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26 Guide for the Geometric Design of Driveways Exhibit 5-2. (Continued). Roadway-Driveway Intersection 33 Angle of intersection with street: O O O flat-angle (turn angle 90 ) 34 Cross slope of street and shoulder, considered with driveway grade 35 Curb threshold treatment (rolled, vertical lip, counterslope, continuous ) 36 Curb-termination treatment (abrupt end, drop-dow n, returned) 37 Entry transition shap e (e.g., radius, flare/taper, straight) 38 Entry transition-shape dim ensions (radius, flare dim ensions) 39 Channelization of right turn from street into driveway 40 Channelization of right turn from driveway into street 41 Channelization in the driveway (e.g., triangular island to prohibit in and out left-turns) 42 Channelization in street - street median prohibits all le ft-turns in/out of driveway 43 Channelization in street - street median prohibits one but not both le ft-turns 44 Drainage: confining the gutter flow 45 Drainage: inlet type and location 46 Clearance from fixed objects, appurtenance s 47 Pavement surface defor mity (corrugation, potholes) Traffic Controls (for drivew ay vehicles) 48 Driveway-roadway intersection control (none, yield, stop, signal) 49 Turn restrictions 50 One-way operation (one-way, do not enter ) 51 Markings (pavem ent, delineators) 52 Other Roadway in Vicinity of the Driveway 53 Right-turn lane attributes: (absence or presence ) 54 lane width 55 lane deceleration, storage length 56 lane entry transition shape 57 lane offset 58 Left-turn lane attributes : (absence or presence ) 59 lane width 60 lane deceleration, storage length 61 lane entry transition shape 62 lane offset 63 Number of driveways per site 64 Driveway spacing from upstream access connection 65 Driveway spacing from downstream access connection Sight Distance and Conspicuity Two considerations that are frequently part of the discussion of many design elements are sight distance and conspicuity. There are many types of sight distance. The basics of stopping sight distance and intersection sight distance are explained in the AASHTO Green Book (5-1, pp. 109114 and 650676), and an understanding of these basics is a mandatory prerequisite for anyone designing a roadway or driveway connection to a roadway. The designer should check that walls, wide utility poles, veg- etation, or other objects do not block the lines of sight that a bicyclist, driver, or pedestrian needs to maneuver safely. Conspicuity is the attribute of standing out so as to be noticed or observed. As applied to drive- ways, conspicuity means that users (whether bicyclists, motorists, or pedestrians) approaching the driveway can detect and recognize the presence of the driveway far enough in advance so as to make any needed adjustments in their travel trajectory or speed. Also, as the user either on the roadway or on the "private side" nears the driveway, the user can detect the precise edge or
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Geometric Design Elements 27 Exhibit 5-3. Driveway considerations generally outside the control of the designer. Shared Elements, Surroundings 1 Land use 2 User and vehicle mix and composition 3 Temporal variation: seas on, day of week, time of day 4 W eather and weather effects Sidewalk-Drivew ay Intersection 5 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 roadway 8 Lanes (num ber, width) 9 Lane type (travel, HOV, bicycle, turn, parking) 10 Cross slope (travel lanes, shoulders) 11 Horizontal alignment of roadway 12 Vertical prof ile of roadway 13 Sight distanc e restrictions User Characteristics - Bicyclist 14 Bicyclist perception-reaction process, time 15 Speed 16 Braking capability 17 Sight distanc e need User Characteristics - Pedestrian 18 Pedestrian perception-reaction process, time 19 Speed 20 Sight distanc e need Special needs groups 21 General - children, elderly 22 Those with disabilities (e.g., mobility, visual) 23 Legal mandates - those related to disabilities User Characteristics - Vehicle, Driver 24 Driver perception-reaction process, time 25 Speed 26 Deceleration characteristics (typical) 27 Braking capabilit y (limiting) 28 Sight distanc e need 29 Vehicle width 30 Vehicle length 31 Vehicle turning radius 32 Vehicle front overhang, wheelbase, rear overhang, and ground clearan ce dimensions other elements that will affect the user's position and path when crossing, entering, or exiting the driveway. Means to improve conspicuity include the following: · Clearly defining the edges of shapes, so as to differentiate between shapes (e.g., the edge between a sidewalk and a driveway); · Providing contrast between light and dark surfaces; · Placing a business sign near the driveway, to reinforce its location; and · Installing artificial illumination. Exhibits 5-4 and 5-5 show undesirable design practices. Exhibit 5-4 shows how a planter and a utility pole near the intersection of a driveway with a roadway restrict the sight distance. Visual
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28 Guide for the Geometric Design of Driveways Exhibit 5-4. Placement of planter and utility pole Exhibit 5-5. Poorly defined edge leads to scrapes. limit the sight distance for a driveway exit. obstructions may also make it difficult for motorists on the roadway approaching the driveway connection to have an adequate preview of the driveway or vehicles in the driveway. In Exhibit 5-5, the driver's view from the street side provides clear definition of the edge. However, for the driver in the parking lot, the curb edge drop off is hidden, so, unable to detect the dropoff, some vehicles drive over the curb. Practices similar to that shown in this exhibit, which create a continuous expanse of pavement and no distinction between the actual driveway and the curb dropoff, should be avoided. A similar problem can occur when a driveway that slopes downward from the roadway edge is located on the high side of a superelevated roadway. Drivers in the roadway attempting to enter the driveway may have difficulty determining where the driveway edges are. A designer may con- sider adjusting the driveway profile so that it rises slightly before descending, or adding delin- eators or soft landscaping to help drivers identify the driveway edges. Exhibit 5-6 offers guidance for sight distance and conspicuity elements. Exhibit 5-6. Visibility design concerns. Concern or Issue Design Response Specific Procedure and/or Information Bicyclists, motorists in At driveway intersections with public Refer to the latest edition vehicles, and pedestrians roadways, have unobstructed lines-of- of the AASHTO Green need to see each other far sight that provide adequate stopping Book for the procedure to enough in advance to sight distance. calculate the needed avoid collisions. Along high-type public roads, adequate stopping sight distance or intersection sight distance also should be intersection sight distance. provided. However, this may not always be practical in built-up areas. Do not place anything in the border that blocks needed sight lines. To have time to react, Have driveway edge color contrast with Curbed driveways provide drivers need to detect the the color of the abutting surface. a clearer delineation of the driveway well in Have driveway pavement color contrast driveway entry shape than advance and be able to with the color of the roadway. "dust pan" designs do. visually define its shape Consider illumination during darkness. before entering or For non-residential, place a monument exiting. sign very close to the driveway intersection with the roadway.