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Geometric Design Elements 55 Driveway Length The following sections address the design of the following elements related to driveway length: 1. Minimum length of driveway to a barrier (e.g., garage door or gate) 2. Minimum length of driveway paving 3. Minimum length of the driveway throat or connection depth 4. Accommodating the need to reverse vehicle direction (i.e., turn around) within the private site. Drawings showing different facets of driveway length were provided at the beginning of Chapter 2 as well as on the following pages. Driveways can be divided into two groups. One group operates in the manner typical of single- family suburban driveways, at which vehicles enter the driveway and then come to a stop and park. The other group operates in a manner typical of commercial driveways--entering vehicles continue to move and proceed for some distance along the driveway, often into a parking lot. Some of the following controls are more likely to apply to the first category, while other controls are more likely to apply to the second category. Minimum Length of Driveway For this section, the length of a driveway is the distance from where the driveway on one end connects to the traveled way, to the other end where the driveway encounters some sort of barrier or terminates. This may be an intersecting circulation road within a site, the end of the pavement, a gate, a garage door, or other barrier that when in place, discourages or pre- vents a motor vehicle from proceeding. Even under the simplest of situations, many driveways will require a certain minimum length in order to avoid creating problems for one or more user groups. Driveway Minimum Length Considerations Problems can result when vehicles entering a driveway cannot proceed far enough into the driveway and parts of the vehicle then block the traveled way, bicycle lanes or paths, or sidewalks. Exhibit 5-50 shows vehicles parked in the driveways of townhouses constructed in the early 2000s. The rears of these vehicles partially block the sidewalk and lumber in the bed of the pickup truck in the foreground extends over the tailgate into the pedestrian path. This situation would be especially dangerous for a pedestrian with visual impairment using the sidewalk. Unless a driveway is so short as to discourage its use for stopping or parking, the minimum length between controlling features on each end of the driveway is the sum of the following three components (see Exhibit 5-51): 1. Setback from the end toward the roadway to clear the outer edge of the traveled way, a bicycle lane or path, or a sidewalk 2. Length of the longest vehicle that typically would park there 3. Clearance buffer from a gate, garage door, or other similar end-barrier The buffer allows a person to walk between the end of the vehicle and the end-barrier. The buffer should also accommodate many drivers' tendencies to shy away from a barrier, rather than pulling close to it. It is hoped that the driver of a vehicle with a load that slightly hangs over the rear will use the buffer to pull forward until the load clears the sidewalk.
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56 Guide for the Geometric Design of Driveways Exhibit 5-50. Insufficient driveway length leads to Exhibit 5-51. Minimum driveway vehicles blocking the sidewalk. length considerations. Where stopping or parking in driveway occurs, Minimum Driveway Length = sum of 1. setback to + 2. design vehicle length + 3. buffer outer edge of sidewalk, or other similar control roadway sidewalk driveway Driveway Minimum Length Design Suggestions Given that the minimum length of the driveway is the sum of three values, two of which can vary greatly, prescribing a single or even a few values is of little benefit. Instead, it is recommended that the designer follow this series of steps: 1. Determine the longest vehicle type likely to use the driveway. 2. Determine the length of that vehicle. 3. Estimate a front buffer dimension. In the case of a smaller design vehicle (e.g., a P-car), esti- mate 2 feet. For a larger design vehicle (e.g., a bus or large truck), select 3 feet. If the front buffer area involves a gate that swings outward, there also should be an allowance for the gate. 4. Estimate a value for the rear clearance. Where a sidewalk exists, this is the distance from the edge of the traveled way to the far edge of the sidewalk. If no sidewalk exists, allow a mini- mum of 2 feet. 5. Sum these values to determine the minimum driveway length. Research would be helpful to better define these dimensions (e.g., the actual buffer taken by the drivers of various sizes of vehicles). Minimum Length of Driveway Paving If the driveway within the private property site is dirt or gravel, how far back from the edge of the traveled way to pave the driveway connection is an issue. A survey of transportation agencies found that practices varied among agencies, and no one practice predominated. Some agencies pave the driveway connection a fixed distance from the edge of the traveled way; others pave to the right-of-way line. The objectives of paving the connection to a gravel or dirt driveway some distance back from the traveled way edge include (1) providing a more stable driveway surface "platform" from which to enter or exit the traveled way and (2) minimizing or eliminating the depositing of dirt, gravel, or mud onto the traveled way. Factors which can affect the extent to which debris from such a private driveway are deposited on the traveled way include · The distance from the traveled way edge to the beginning of the gravel or dirt surface; · The grade of the driveway; · Surface drainage patterns, combined with the amount of precipitation; and · The volumes and types of traffic using the driveway.
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Geometric Design Elements 57 Given the lack of consensus among agencies and the factors that affect the extent to which debris from unpaved driveways is deposited on the traveled way, guidance on this matter is limited to advising designers to pave driveway connections some distance back from the edge of the traveled way. A designer may find guidance by observing the extent of the debris ema- nating from existing unpaved driveways in the vicinity of the driveway under consideration. One specific suggestion is to pave a length at least as long as the length of the vehicles expected to use the driveway, plus a clearance from the edge of the traveled way or sidewalk. This will encourage vehicles that pull off the road to clear the roadway and sidewalk. Another suggestion is that local governments require on-site paving or other mitigation actions to prevent debris from washing onto the public roadway. Minimum Length of Driveway Throat The driveway throat length is the distance from the outer edge of the traveled way of the inter- secting roadway to the first point along the driveway at which there are conflicting vehicular traf- fic movements. Similar or related terms include the driveway connection depth, reservoir length, stacking distance, and storage length. Exhibit 5-52 illustrates a driveway throat. Sources differ as to what point actually defines the internal (i.e., within the private site) end of the throat. Examples of these variations include "end of the driveway inside the land devel- opment" (5-17), "the parking lot served by a driveway" (5-18), and "the furthest end of the driveway" (5-19). Given that an impetus for providing an adequate length for the driveway throat is related to allowing smooth traffic flow along the driveway between the street and on- site roadways or parking lots, the point at which conflicting traffic movements are encountered was selected to define the end of the driveway throat that lies within the site. So, implicit in the definition used herein is "non-conflicted" throat length. Design Considerations Providing an adequate driveway throat length or connection depth in which there are no con- flicting movements can help create smoother traffic flows in and near the driveway throat and avoid conflicts to which drivers may not have adequate time to react (which in turn may lead to colli- sions). As Exhibit 5-52 shows, an inadequate throat length can produce traffic situations that adversely affect the flow of traffic on the public roadway. In this drawing, vehicles that have entered the driveway have formed a queue that blocks the sidewalk. Additional vehicles trying to enter the driveway will not be able to proceed; therefore, they will stop in and block the public roadway too. Exhibit 5-53 portrays a different situation that can affect the length of the right or entry side of the throat. All parking spaces should be far enough from the roadway, bike path, and/or sidewalk, so that vehicles backing out of parking stalls do not encroach into the projection of the sidewalk or bike path across the driveway, or into the roadway. Even if a vehicle backing out of a parking space into the driveway throat does not encroach into one of these areas, the backing vehicle will Exhibit 5-53. Driveway Exhibit 5-52. Driveway throat. entry throat and parking stalls. THROAT LENGTH or CONNECTION DEPTH parking sidewalk lot roadway internal (site) road driveway VEHICLE BACKING OUT roadway OF PARKING STALL sidewalk
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58 Guide for the Geometric Design of Driveways Exhibit 5-54. Undesirable speed control in driveway throat. still block the driveway entry for the duration of the backing maneuver. In many situations, such a blockage would have undesirable effects on traffic, so in those cases, parking should not be allowed in the driveway throat area. Exhibit 5-54 shows yet another type of conflict, a speed hump installed in a driveway entry. Observations of traffic at this driveway revealed that as vehicles turned into the driveway, drivers were surprised by the need to rapidly decelerate over a short distance. Such a rapid deceleration increases the driver's exposure to being struck on the side by through vehicles and in the rear by following vehicles. The process of slowing or stopping before turning into this driveway constitutes adequate traffic calming. When turning into a driveway while watching for conflicting bicyclists, pedestrians, and other vehicles, motorists should not be confronted with additional driving tasks until they have had time and distance to reorient themselves. The following factors affect the distance needed to provide an adequate driveway throat length: · The positions of bike paths and sidewalks. · The queuing or stacking space needed for exiting vehicles. If the exit is signalized, then suffi- cient queuing length is needed to supply the green phase with vehicles proceeding at the sat- uration flow rate, accounting for lost time due to weaving on the driveway approach during the green phase. · In cases of a multilane exit, the length needed for exiting vehicles to make weaving maneuvers as they change lanes in the driveway. · The distance needed to provide motorists entering the driveway with time to reorient them- selves and detect conflicting traffic movements from crossroads, parking spaces, bicycle routes, or pedestrian paths they encounter. · The queuing or stacking space needed for entering vehicles. · For a multilane entry, the length for entering vehicles to make weaving maneuvers as they change lanes in the driveway. · For a one-way driveway, enough length to place Wrong Way/Do Not Enter signs so that the intent is obvious to motorists. Other considerations for the length of the throat found in the literature review include the · Functional category of the intersecting roadway, · Type of driveway intersection traffic control (stop sign or signal) and traffic signal timing, · Type and intensity of land use activities served, and · Number of parking spaces (within the site) per exit lane.
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Geometric Design Elements 59 Given that one of the underlying factors to consider is the driveway volume, both entering and exiting vehicles during the peak time period, it may be necessary to estimate likely driveway usage before designing the length of the driveway throat. Design Suggestions The throat length must be long enough to avoid internal site conflicts associated with cross- ing or weaving movements. It also must be adequate to avoid spillback onto the public road or internal circulation system. There are different controls: 1. Designing sufficient length to react to conflicts, 2. Designing sufficient length to accommodate traffic queues, and 3. Designing sufficient length to accommodate weaving. Different sources have developed different approaches for establishing minimum throat lengths. The following narrative presents the approaches from various sources. Koepke and Levinson Throat Length. When more detailed, site-specific information is available, one could apply the recommendations by Koepke and Levinson in NCHRP Report 348 (5-20). For signalized driveways, suggested on-site throat lengths (per lane) were based on the equation N = 2qr, where N = number of cars to store, q = vehicles per hour per lane, and r = effective red time per cycle. Alternative guidelines were cited based on the number of parking spaces per exit lane for multi-family, residential, retail, office, and industrial uses. The following suggested guidelines were based on both sets of criteria: · 50 feet for minor driveways that serve 50 to 100 apartments, less than 50,000 square feet of retail, or a quality restaurant; · 150 feet, with at least two exit lanes, for shopping centers of up to 700,000 square feet, and office complexes up to 500,000 square feet; and · 200 feet or more, with at least two exit lanes, for larger commercial complexes. Stover and Keopke Throat Length. In Transportation and Land Development, Stover and Koepke (5-13, p. 728) state that the exit condition controls the throat length for high-volume traffic generators, while the entry condition controls the throat length for low-volume traffic generators. The exit side of a driveway should be designed to enable traffic to efficiently leave a site. The throat length and cross section are interrelated: the wider the cross section, the longer the exit throat length needed to accommodate the associated weaving maneuvers. Exhibit 5-55 presents the minimum throat length for stop-controlled and for signalized-access drives, based on the number of egress or exit lanes. Exhibit 5-55. Minimum throat length based on the type of control and number of lanes. Type of Number of Exit Lanes Present Control 1 Exit Lane 2 Exit Lanes 3 Exit Lanes 4 Exit Lanes STOP sign 30 to 50 ft 50 ft (2 cars) -- -- Signal -- 75 ft 200 ft 300 ft NOTE: -- indicates no value given Sources: Transportation and Land Development, 2nd ed. (2002), p. 7-28 (5-13) and Access Management Manual (2003), p. 184-185 (5-21)
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60 Guide for the Geometric Design of Driveways The throat length for a one-way exit driveway needs to be sufficient to allow for DO NOT ENTER or WRONG WAY signs to be installed to be effective in warning drivers turning from the roadway. This is related to liability, not queue length. The throat length based on the entrance side of a driveway needs to minimize the potential for the conflicts on the access drive from adversely affecting the intersecting roadway. Drivers enter- ing the site should clear the roadway intersection before encountering decision points and poten- tial conflicts along the driveway. Transportation and Land Development indicates the minimum throat lengths for unsignalized access drives based on two driveway configurations 1 entering/ 1 exiting lane and 1 entering/2 exiting lanes. For the 1 entering/1 exiting lane configuration, the minimum throat length is 75 feet to the first parking spaces on site or 30 feet to the first intersection on site. For the 1 entering/2 exiting lane configuration, the minimum throat length is 75 feet to the first parking spaces on site or 50 feet to the first intersection on site. For high- volume traffic generators, it is the exit condition that governs the needed throat length. Roseville Throat Length. The Roseville, California, design standards (5-22) present a detailed procedure for estimating the needed length of the driveway throat. Agencies may apply the throat length criteria from other sources to help establish a similar type of procedure. The Roseville procedure is part of the traffic impact study process that applies to proposed proj- ects estimated to generate more than 50 PM peak-hour trip ends. The traffic study includes an eval- uation of the Minimum Required Throat Depth (MRTD) needed on-site for each access point for a proposed development. The MRTD requirement does not apply to single-family residential or duplex uses. The MRTD is measured from the back of the sidewalk to the first drive aisle or park- ing stall. The purpose of the MRTD is to allow enough stacking distance for egressing vehicles so that the first drive aisle or parking stall is not blocked. This minimizes the possibility of incoming vehicles queuing out into the traveled way of the main street thereby creating a safety concern. The MRTD is measured in car length increments of 25 feet and rounded up to the nearest multiple of 25 feet. The City does not allow a MRTD of less than 25 feet for any project. Throat depths greater than the calculated MRTD are encouraged. On-site parking is not permitted within the MRTD area. The MRTD is a function of the length of the queue of vehicles waiting to exit the driveway. The length of this queue is a function of two variables: the number of vehicles desiring to egress dur- ing a given time period versus the number of vehicles that can enter the traffic stream of the main road during that same time period. If the calculated MRTD is physically or unreasonably too long for the proposed development, then the traffic study can suggest ways to reduce the MRTD by either decreasing the egress demand volume, or by increasing the movement capacity. There are cases when an MRTD of 25 feet is acceptable, for example, when the first drive aisle is "one-way only" as shown in Exhibit 5-56. Another scenario where a MRTD of 25 feet is accept- able is when a raised center median is constructed in the driveway throat from the back of the sidewalk to the calculated MRTD distance. In this case, the nearest drive aisle can be two-way, but turning movements into and out of the drive aisle are restricted by the raised median. Because of the different operations at signalized and unsignalized driveways, two different methodologies apply. At unsignalized project driveways, the MRTD is based on a series of regression equations that the City uses to estimate maximum queue lengths at minor stop- controlled intersections. These equations apply the methodology presented in "Estimation of Maximum Queue Lengths at Unsignalized Intersections" from the November 2001 ITE Journal. Exhibits 5-57 and 5-58 present the methodologies used for calculating the MRTD for various unsignalized driveway conditions. Major street volumes are based on projected future traffic volumes from the latest version of the citywide traffic model. Alternative methodologies for cal-
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Geometric Design Elements 61 Exhibit 5-56. Roseville explanatory drawing. culating unsignalized MRTD lengths may be considered, but need to first be approved by the Public Works Department prior to incorporation into traffic studies. At signalized project driveways, MRTD lengths are a function of egressing traffic volumes, lane geometrics, and traffic signal timing. Typically, signalized access locations will have more than one approach lane for egressing vehicles; therefore, the MRTD is determined from the lane with the longest queue. The MRTD is based on the Operational Analysis methodology contained in Exhibit 5-57. Minimum required throat depth regression equations.
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62 Guide for the Geometric Design of Driveways Exhibit 5-58. MRTD for right-turn-only movements. the latest version of the Highway Capacity Manual or other methodology as approved by the City's Public Works Department. Major street volumes are based on projected future traffic volumes from the latest version of the citywide traffic model. For existing traffic signals, it is recommended that the consultant discuss likely signal timing parameters with City staff. There may be some restrictions to signal timing parameters for existing signals because of progression and so forth. The City also has provisions to help ensure that sufficient onsite storage is provided for drive- through service uses to ensure that vehicles will not queue into the public right-of-way. The following definitions are for the terms used in the MRTD equations: AppVol = hourly traffic volume divided by peak-hour factor (PHF) for subject movement ConflVol = hourly traffic volume divided by PHF that conflicts with subject movement (refer to the Highway Capacity Manual to identify movements that conflict with subject approach) TS = a dummy variable with a value of 1 if a traffic signal is located on the major street within one-quarter mile of the subject intersection and 0 otherwise Lanes = number of through lanes occupied by conflicting traffic Speed = posted speed limit on major street (in miles per hour) RT% = Percentage of vehicles on shared left/through/right minor street approach that turn right The following scenario employs several assumptions to illustrate another facet of principles related to adequate throat length--minimizing traffic conflicts of the entry side of the driveway throat. Assuming a level, 90-degree entry, it was hypothesized that as drivers turn right into a driveway, the eventual 90-degree reorientation of drivers' lines of sight is affected by factors such as the following:
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Geometric Design Elements 63 1. Human factors. The span or width of drivers' fields of vision and the degree to which drivers can turn their heads. 2. Vehicle factors. Limitations imposed by the structure of the vehicle (e.g., the position and width of the front windshield posts). 3. Operational factors. Informal observations suggest that drivers maneuvering vehicles into a driveway are not free to devote attention to conflicts in the driveway throat length ahead until after the entering vehicle has cleared any conflicts at the entry. This includes conflicts with pedestrians and "sideswipe conflicts" between the left front corner of the entering vehicle and the left side of any vehicles exiting the driveway onto the public street. Assume that drivers turning right into a driveway with a 25-ft radius at a speed of 15 mph or 22.0 ft/s give full attention to the driveway ahead after completing 60 degrees of the turn. At this point, drivers have sufficiently squared-up their lines of sight and can detect a conflict (e.g., a vehicle backing out of a parking stall or cross traffic within the site). The current AASHTO Green Book guidelines (5-1, pp. 110114) allow 2.5 seconds for a driver to react to an unexpected sit- uation ahead requiring the vehicle to stop; whether or not a conflict in the driveway ahead would constitute an unexpected situation is arguable. For this illustration, assume that the driver has a narrow focus on the driveway ahead and requires only 1.0 second of perception-reaction time and a deceleration rate of 11.2 ft/s2. This leads to the following calculations: 25 ft radius × tan (90° - 60°) = 14.4 ft 25 ft tangent - 14.4 ft = 10.6 ft into driveway from edge of traveled way Distance from driver to front bumper of vehicle: 6 ft Distance into driveway + perception reaction distance + braking distance 10.6 ft + 6 + [22.0 1.0] + [0.5 × 22.02 / 11.2] = 60 ft of entry throat length Therefore, the designer would require a minimum of 60 feet of driveway connection depth from the outer edge of the traveled way to the first crossroad or other conflicting movements within the site. If the first conflict encountered is with a vehicle backing out of a parking stall, then the position of the rear bumper of the vehicle that has just backed out of the stall will also need to be taken into account. Even with this non-conflicting connection depth, if a second vehicle is closely following the first and also turning right into the driveway, the driver of the second vehicle may not be able to react and stop or the second vehicle may come to a stop with its rear still in the through roadway. Other Throat Lengths. For a comparison, Exhibit 5-59 presents minimum throat length criteria from two states, New Mexico and Florida. In both cases, the minimum requirement is 30 feet. Exhibit 5-59. Throat length criteria from two states. Source: New Mexico DOT, State Acc. Mgmt. Manual Ch. 8, Sec. 18, p. 91, Sept. 2001 Source: Vergil Stover unpublished course notes FDOT Driveway Handbook, p. 54, Mar. 2005 (a) (b)
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64 Guide for the Geometric Design of Driveways Providing Onsite Turnaround Capability It is often undesirable or unsafe for vehicles to perform a backing maneuver from a driveway into a public roadway. Therefore, most sites should be designed so that once entering the site, a vehicle can be re-oriented and leave in a forward direction. This is highly desirable for all sites except for single-family and duplex residences along lower volume, lower speed streets. The type and design of needed turnaround facilities depend on the size of and types of activ- ities conducted on the site, the likely mix of vehicles, the building arrangements, and the circu- lation system for each site. Sometimes, turnaround needs can be accommodated by circulation on internal road systems or through parking areas. In other situations, a site needs a specific turn- around facility, such as a circle or other shape shown in Exhibit 5-60. Internal Roadway Systems Many developments, especially larger ones, have internal circulation systems that allow vehi- cles to enter the site and then, through a series of normal driving maneuvers, assume an orienta- tion that allows the vehicle to head out of the site. Circulation Through Parking Lots Somewhat similar to an internal roadway system but on a smaller scale, other sites have a layout that allows vehicles to circulate through the parking lot and leave the site in a forward direction. This type of turnaround works well with cars and smaller trucks, but may not be adequate for large trucks, unless greater maneuvering spaces are provided. One form of this is "Loop Routing," shown in Exhibit 5-60 (a and b). Where two parallel drive- ways enter a site, it is sometimes practical to turn around via an inverted "U" movement. For traf- fic driving on the right side, a counter clockwise movement has less internal traffic conflict, and left turns around corners are easier for larger vehicles to negotiate than are right turns. Sometimes, a single driveway access point can be "split" via a loop road to provide the turnaround. This pat- tern is often seen at fast food restaurants, with the building located inside the loop. Exhibit 5-60. Turnaround design schematics. U-shaped U-shaped internal internal circulation circulation pattern pattern roadway roadway (a) (b) Cul-de-sac should have a Offset reverse curve cul-de-sac T-shape Hammerhead (d) (e) (f) (c)