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20 Guide for the Geometric Design of Driveways Exhibit 4-4. Driveways too close to exit ramp Exhibit 4-3. Driveways too close to roadway terminals allow more conflicts to occur. intersections allow more conflicts to occur. Where two lower-volume sites are adjacent, access to both can be provided by a single shared driveway. When access from the major roadway is required, sharing access with adjacent tracts reduces the overall number of connections to the major roadway. Shared access arrangement should be implemented by an appropriate joint easement. For higher volume sites, additional access points may be needed. The assessment of this need must consider (1) whether or not good site planning principles have been applied and (2) the traffic safety and operational effects of the additional access. Along major roadways, the left-turn exit movement from driveways should be kept to a min- imum. If a roadway is converted from undivided to divided, left-turn access may be closed in one or both directions. Where physically practical, direct left-turns can be replaced by right turns followed by u-turn movements. When access is not available from parallel or cross streets, or across adjacent tracts, it may be necessary to provide property access from the major roadway. This access often should be lim- ited to right turns only. However, in some situations, limiting access to only right turns will result in left-turning movements migrating to and overloading a nearby intersection--in such cases, it may be better to allow left-turn movements at the subject access point. An assessment may be needed as to which arrangement helps preserve the functionality of the roadway and the mobil- ity of the traffic. Driveway Location and Spacing Experience has shown that certain driveway locations tend to be problematic, and that for bet- ter safety and mobility, the frequency of driveways should be minimized. This section discusses the following four types of driveway spacing: Spacing between unsignalized connections; Spacing of driveways from signalized intersections (corner clearance); Spacing for a signalized driveway; and Spacing of a driveway from an interchange ramp.

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Driveway Location and Spacing 21 Spacing Between Unsignalized Connections Spacing between unsignalized connections (whether between two driveways or a driveway and a roadway) should not interfere with safe and relatively unimpeded movement on the through roadway. Driveway spacing practices should provide reasonable access to abutting private property. General guidelines pertaining to unsignalized driveway spacing follow: The needed distance between successive connections (both driveways and side streets) increases with higher operating speeds, higher access classifications for the public roadway, and higher driveway volumes. A driveway should not be located within the functional area of an intersection or in the influ- ence area of the upstream and downstream driveways. Left-turn lane storage requirements should be considered when determining the driveway influence area and can limit how closely driveways can be spaced. On roadways that are undivided or have TWLTLs, the alignment of driveways on opposite sides of the road needs to be considered. Driveways on opposite sides of a lower-volume road- way may be aligned across from each other. Alternatively, they should be spaced so that those drivers desiring to travel between the driveways on opposing sides of the roadway need to make a distinct right turn followed by a left turn (or a left followed by a right). A much longer separation is needed on a higher-speed, higher-volume roadway (4-4). On roadways with restrictive medians, the spacing between right-turn access points on oppo- site sides of the road can be treated separately. Ideally, driveway access for a major development involving left-turn egress movements should be located where effective coordination of traffic signals would be achievable if there is a need to signalize the driveway. Driveway connections to public roadways are subject to the same intersection control device analyses as are street intersections. If existing or future volumes warrant installing a traffic sig- nal, and signalized spacing requirements cannot be met, left-turn access should be subject to closure in one or both directions. Driveway spacing from roundabout considerations are similar to those of other types of inter- sections, but driveways may be closer to a roundabout because of shorter queuing. Driveways should not interfere with operation of the roundabout. General guidelines for unsignalized access spacing are contained in the Access Management Manual (4-1) and NCHRP Report 348 (4-2). Spacing of Driveways from Signalized Intersections The needed minimum separation distance (i.e., corner clearance) from a driveway to a signal- ized upstream or downstream location will depend on the function, operation, and design features of the roadway and the characteristics of the access connection. The basic principle of locating one connection outside of the functional area of another connection applies to driveways. For a driveway upstream of or approaching a signalized location on a major road, the func- tional area includes the perception-reaction time, maneuver distance, and storage length of the traffic on that approach. The spacing should provide separation between the conflicting move- ments occurring at the signal and the conflicting movements occurring at the driveway. In addi- tion, this spacing would enable the driveway to operate without being obstructed by the traffic backing up from the signal.

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22 Guide for the Geometric Design of Driveways The spacing for a driveway downstream of the departure leg (i.e., far side) of a signalized location on a major road should be sufficient to minimize the adverse effects of the driveway operations on the intersection. According to Transportation and Land Development (4-4, p.628), the minimum downstream corner clearance should be no less than the stopping sight distance. Along the far side of an intersection of a crossroad with an arterial, the corner clearance distance to the first driveway varies. If the arterial does not have a channelized right-turn lane for traffic turning onto the crossroad, one source recommends that the driveway be spaced a minimum of 120 feet from the intersection. If the arterial has a channelized right-turn lane for traffic turning onto the crossroad, the clearance distance should reflect the inside corner radius. The clearance should be 200 feet for a 50-foot radius, 230 feet for a 75-foot radius, and 275 feet for a 100-foot radius (4-4, p. 635). The stopping sight distance principle also applies to driveways connecting to crossroads, along the far side of the intersections of cross- roads with major roads. For crossroads, the near side corner clearance should extend beyond the normal queuing dis- tance along the crossroad. Spacing for a Signalized Driveway Signal spacing is a function of travel speed and signal cycle length. The same criteria for sig- nal spacing apply to both a signalized driveway and a signalized public roadway intersection. If a driveway is going to be signalized, then it should be located to "fit" into the traffic signal progression along an arterial roadway and not interfere with the progression of traffic from one signalized intersection to the next. Desirable spacing is shown in Exhibit 4-5. When signalized driveways and intersections can be placed at these distances, there is no loss in green-band (through band) width. Small devia- tions (e.g., less than 10%) will have minimal negative effects on the progression. Further guidelines for green-band width are contained in NCHRP Report 348 (4-2, p.5658) and in the Access Management Manual (4-1, p.140149). Where the recommended spacing in the table exceeds 1/2 mile (2,640 feet), designers can limit the actual spacing to 2,640 feet. Spacing of a Driveway from an Interchange Ramp The needed driveway separation distance from an interchange area depends on the geomet- ric design of and methods of traffic control at the freeway ramp joining the roadway. It is also affected by the speed, volume, and number of lanes on the through roadway, the ramp volume Exhibit 4-5. Signalized intersection spacing for various progression speeds and cycle lengths. Speed (mph) Cycle 25 30 35 40 45 50 55 length 60 1100 1320 1540 1760 1980 2200 2420 70 1280 1540 1800 2050 2310 2570 2820 80 1470 1760 2050 2350 2640 2640 2640 90 1630 1980 2310 2640 2640 2640 2640 120 2200 2640 2640 2640 2640 2640 2640 NOTES: Spacing distances are in feet. Where the recommended spacing in the table exceeds mile (2,640 ft), designers can limit the actual spacing to 2,640 ft.