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Geometric Design Elements 33 Exhibit 5-13. Driveway design considerations related to nearby bus stops. Concern or Issue Design Response Specific Procedure and/or Information Do not block drivers' Provide adequate stopping Refer to the latest edition of the lines-of-sight sight distance or AASHTO Green Book for the intersection sight distance. procedure to calculate the needed Provide separation from stopping sight distance or intersection driveway edge to bus stop. sight distance. Do not place bus shelters in a location that blocks needed sight lines. Space so that the bus Provide distance from Length of city transit bus: does not physically block driveway edge to bus stop typical bus, about 40 ft. the driveway articulated bus, about 60 ft Additional length along roadway needed for maneuvering: where buses do not change lanes: 20 ft where a lane change is required: 50 ft Patrons loading or Provide a pedestrian unloading from transit standing area separate from vehicle do not occupy the and removed from the driveway driveway Connect the bus loading area Pedestrian connection at least 5 feet to the sidewalk wide, where possible Driveway Plan and Cross-Section Elements The following sections discuss driveway plan and cross-section elements, such as the type of entry/ exit geometry, the amount of flare or radius, the driveway width, and driveway channelization. Driveway Width, Number of Lanes, and Connection Transition This section discusses and presents recommendations for three plan view elements: 1. The normal width of the driveway throat, which does not include the normally found widen- ing or transition with a radius or a flare near the driveway intersection with the roadway; 2. The number of driveway lanes needed; and 3. The shape and dimensions of the shape at the connection (throat entry/exit) transition. The driveway width and the driveway connection transition are separate elements, but the design of each can affect the design of the other, so the discussion of these elements has been combined. Objectives for designing the driveway entry and exit geometry include the following: 1. Define the edge so it is visible for bicyclists, drivers, and pedestrians. 2. Minimize the width of the driveway that bicyclists and pedestrians will need to cross. 3. Design a shape that conforms to the path of the turning vehicle, which enables vehicles to enter a driveway without encroaching into other lanes. 4. Design to enable vehicles to enter the driveway without significantly impeding the upstream flow of through traffic on the roadway. 5. Provide adequate driveway capacity, including providing separate right- and left-turning exit movements, when needed. 6. Design for easy construction. Exhibit 5-14 raises questions that address the design of the connection transition, and Exhibit 5-15 shows the effects of inadequate geometry in this area. The geometry should not force normal right entry or exit movements to cross over the driveway curb or edge, drive on

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34 Guide for the Geometric Design of Driveways Exhibit 5-14. Design issues for a vehicle turning right into or from a driveway. Does a vehicle turning into roadway Does a vehicle turning out the driveway encroach into of the driveway encroach the adjacent lane? into the adjacent lane? sidewalk sidewalk Does vehicle encroach Does vehicle encroach on the curb or sidewalk? on the curb or sidewalk? Does vehicle encroach Does vehicle encroach into the adjacent lane? to the adjacent lane? driveway Exhibit 5-15. Effects of inadequate driveway radii. (a) (b) (c)

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Geometric Design Elements 35 the sidewalk, or swing wide so that the left side encroaches into adjacent lanes. But excessive width unnecessarily increases the distance across the driveway that bicyclists and pedestrians must cross. Interrelated Factors Affecting the Design Various factors act in concert as a vehicle turns into or out of a driveway and as the vehicle crosses a bicycle lane or sidewalk parallel to the roadway. The following interrelated elements come into play as a driver turns into or out of a driveway intersection (5-7): 1. Visibility and conspicuity of the features that shape the driveway (e.g., opening, edges, markings); 2. Vehicle turning radius; 3. Vehicle tracking width and offtracking characteristics; 4. Intersection-corner treatment and treatment dimensions (e.g., radius or taper dimensions); 5. Width of the lane from which the turn is made (includes offsets, edge flares); 6. Width of the lane into which the turn is made (includes offsets, edge flares); 7. Angle of the intersection; 8. Cross slope of the pavement surface in the turn; 9. Pavement surface condition (e.g., in extreme cases, a corrugated surface or pothole can impart vertical acceleration to a turning vehicle, decreasing the available side friction); 10. Turning speed; 11. Driver's tolerance of lateral acceleration; and 12. Driver's ability to perceive these elements. The vertical profile also affects the driving experience. Before selecting the dimensions of the connection transition, the designer should identify design vehicles for the particular driveway. Chapter 3 includes a discussion of design vehicle con- siderations. Where heavy vehicles may run over area behind the curb and damage surfaces such as a sidewalk or a driveway median, the designer should consider a strengthened pavement sur- face for the affected area behind the curb. Number of Lanes A basic driveway design question is "how many driveway lanes should be provided?" Typically, driveways serving a single-family residence are one or two lanes wide, often reflecting the width of the garage. Driveways serving farms and fields are typically one lane wide, although the width of that lane is quite wide, reflecting the widths of farm machinery. In general, driveways serving commercial and industrial sites should have at least two lanes (one-way driveways are an obvious exception), operating with one lane in each direction. With increasing driveway volume, adding a second exit lane becomes highly desirable in order to avoid excessively long queues and delay. Without two exit lanes, a motorist waiting for gaps in both traffic directions before turning left out of the driveway will unnecessarily block other motorists in the exit queue who could otherwise turn right when there are gaps in the traffic from the left. However, the number of lanes exiting from the development and turning in one direc- tion must not exceed the number of available traffic lanes on the roadway in that direction. For example, for a driveway entering a two-lane two-way roadway, no more than one lane in each direction (a total of two exit lanes) should be allowed to exit the driveway. Generally, dual exit- lane driveways are desirable when the exit volume reaches the level that more than one vehicle will want to exit the driveway within the time interval it takes one left-exiting vehicle to wait for and accept an adequate gap in roadway traffic, or when the driveway intersection with the pub- lic road is signalized. Exhibit 5-16 shows some of the more common commercial driveway con- figurations, excluding those for very high volumes.

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36 Guide for the Geometric Design of Driveways Exhibit 5-16. Common choices for a range of commercial driveway designs. yellow white white Width for 2 lanes. (Without Width for 1 entry and 2 exit Width for 1 entry and 2 exit a marked center line, there lanes; separated by yellow lanes; separated by median may be more encroachments.) markings If the driveway forms the fourth leg of an intersection, additional lanes may be needed. In such cases, a configuration of three exit lanes (left turn, through, and right turn) and/or two entry lanes may be desirable. At what might be considered the high-volume end of the spectrum, sites such as major shopping centers and urban activity centers, even more lanes may be needed. If there is a question of whether additional lanes are needed, an operational analysis of the inter- section between the driveway and the roadway, perhaps using calculations from the Highway Capacity Manual (5-8), could be performed. Variables reflected in the operational analysis include the volume on the main roadway, the volume and directional distribution of the driveway traffic, the number of adequate size gaps in through street traffic, and the form of traffic control at the driveway/roadway intersection. Driveway Width The width of a driveway is its normal width, measured some distance back from its intersection with the roadway. It is not the width that includes widening near the intersecting roadway. The width of a driveway is a function of the number of driveway lanes, the widths of those lanes, and the presence and width of a median. The width of a driveway should reflect the needs of both vehicular and pedestrian traffic. The competing goals of reducing vehicle delay by adding lanes and reducing pavement width to facil- itate pedestrian crossings need to be recognized. Exhibit 5-17 shows a wide-open, undefined driveway across what appears to be the full frontage of the tract. These designs are particularly unfriendly to bicyclists and pedestrians crossing the excessive driveway opening width. Because of the lack of lane definition, vehicles enter and leave such sites in random positions and are more likely to cross paths. Such a design should be avoided. Exhibit 5-17. Undesirable wide-open driveway.

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Geometric Design Elements 37 Exhibit 5-18. Range of reported driveway widths and radii. Normally, Use . This in Most Commercial (ft) Residential (ft) Situations (ft) Smallest Smallest Smallest reported reported reported reported reported reported Average Average Average Largest Largest Largest Width for 2-way: normal maximum (ft.) 24 34 40 35 40 46 12 23 30 Width for 2-way: normal minimum (ft.) 12 24 35 12 22 30 8 12 15 Entry-shape plan-view dimensions for curved radius, maximum R (ft.) = 20 41 75 40 50 70 10 23 35 for curved radius, minimum R (ft.) = 3 16 25 15 21 30 3 11 15 NOTE: These values reflect survey responses from 1 local and 16 state transportation agencies. Exhibit 5-18 presents ranges of driveway widths and radii given in response to a survey of transportation agencies. Connection Transition Shape For a driveway to intersect a public roadway, a break in the curb line or the edge of the road- way is required. This section discusses aspects of the transition that occur past the break, within the first few feet of the driveway. This transition may be designed with a perpendicular edge, rec- tangular apron, flare or taper, or curved radius. Exhibit 5-19 shows basic types of driveway con- nection transition geometries. The driveway connection transition, where turning vehicles enter and exit the driveway, is usu- ally a critical design area, given that it is the location of potential interaction of entry and exit movements (5-9). One of the key aspects of good driveway design is accommodating the entry and exit movements so that they do not encroach on one another or vehicles in other lanes (5-9). In some situations, this requires that the driveway radius be almost as large as the turning radius of the selected design vehicle. Except on low-volume, low-speed roadways, the curb radius or flare dimensions should be designed so that normal right-turn entry movements do not have to slow down to a near stop in the through travel lanes on the roadway. The dimensions should also allow drivers to turn into or from a driveway without encroaching into conflicting lanes of traffic. Where the roadway is curbed, the entry shape also acts in concert with the curb termination treatment at a driveway entry. Curbs may be terminated abruptly, by means of a drop-down curb, or by a return curb. Exhibit 5-20 shows curb termination treatments. (When the drop-down curb design accompanies a flare/taper transition edge shape, this is sometimes called a "dust pan.") Exhibit 5-21 compares the connection transition shape alternatives. The perpendicular edge is the easiest to construct, but the least conspicuous to both motorists and pedestrians and does not conform to or help define the path of a turning vehicle. The rectangular apron is better than Exhibit 5-19. Types of driveway connection transition geometry. roadway roadway roadway roadway back back back back driveway of driveway of driveway of driveway of curb curb curb curb Perpendicular edge Rectangular apron Flare/Taper Curved radius THESE ARE PLAN (TOP) VIEW DRAWINGS

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38 Guide for the Geometric Design of Driveways Exhibit 5-20. Methods to terminate the curb. Method to terminate the curb: Method to terminate the curb: Method to terminate the curb: RETURN CURB ABRUPT END DROP-DOWN CURB curb driveway curb driveway curb driveway roadway roadway roadway curb driveway roadway (a) (b) (c) the perpendicular edge in terms of functionality, but slightly more difficult to construct. The use of both types should be limited to single-family or duplex residential units. The flared taper is easier to build than the curved radius, but is less effective in terms of conspicuity and conform- ing to the path of a turning vehicle. Therefore, the use of the flared taper generally should be lim- ited to low intensity or medium intensity uses. It has been stated that "Flared driveways are preferred because they are distinct from intersection delineations . . . " (5-1, p.398); 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 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. As for curb termination treatments, an abrupt end is more likely to snag a vehicle tire, and therefore is undesirable. A returned curb has a vertical face, which provides entry-edge definition for an approaching motorist. A discussion of design treatments for sidewalks crossing the driveway in this area is in Pedestrians and Pedestrians with Disabilities. Exhibit 5-22 presents a table from the 2005 Florida Driveway Handbook (5-9, p.31), which was derived from much older sources. The numerical values illustrate the inverse relationship between entry radius and the width of the entry lane: as the size of the radius increases, less entry lane width is needed. Based on recent experience, these dimensions may be generous for many drivers of passenger cars. Connection Transition Design Suggestions The preceding discussion of driveway transition shapes leads to the suggestions in Exhibit 5-23. Driveway Width Design Suggestions When establishing driveway widths and driveway opening treatment dimensions (e.g., size of radius or flare), it is not uncommon to encounter opposing viewpoints; there may be conflict- ing objectives between the various driveway users, such as pedestrians and motorists, with some Exhibit 5-21. Comparison of connection transition shape alternatives. Design Objectives Perpendicular Rectangular Flare/Taper Curved Edge Apron Radius Conforms to path of turning worst (1) poor (2) better (3) best (4) vehicle Definition of edge for motorists poor (2) worst (1) better (3) best (4) Definition of edge for pedestrians best (4) worst (1) poor (2) better (3) Ease of construction best (4) better (3) better (2) worst (1) Overall score best (12) NOTE: Cannot compare scores directly, because the importance or weight of each objective is not equal.

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Geometric Design Elements 39 Exhibit 5-22. Inverse relationship between entry radius and entry lane width. Exhibit 5-23. Driveway transition shape design guidelines. Category Description of Common Applications * Suggested Driveway Transition Shape Design (assuming curbed roadways in urban area, uncurbed in rural area) STANDARD DRIVEWAYS Very high Urban activity center, with almost constant Design as a street intersection. Provide intensity driveway use during hours of operation. separate right- and left-turn lanes on Typified by a driveway serving a post-1950 approaches to public roadways. major shopping center or office complex. Not uncommon for such driveways to be signalized. Higher Medium-size office or retail, such as community Use curb radius design. Consider intensity shopping center, with frequent driveway use separate right- and left-turn lanes on during hours of operation. Also includes land approaches to public roads. uses with extreme peaking patterns, such as public schools, worship assemblies, and employee parking lots. Medium Smaller office or retail, such as convenience Curb radius design is preferred. intensity stores, with occasional driveway use during hours of operation. Also includes some apartment complexes. Lower Typical applications include single-family or Use either the curb radius or the intensity duplex residential, other types with low use. flare/taper design. May not apply to rural residential. SPECIAL SITUATION DRIVEWAYS Central Building faces are close to the street. May have Design will vary depending on location, business on-street parking or bus stops, a continuous land use, and traffic volumes. district sidewalk from the curb to faces of buildings, and higher pedestrian usage than in most other environments. Many situations will serve P-cars and some single-unit trucks. Farm or May be a mixture of residential and industrial Design uncurbed radius or taper to ranch characteristics, used by a mix of design vehicles, accommodate farm/ranch vehicles. such as P-car, single-unit truck, and agricultural equipment. Field Serves a field or other similar rural land area that Design uncurbed radius or taper to is seldom trafficked. Higher-clearance P- accommodate farm/ranch vehicles. vehicles or heavy vehicles are expected. Industrial Driveways frequently used by buses, tractor with Design for trucks. The driveway may semi-trailers, and other vehicles longer and need a special design to accommodate wider than the design passenger car. the extra axles and longer wheelbase that will lead to much greater offtracking of vehicles entering the driveway. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.

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40 Guide for the Geometric Design of Driveways calling for smaller dimensions to make crossing the driveway easier for pedestrians, and others wanting larger dimensions to facilitate motor vehicle ingress and egress. An operational analysis of the intersection between the driveway and the roadway provides a basis for decisions regarding the number of driveway lanes. The connection transition and the driveway width dimensions should complement each other to produce good driveway opera- tions. The driveway width and the curb radius can perform in concert, so to some degree one can increase as the other decreases. In other words, a wide driveway can be used together with a small radius or flare to achieve similar operations to a narrower driveway with a larger radius or flare. When only one vehicle is expected to be using the driveway at any given time, such as a res- idential driveway serving a two-car garage, the smaller radii are suitable with the greater widths. Exhibit 5-24 offers guidelines for driveway width and radius. These dimensions do not con- sider the presence of an offset between the outer edge of the traveled way and the end of the Exhibit 5-24. Driveway width and curb radius guidelines. Category Description of Common Driveway Width Driveway Curb Radius (in ft) Applications (Note: These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.) Higher Moderate Lower speed speed speed road road road STANDARD DRIVEWAYS Very high Urban activity center, with Many justify two lanes 3050 2540 NA intensity almost constant driveway use in, two to three lanes during hours of operation. out. Refer to street design guides. Higher Medium-size office or retail One entry lane, 1213 ft 2540 2035 NA intensity (e.g., community shopping wide center) with frequent driveway Two exit lanes, 1113 ft use during hours of operation. wide. Medium Smaller office or retail, with Two lanes, 2426 ft total 2035 1530 NA intensity occasional driveway use during width hours of operation. Seldom more than one exiting vehicle at any time. Lower Single-family or duplex May be related to the 1525 1015 510 intensity residential, other types with low width of the garage, or use, on lower speed/volume driveway parking. roadways. May not apply to Single lane: 912 ft rural residential. Double: 1620 ft SPECIAL SITUATION DRIVEWAYS Central Building faces are close to the Varies greatly, depending NA 2025 1015 business street. on use district Farm or A mix of design vehicles; some Min. 16 ft, desirable 20 3040 2030 NA ranch; may be very low volume. ft. Affected by widths Field of field machinery. Industrial Driveways are often used by Minimum 26 ft 5075 4060 4060 large vehicles. 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 90 degrees, check the radius design with turning templates. For connection corners at which a turn is prohibited, a very small radius is appropriate. Also see the section, Driveway Horizontal Alignment and Angle. Driveways crossing an open ditch should have a minimum 2 ft shoulder on each side. (source: Statewide Urban Design and Specifications, Iowa State U., Ames, IA (October 21, 2008) p. 4. If the roadway has a usable shoulder, a somewhat smaller radius may perform acceptably.

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Geometric Design Elements 41 Exhibit 5-25. One-way driveway widths from selected states. Agency Source Category Width for one-way Florida Driveway Handbook Urban 12 ft. minimum Missouri 940.16 (5/13/09) Driveway 2030 ft New Jersey C-11 (6/20/07) Driveway 2023 ft New York 608-03 (1/8/09) Minor Commercial 1224 ft; 16 ft normal Utah 12.1.1601.10 Driveway 1232 ft driveway, i.e., the driveway threshold. There are arguments for and against adjusting the radius when an offset is present. Some agencies reduce the required radius when an offset is present, expecting turning vehicles to follow an effective radius that utilizes the space between the outer edge of the traveled way and the threshold. Arguments against this practice include an assump- tion that some drivers may not follow the imaginary effective radius, but instead try to follow the visible physical connection transition edges. Also, it is possible that, in future, the roadway cross section width may be reallocated and the offset eliminated, resulting in an undersized connection transition. One-Way Driveway Widths Only a small fraction of driveways operate in a one-way mode. Information on which to base guidance for the design of one-way driveways is limited and, as Exhibit 5-25 shows, current agen- cies' standards differ considerably. Structured studies of one-way driveway design elements would be helpful. Throat Transition Design for Larger Vehicles The offtracking of even-turning single-unit trucks can result in tires running over the curb or the sidewalk behind the curb. But if the designer accommodates turning trucks with a sim- ple radius design, this accommodation may create a very wide entry opening. To better accom- modate the wheel paths of turning trucks without paving such a wide area, refer to the AASHTO Green Book's (5-1, p. 583621 ) discussion of designing simple curves with a taper and design- ing three-centered compound curves. Exhibit 5-26 dissects the geometry of a three-centered curve at a 90-deg. intersection. Throat Width for Curved Driveways If the driveway horizontal alignment is curved instead of straight, then additional driveway width may be required to account for the effects of vehicle offtracking. Refer to the AASHTO Green Book (5-1, p. 202223) for procedures to determine how much additional width is needed. Throat Transition Widening Some driveways are constructed with a wider section close to the intersecting roadway, then Exhibit 5-26. Geometry of a sym- the width tapers to a narrower section some distance back from the intersecting roadway. Two metrical 3-centered of the reasons for doing this are to widen the driveway curve. 1. To provide additional lanes at the intersection with the public roadway; and Offset Offset 2. To accommodate the offtracking and swept paths of turning vehicles entering and exiting driveways at the entry/exit area. 2 R R1 In either case, the designer will need to design a transition from the wider cross-section width R2 to the narrower cross-section width. Exhibit 5-27 shows a schematic of this concept. A 6:1 taper would be adequate for an assumed design speed of 19 mph. Tapers of 8:1 to 12:1 R1 should be more than adequate for the typical driveway, excluding those that look and operate like public roadways.

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42 Guide for the Geometric Design of Driveways Exhibit 5-27. Taper to effect Exhibit 5-28. Example taper design. throat transition widening. O O Taper angle 4.764 4.764 / 2 42.618O 42.618O R R 15 ft 20 ft PC roadway TAN 13.80 ft 18.40 ft 2 Q 13.75 ft 18.34 ft Offset at PT 1.85 ft 1.47 ft R TAN Offset PT TAN 12:1 taper Y = 3' driveway Q X = 36' Exhibit 5-28 shows an example 12:1 taper design. For a driveway having 12-ft-wide lanes, the 15-ft radius provides a motorist with a 13.9 ft (12.0 + 1.9) opening at the point of tangency (PT), and the 20-ft radius produces a 13.5 ft (12.0 + 1.5) opening at the PT. Channelization Various types of channelization are sometimes incorporated into driveway designs. These include medians in the driveway, islands in the driveway, and channelization in the roadway at the intersection with the driveway. The general design objectives for channelization are to Separate conflicting movements (including opposing directions of travel) Control the angle of conflict Reduce excessive pavement area Regulate traffic and indicate proper use of driveway/intersection Provide pedestrian refuges/protection Provide for protection and storage of turning and crossing vehicles Where channelization is desired but there is not sufficient space to accommodate the width of a median or island, some agencies have installed channelizing devices such as tubular markers. These are discussed later in the section, Traffic Controls. Channelization in the Roadway Medians are sometimes labeled as being either non-restrictive or restrictive. A non-restrictive median is a median or painted centerline that does not provide a physical barrier between center traffic turning lanes or traffic lanes traveling in opposite directions; examples include continuous center turn lanes and undivided highways. Restrictive medians physically separate vehicles travel- ing in opposite directions and restricts the movement of traffic across the median; (e.g., a concrete barrier or guard rail, a raised curb island, or a grassed or swaled median). Either type can be designed to provide some degree of separation between opposing traffic flows and provide space for left- turning vehicles out of the through lane. Restrictive medians offer several safety benefits. Restrictive medians in the roadway provide refuge for pedestrians crossing the roadway. Some median openings allow all traffic movements to be made. Other openings restrict left-turn and other movements across the median. Prohibiting movements translates into fewer conflicts, greater safety, and more uniform travel speeds along the arterial. However, these benefits may be somewhat offset by the increased turning volumes where there are full-movement median openings. Restrictive medians are often used because, as

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Geometric Design Elements 43 Exhibit 5-29. Driveway crash types related to maneuver and orientation. Percent of Total Maneuver______Turn____Collision_______Driveway Crashes Entering Left Rear-end 26 Leaving Left Right-angle 24 Entering Left Head-on angle 15 Entering Right Rear-end 12 Leaving Right Right-angle 7 Leaving Right All other 8 Leaving Left All other 3 Entering Right All other 3 Entering Left All other 2 100 Source: Box, Public Safety Sys., 1969 indicated in Exhibit 5-29, more than two thirds of driveway crashes are related to left-turn entry or exit movements (5-10). Exhibit 5-30 shows how restrictive medians may be used to eliminate some or all of the left- turn movements in and out of driveways. They may be channelized to allow for left-turns from the roadway, while prohibiting left-turns out. Alternatively, they may be channelized to allow for left-turns from the driveway, while prohibiting left-turns in. The following items are aspects of good median approach-island design (5-11): The approach nose should be offset from the approach lanes to minimize accidental impacts. The shape of the island should be based on the turning path of the design vehicle and the island function. The length of the island should be related to approach speed and reflect available width, taper design, and local constraints. The width of the island should adequately serve its intended functions (e.g., access control, pedestrian refuge, separation of conflicts, and shielding left-turn lanes). Median islands should begin on tangent alignment and on upgrades or well past crest vertical curves. It may be appropriate to extend a median island to avoid its introduction on a hori- zontal curve or within an area of limited sight distance. Designs that prohibit some left-turn movements are often accompanied by an analysis of traffic patterns that identifies alternate means of accomplishing a left turn, such as downstream U-turns or other indirect means. U-turns are used as an alternative to direct left turns to reduce conflicts and improve safety along arterial roadways.They make it possible to eliminate left-turn movements into and out of driveways. They also make it possible to eliminate the need for certain traffic signals (or reduce the number Exhibit 5-30. Using restrictive medians to eliminate some left-turn movements. Source: Transportation and Land Development, Second Edition.

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44 Guide for the Geometric Design of Driveways of signal phases) that would not fit into a progression pattern (5-12). The median openings where U-turns would be made need to be designed to accommodate the additional turning movements. Requests for a median opening or opposition to closing a median opening may be based on an assumption that a direct left-turn egress maneuver is preferable to a right turn followed by a U-turn. However, observations show that drivers often make a right turn followed by a U-turn where the median opening design permits a direct left turn from a driveway (5-13). The addi- tional travel distance of turning right and then making a U-turn is offset by travel time savings by not having to wait for a gap in both directions that is needed for direct, left-turn egress. Medians in the Driveway The benefits of restrictive medians in a roadway can also accrue when medians are installed in driveways. Medians in a driveway may be appropriate where one or more of the following conditions exists: The driveway has two or more entrance lanes. The driveway has two or more exit lanes. There is a large pavement area that may confuse drivers. The driveway operates as right-in/right-out, and this may be unclear to some drivers. The driveway serves a high volume of traffic. The driveway is or will be signalized. A median in a driveway that separates the ingress and egress movements is appropriate for very high-intensity driveways, where the median may provide refuge for pedestrians, separate the opposing traffic flows, and channelize the traffic movements. Exhibit 5-31 provides guidance for when a median in a driveway may be beneficial. The presence of a median will make the overall length of the pedestrian crossing wider. However, this may be more than offset by the pedestrian refuge effect the median creates in the middle of the driveway. Where a driveway median is needed, there are minimum dimensions that apply to avoid hav- ing a median that is too short and narrow. There is also a possibility that if the median island is too wide, drivers may mistake one driveway with a median for two separate driveways (5-9, p. 46). Exhibit 5-31. Driveway median use recommendations. Driveway Description of Common Applications Applicability of Median in Driveway Category (Note: These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.) STANDARD DRIVEWAYS Very high Urban activity center, with almost constant Applicable intensity driveway use during hours of operation. Higher Medium-size office or retail, such as community May be applicable intensity shopping center, with frequent driveway use during hours of operation. Medium Smaller office or retail, some apartment Usually not applicable, but may be intensity complexes. applicable for some wider driveways Lower Single-family or duplex residential, other types Not applicable intensity with low use. May not apply to rural residential. SPECIAL SITUATION DRIVEWAYS Central business Building faces are close to the street. Usually not applicable, but may be district applicable for some wider driveways Farm or ranch; A mix of design vehicles; some may be Usually not applicable Field very low volume. Industrial Driveways are often used by large Often not applicable, but may be vehicles. applicable for some wider driveways

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Geometric Design Elements 45 Exhibit 5-32. Driveway median design guidelines. Aspect Suggested Design Rationale Length Minimum 40 ft, preferable 50 ft or more Need adequate length for conspicuity, effectiveness. Width Absolute minimum: 4 ft Absolute minimum based Minimum to provide pedestrian refuge: 6 ft on the Green Book. Width for visibility of landscaping: 8 to 10 ft Maximum based on Maximum for a driveway divisional island (width of potential for drivers to the part that is unavailable for travel, i.e., not mistake one driveway including turn lane widths): 12 to 16 ft. with a median for two separate driveways. End The 2004 AASHTO Green Book states that for a To fit the wheel path of a treatment median island less than 10 ft wide, a semicircular turning vehicle, per the end shape is adequate. For median island widths of 2004 Green Book 10 ft or more, a bullet nose end shape is suggested. (p.697). From observations, a bullet nose shape may be desirable for widths of less than 10 ft. Geometry of a bullet nose median-end shape Half-bullet nose median-end shape Stop line Median width Radius of nose 2 Radius of nose Exhibit 5-32 lists suggested minimum dimensions and presents two versions of bullet nose end geometry. The half bullet nose provides a larger radius to accommodate the path and offtrack- ing of a vehicle nearing the end of its left turn. If the stop line and stopped position for vehicles leaving the site is close to the median nose end, then a lesser radius may be adequate for that move- ment. Examine the turning paths of left-turning vehicles to ascertain what shape will suffice. Islands in the Driveway Driveway triangular islands (pork chops) can be constructed in the driveway entry throat at driveway intersections with both divided and undivided roadways to Channelize right turns, Discourage or prohibit one or both left turns, or Provide refuge for pedestrians. Exhibit 5-33 illustrates three different scenarios for using islands to discourage left turns. A triangular island and an angled driveway can have some design objectives and features in common. The objectives of either design can include facilitating right turns and discouraging left turns. With both, the design can attempt to align vehicles at a skewed angle rather than per- pendicular to the public roadway. Exhibit 5-34 shows two schematics for islands to channelize right turns exiting a driveway (5-13). In the (a) schematic, a flatter entry-angle combined with a larger radius may increase the speed at which right-turning vehicles leave the driveway. The flatter entry-angle requires the driver's head to turn a greater number of degrees to the left to monitor oncoming traffic from the left. If the design evokes a subconscious association with a freeway entry ramp, it could theoretically give the driver a false sense of a free entry into the through roadway. This arrangement has been criticized for being

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46 Guide for the Geometric Design of Driveways Exhibit 5-33. Using islands to discourage left-turn movements. Source: Transportation and Land Development, Second Edition. unfriendly to pedestrians because of the relatively high speed of the right turn and the need for drivers to scan a wide angle for pedestrians. The practice of placing the pedestrian crosswalk in the middle of the curve, however, affords drivers an improved view of pedestrian crossings. The second schematic (b) shows an alternative design that has several advantages. Motorists turn- ing right from a driveway can more easily see approaching through traffic. This design is more pedes- trian friendly because drivers have a better view of the sidewalk and the speeds are relatively slow. The island area should be sufficiently large to command attention. Refer to the AASHTO Green Book (5-1) to find the recommended minimum area and dimensions on a side. At loca- tions where there is a likelihood of traffic, especially large trucks, overrunning the island, there may be a need for a mountable curb and structural pavement within the island. Along roadways lacking a restrictive median, observations of traffic movements at triangular islands indicate it is not uncommon for drivers to make unusual maneuvers to circumvent the island and make prohibited movements. It is challenging to identify a shape, size, radius, and other features that will effectively discourage drivers from circumventing the island. Florida DOT does not use driveway triangular islands on undivided roadways (5-9). Lakewood, Colorado, uses a 40- by 18-ft island (360 sq ft) where all left turns are prohibited and a 20- by 18-ft (180 sq ft) island where only the left turns leaving the driveway are prohibited. A design recommended for South Exhibit 5-34. Comparing two right-turn island designs. Source: Transportation and Land Development, Second Edition.

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Geometric Design Elements 47 Dakota incorporated a long stem on the driveway-end of the triangular island to discourage Exhibit 5-35. Design wrong-way movements (see Exhibit 5-35). To better achieve the objective, triangular island proposed to discourage installations have been accompanied by the installation of a barrier median or by vertical pylons circumventing (traffic posts) along the middle of the public roadway. triangular island. Inappropriate Channelization roadway Although channelization can be beneficial, it can also be ineffective or inappropriate in some situations. One does not have to look far to find examples (e.g., Exhibit 5-36) of questionable Source: Dye Mgmt. CHANNELIZING Group, Review of driveway ISLAND SDDOT's Hwy. Access Control Process, Feb. 2000 Exhibit 5-36. Channelization with dubious benefits. (a) (b) (c)

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48 Guide for the Geometric Design of Driveways island designs. The islands in the photos seem too small. It appears that the island in the upper photo is aligned in a way that forces right-turning vehicles off of the pavement into the dirt. One can guess at the intent for islands in photos (b) and (c), but one wonders if they have any posi- tive effects on traffic, or if they are just obstacles and nuisances. Visual and Tactile Cues Providing visual and tactile cues that distinguish the sidewalk and define it separately from other driveway areas can assist pedestrians having visual impairments to cross the driveway effi- ciently and safely. Texture, visual contrast, and slope differences are desirable. Exhibit 5-37 shows a sidewalk crossing a driveway. The driveway has a distinct slope toward the street. The slope between the street edge and the sidewalk edge is much greater than the slope across the sidewalk. The difference between the slopes may help pedestrians with vision impair- ments 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 and the driveway throat area and a slight texture difference between the sidewalk and asphalt which can be detected by some pedestrians using a long cane. Except for signalized driveways or a few other cases, the use of detectable warning surfaces, such as truncated domes, is discouraged because overuse of detectable warnings surfaces may make it more difficult for pedestrians with vision impairments to recognize streets and to maintain their orientation (see Exhibit 5-38 for further discussion). Exhibit 5-39 shows a typical driveway con- struction plan for a detectable warning surface on a sidewalk at the edge of a signalized driveway. Driveway Cross Slope Where the driveway intersects the roadway, one side of a driveway will be higher than the other side, unless the roadway that the driveway intersects is perfectly level. Proceeding from the trav- eled edge toward the private property, the designer can alter the relative difference in elevation between the two outer edges of the driveway by having different edge profile grades. Throughout Exhibit 5-37. Sidewalk with visual, tactile, and geometric cues crossing a driveway.

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Geometric Design Elements 49 Exhibit 5-38. When to use detectable warning surfaces. Advisory R221 Detectable Warning Surfaces. Detectable warning surfaces are required where curb ramps, blended transitions, or landings provide a flush pedestrian connection to the street. Sidewalk crossings of residential driveways should not generally be provided with detectable warnings, since the pedestrian right-of-way continues across most driveway aprons and overuse of detectable warning surfaces should be avoided in the interests of message clarity. However, where commercial driveways are provided with traffic control devices or other- wise are permitted to operate like public streets, detectable warnings should be provided at the junction between the pedestrian route and the street. Source: Revised Draft Guidelines for Accessible Public Rights-of-Way, November 23, 2005 [http://www.access-board.gov/prowac/draft.htm#221 ] the length of the driveway, one edge may be higher than the other, or the center line may be higher than the edges, creating driveway cross slope. Where the driveway and sidewalk intersect, the driveway cross slope is the same as the sidewalk grade. In the absence of information specifically developed for driveways, these guidelines have incor- porated cross slope recommendations from AASHTO. A minimum cross slope of 2% is recom- mended to provide surface runoff drainage. Where feasible, a maximum cross slope of 8% is recommended in areas where snow or ice can occur. Exhibit 5-39. Example of a detectable warning surface at edge of signalized driveway.

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50 Guide for the Geometric Design of Driveways Driveway Horizontal Alignment and Angle The alignment of a driveway near the connection with the public roadway affects traffic operations and safety on both the driveway and the roadway. This section addresses driveway horizontal alignment and the angle of intersection with the roadway. When there are driveways on opposite sides of the roadway from each other, the designer should check the alignment of through lanes, turning lanes, and medians on both the intersect- ing roadway and the driveway for potential operational and safety problems. The through lanes should not be offset, but aligned. The review should include a check for whether the lane and median locations would create an offset that would obstruct sight distance (e.g., the sight distance between a vehicle turning left and an opposing through vehicle). Angle of Intersection Just as it is undesirable for two roadways to intersect at highly skewed angles, it is undesirable for most driveways to intersect the roadway at a large skew. When a skew angle forces drivers to deal with a turning angle that is much less than or greater than 90 degrees, drivers will have greater difficulty turning their heads to scan the through roadway for an adequate gap, and more distance and time is required to complete an acute angle turning movement. If the crossing is perpendi- cular to the driveway, it will also be more difficult for bicyclists and pedestrians about to cross a driveway to look over their shoulders to spot vehicles turning from the main roadway. Research studies have concluded that the intersection angle should not be skewed from 90 degrees by more than 15 to 20 degrees (5-14 through 5-16). One-way driveways are an exception to this, and they have operated successfully with skew angle intersections with the roadway. Exhibit 5-40 lists minimum allowable angles reported in a survey of transportation agencies. For two-way driveways, the average value allowed no more than about 20-deg. deviation from 90 degrees. Where the one-way driveway is intended to operate in a right-turn entry-only or a right-turn exit-only manner, there are tradeoffs. One theory is that a flatter angle (e.g., 45 degrees) makes it less likely that drivers will violate the right-turn-only intention or use the driveway in the wrong direction. On the other hand, the flat exit requires that drivers entering the roadway turn their heads much more than 90 degrees to see oncoming traffic from the left. Also, the greater the skew angle, the greater the crossing distance parallel to the roadway for bicyclists and pedestrians. A review of state standards indicates that few allow an angle less than 60 degrees at one-way driveways. Given strictly as an example, the following passage and drawing from the Ohio DOT manual regulate the angle at which a driveway intersects the roadway: 803.21 Drive Intersection Angle New drives should intersect the highway at an angle between 70 and 90. However, in some cases, it may be necessary to retain existing drive angles that vary from these desirable angles. Exhibit 5-40. Range of reported allowable driveway intersection angles. Normally, Use This in Most Commercial Residential Situations Smallest Smallest Smallest reported reported reported reported reported reported Average Average Average Largest Largest Largest For 2-way drive, minimum angle with the 60 68 90 60 69 75 60 70 90 roadway allowed (90O is right-angle) For 1-way drive, minimum angle with the 45 64 90 45 68 90 45 66 90 roadway allowed (90O is right-angle) NOTE: These values reflect survey responses from 1 local and 16 state transportation agencies.

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Geometric Design Elements 51 Exhibit 5-41. Example of a skew-angle driveway. Exhibit 5-41 shows a guideline for angled, one-way driveways, from Ohio DOT design details. Exhibit 5-42 lists suggested minimum allowable intersection angles for driveways. Driveway Horizontal Alignment Past or back from the driveway connection transition area (the intersection with the roadway), the horizontal alignment (i.e., plan view) of a driveway should be straight, not curved. One rea- son for this is so the driver of a motor vehicle entering or leaving the driveway does not have the added task of steering in a compound or reverse or multiple curves, which diverts more atten- tion from the task of monitoring crossing bicyclists, pedestrians, and vehicles. Another reason is that a straight alignment makes it easier for drivers to position and align their vehicles as they approach the intersection and make turning maneuvers and not sideswipe other vehicles. A third reason is to avoid creating situations where the vehicle exiting the site is unintentionally posi- tioned at a skew angle to the roadway. Exhibit 5-43 recommends minimum lengths of straight approaches in advance of the actual physical intersection of a driveway with a roadway. Space for Bicyclists and Pedestrians Motor vehicles are not the only form of traffic traveling perpendicular to the roadway to and from a traffic generator set back from the roadway. Bicyclists and pedestrians also make these movements at many locations and, in the absence of a separate facility, they may bike or walk in the driveway. Exhibit 5-44 shows a pedestrian on a gray, overcast day with light rain, forced to walk in the driveway because of the lack of a sidewalk. At this particular location, the lack of a sidewalk con- tributes to occasional conflicts between vehicles and pedestrians. Exhibit 5-42. Suggested driveway intersection angles with roadway. Driveway Category Description of Common Minimum Allowable Applications* Driveway Intersection Angle in Degrees STANDARD DRIVEWAYS Very high intensity, Higher 70 intensity, Medium intensity Lower intensity Very infrequent use, such as 60 single-family or duplex residential, on urban lower volume, lower speed roadways SPECIAL SITUATION DRIVEWAYS CBD, Farm or ranch, Field, 70 Industrial One-way, for either right-turn Flat, acute angle 45 to 60 entry-only or right-turn exit-only may discourage wrong-way use * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category.

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52 Guide for the Geometric Design of Driveways Exhibit 5-43. Suggested minimum lengths of straight driveway alignment. Driveway Description of Common Applications* Minimum Length of Straight Alignment Category on the Driveway Approach Adjacent to the Connection Transition with a Public Roadway STANDARD DRIVEWAYS Very high Urban activity center with almost constant 75 ft (based on length of 3 P-car) intensity driveway use during hours of operation. Local requirements for tangent at a signalized intersection may apply. Higher Medium-size office or retail (e.g., community 50 ft (based on length of 2 P-car) intensity shopping center) with frequent driveway use during hours of operation. Medium Smaller office or retail, some apartment 25 ft (based on length of 1 P-car) intensity complexes, with occasional driveway use during hours of operation. Lower Single-family or duplex residential, other types 25 ft (based on length of 1 P-car) intensity with very low use. May not apply to rural residential. SPECIAL SITUATION DRIVEWAYS CBD, Farm or ranch, A mix of design Length equal to the design vehicle Field, Industrial vehicles. length, plus 5 ft NOTE: The recommended lengths are based on orienting a likely number of vehicles to be present up to and through the driveway-roadway intersection. Further study to develop these values is needed. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category. Exhibit 5-45 suggests situations where a separate facility parallel to the driveway may be needed and where it may be acceptable for the bicyclist or pedestrian to share the driveway with motor vehicles. Driveway Edge and Border Treatments A driveway edge should be clearly defined and visible to all users, so users can ascertain the lat- eral limits of motor vehicle operation. From observations such as in Exhibit 5-46, a vertical wall at the edge of a driveway causes drivers to shift their vehicles toward the center line. It is suggested that no vertical face (e.g., a retaining wall) be within 2 feet of the edge of the intended way for vehi- cle use. A wider offset must be provided if there will be a sidewalk parallel to the driveway. Exhibit 5-44. Lack of a sidewalk forces the For driveways with flat edges (i.e., no curb) in a fill, drivers will find it harder to determine pedestrian into the where the edge is in rain, fog, or darkness. The designer should not place a sudden drop off at such driveway. an edge. A relatively flat shoulder with a minimum width of 2 ft (after any rounding) is suggested before the side slopes downward. Some property owners install reflectors or other similar devices at the edge to help deal with this problem. As shown in Exhibit 5-47a, the toe of a slope should not extend to the base of a driveway or sidewalk edge, because runoff and erosion can lead to a mud-covered driveway or sidewalk surface. Exhibit 5-47b shows the toe of the slope recessed from the pavement edge, a method which yields better results. Edge Clearance from Fixed Objects Fixed objects such as utility poles, fire hydrants, and drainage inlets should be set back from the edge of the driveway and from the edge of the roadway. Reasons for this include allowing

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Geometric Design Elements 53 Exhibit 5-45. Suggestions for separate facilities for bicyclists and pedestrians. Driveway Description of Common Applications* Need for a Facility Parallel to Driveway Category for Bicyclists or Pedestrians STANDARD DRIVEWAYS Very high Urban activity center with almost constant Bicycle the need for separate lane or intensity driveway use during hours of operation. path depends on bicycle use in the area Pedestrian often need sidewalk Higher Medium-size office or retail (e.g., community Bicycle shared use may be adequate intensity shopping center) with frequent driveway use Pedestrian may need sidewalk during hours of operation. Medium Smaller office or retail and some apartment Bicycle shared use usually adequate intensity complexes with occasional driveway use during Pedestrian may need sidewalk hours of operation. Lower Single-family or duplex residential, other types Shared use is adequate intensity with low use. May not apply to rural residential. SPECIAL SITUATION DRIVEWAYS CBD Building faces are close to the street. Seldom applicable, because buildings are close to the street Farm or Seldom used, very low volume. Shared use is adequate ranch, Field Industrial Driveways are often used by large vehicles. Depends on the specific site plan and May have separate driveways for employees transportation modes used by the and/or customers. employees. * These descriptions are intended to help the designer form a mental image of some of the more common examples of the category. Exhibit 5-46. Effects of a vertical wall too close to the driveway. retaining wall is too close to driveway traffic; to have a comfortable distance away from the wall, the driver moves over, across the centerline

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54 Guide for the Geometric Design of Driveways Exhibit 5-47. Locating the toe of a slope near a driveway. (a) y (b) y ewa ewa driv driv Toe of Slope roadway Toe of Slope roadway if the toe of the slope abuts the edges of moving the toe of the slope back from the the driveway or the sidewalk, then mud edges of the driveway and the sidewalk running down the slope can accumulate leaves space for run off to accumulate, on the driveway and sidewalk, leaving a making it less likely that mud will cover messy area the driveway or sidewalk clearance for vehicle side mirrors and to account for the wheel and body paths of offtracking turning vehicles. Exhibit 5-48 shows drainage inlets flanking a driveway, with both inlets show- ing what appears to be damage from turning vehicles. Example design criteria such as Exhibit 5-49 suggest a clearance from vertically projecting fixed objects (e.g., poles and fire hydrants) to the edge of the driveway of 5 feet or more. Objects such as curb inlets should clear the paths of vehicles turning into and out of the driveway. Clear zone design practices affect the lateral placement of objects with respect to the edge of the traveled way of a street or highway. The minimal urban clear zone may be inadequate in the immediate vicin- ity of the roadway-driveway intersection, and a larger dimension may better accommodate turn- ing vehicle offtracking. The adequacy of any given driveway design can be checked with the turning templates of the design vehicle. The designer should also check to ensure that roadside objects do not become obstacles for other users (e.g., bicyclists and pedestrians). Exhibit 5-48. Drainage inlets too close to the edges of the driveway. Exhibit 5-49. Driveway edge clearance from fixed objects.