radius is appropriate. . If the roadway has a usable shoulder, a somewhat smaller radius may perform acceptably.

Adapted from NCHRP Report 659, p. 40

2 4 6 8



driveway

sidewalkroadway

Where stopping or parking in driveway occurs,Minimum Driveway Length = sum of

1. setback to outer edge of sidewalk, or other similar control

+2. design vehicle length 3. buffer+

(a) (b)

FIGURE 7 Minimal Driveway Lengths 2

FIGURE 8 Roadside Objects Restrict Sight Distance 4 6 Connection Transition Vertical Alignment The vertical profile of driveway connections can create impediments. A vertical lip at a 8 driveway threshold causes an unpleasant bump for not only drivers but also for bicyclists and pedestrians with disabilities. The bump may cause a bicyclist to lose control. 10 Excessive grades and breakover grades (changes of grade between roadway cross slope and the driveway grade) create operational problems. The dragging and scraping of vehicles’ 12 undersides (see Figure 9) where the breakover is too steep may damage vehicles or even cause vehicles to become hungup, requiring a wrecker to extricate the vehicle. The number of 14 collisions that result when a driver abruptly decelerates upon perceiving the abrupt geometry of the driveway they are about to enter is unknown. 16



FIGURE 9 Excessive Breakover Grade Results in Vehicles Dragging on the Pavement 2 The previously mentioned 12 driveways were assigned to one of the following three 4 categories. Steeper: grades up from the gutter line of 12.5% to 15.5%, and with breakovers between 6

13.5% and 19.0% Moderate: grades up from the gutter line between 6.0% and 9.0%, with breakovers 8

between 5.0% and 10.5%. Flatter: grades up from the gutter line between 1.5% and 5.0%, with breakovers between 10

5% to 6.5%. The research team measured ground clearance dimensions of four vehicles (Camaro, 12 Corvette, Class A diesel motor home, and tractor connected to a 10-bay beverage trailer), and obtained manufacturers literature to define the dimensions for a pickup truck with a trailer. The 14 team also made measurements to define the vertical profiles of 31 driveways with visible scrape marks on the pavement surface. 16 The recommendations shown in Figure 10, applicable to driveways where the design vehicle is a passenger car, were based on a combination of the entry speed and vehicle ground 18 clearance studies. Although a much more exhaustive study would have been beneficial, from the information collected, it was concluded that these recommended dimensions are not extremely 20 conservative; a smaller SAG angle may be desirable for some vehicle-trailer combinations.

Driveway

Maximum breakover sag = 9%

Maximum breakover

crest = 10%* Maximum breakover is the maximum without a vertical curve.

- ELSE - Vertical curve

22 FIGURE 10 Suggested Vertical Profile Criteria 24



SUMMARY Making use of a variety of input sources, the NCHRP project addressed a wide range of 2 driveway design issues. The report discusses other issues that were outside of the scope of this paper. 4 Summarizing the issues and problems that were examined, from the collision data reviewed from urban environments, the following were found. 6 1. Left-turn movements are overrepresented. 2. Driveway related crashes comprise anywhere from 11% to 19% of all crashes. 8 3. Crashes involving bicyclists or pedestrians at driveway or alley connections comprise a

very small percent (somewhere between 0 and 3%) of the total. 10 4. It may be that if one relies on summary data, the percent of crashes involving driveways

will be underreported. A detailed study of re-examination of each crash record report is 12 required to get a better estimate.

In order to assess and evaluate trade-offs among different user groups, it would be desirable to 14 have more information about crash severity, and to determine if driveway related crashes with bicyclists or pedestrians tend to occur in certain environments, or if they are randomly dispersed 16 across the entire urban landscape. Opportunities exist for research and development of treatments to accommodate 18 pedestrians with visual and mobility disabilities as they cross driveways. Recognizing current situations should lead to better accommodations of bicyclists and pedestrians traveling parallel to 20 driveways. In addition to the considerations discussed in the body of the paper, designers should be cognizant of what might be called the driveway border: the width past the edge of the 22 driveway. Is there adequate room to accommodate pedestrians or sidewalk, or is the toe of slope is too close to the driveway? 24 AASHTO and predecessor publications have presented stopping sight and intersection sight distance principles and dimensions for many decades; these principles should be applied to 26 driveway connections. Similarly, existing documents and the Guide for the Geometric Design of Driveways provide designers with information to create driveways that better conform to 28 motorist turning paths. Problems that arise from inadequate driveway length illustrate the principle that in order to create safe and efficient roadway operations, controls past the right-of-30 way line are necessary; a minimum setback distance could be established so that vehicles do not block the sidewalk. 32 A wide variety of vehicle types with different ground clearance requirements travel roadways. While certainly not addressing the many types, the NCHRP 659 guidelines do 34 address the great majority of these vehicles, passenger cars. 36 ACKNOWLEDGMENT This paper is based on findings from NCHRP Project 15-35, Geometric Design of Driveways. 38 REFERENCES 40 1. American Association of State Highway Officials. An Informational Guide for Preparing

Private Driveway Regulations for Major Highways. Washington, D.C., October 10, 1959, 42 copyright 1960, 31 pp.

2. Institute of Transportation Engineers. Guidelines for Driveway Location and Design. 44 Washington, D.C., 1987, 23 pp.



3. Stover, V.G., W. G. Adkins, and J. C. Goodknight. Guidelines for Medial and Marginal Access Control on Major Roadways, NCHRP Report 93. Highway Research Board, 2 Washington, D.C ., 1970, pp 39-42 and 117-133.

4. Richards, S. H. Guidelines for Driveway Design and Operation, Vol 2. Research Report 4 5183-2. Texas Transportation Institute, College Station, Tex, April 1980, 214 pp.

5. Stover, V.G. Guidelines for Spacing of Unsignaled Access to Urban Arterial Streets, 6 Technical Bulletin 81-1. Texas Engineering Experiment Station, Texas A&M University System, College Station, Tex, January 1981, pp. 19-32. 8

6. Gattis, J.L., J. S. Gluck, J. M. Barlow, R. W. Eck, W. F. Hecker, and H. S. Levinson. Geometric Design of Driveways, NCHRP Web-Only Document 151. Transportation 10 Research Board of The National Academies, Washington, D.C., July 2009.

7. Gattis, J.L., J. S. Gluck, J. M. Barlow, R. W. Eck, W. F. Hecker, and H. S. Levinson. Guide 12 for the Geometric Design of Driveways, NCHRP Report 659. Transportation Research Board of The National Academies, Washington, D.C., 2010, 91 pp. 14

8. Box, P.C. Driveway Accident and Volume Studies, Part I-General Relationships, Public Safety Systems, May/June 1969, pp. 18-22. 16

9. Box, P.C. Driveway Accident and Volume Studies, Part II-Service Stations. Public Safety Systems (July/August 1969) pp. 15-18. 18

10. McGuirk, W.W. and G.T. Satterly. Evaluation of Factors Influencing Driveway Accidents, JHRP 76-1, Purdue University, W. Lafayette, Ind., 1976. 20

11. Richards, S.H. Guidelines for Driveway Design and Operation, Vol 2, Research Report 5183-2. Texas Transportation Institute, College Station, Tex., April 1980. 22

12. Hunter, W.W., J.C. Stutts, W.E. Pein, and C.L. Cox. Pedestrian and Bicycle Crash Types of the Early 1990's, FHWA-RD-95-163. Federal Highway Administration, Washington, 24 D.C., June 1996, pp. 131-138.

13. Stutts, J.C., W.W. Hunter, and W.E. Pein. Pedestrian Crash Types: 1990s Update, 26 Transportation Research Record 1538. Transportation Research Board of the National Academies, Washington, D.C., 1996, pp.68-74. 28

14. Wessels, R.L. Bicycle Collisions in Washington State: A Six-Year Perspective, 1988-1993, Transportation Research Record 1538. Transportation Research Board of the National 30 Academies, Washington, D.C., 1996, pp. 81-90.

15. Rawlings, J. and J. L. Gattis. A Detailed Study of Driveway Collision Patterns in an Urban 32 Area, Compendium of Papers DVD, Paper #08-0710, 87th Annual Meeting. Transportation Research Board of the National Academies, Washington, D.C., 2008, 15 pp. 34

16. American Association of State Highway and Transportation Officials. A Policy on Geometric Design of Highways and Streets, Washington, D.C., 2011, p5-19. 36

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Documents

CONSIDERING AND APPLYING DRIVEWAY DESIGN …docs.trb.org/prp/13-2223.pdf · Gattis, Gluck, Barlow, Eck, Hecker, Levinson 13-2223 11 November 2012 2 2 ABSTRACT NCHRP Project 15-35,