Geometric Design of Driveways (2010)

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ES-1 EXECUTIVE SUMMARY Geometric Design of Driveways Driveways are private roads that provide access (both ingress and egress) between a public way and abutting properties, and any facilities on those properties. The roadway engineers’ focus is often on a part of the driveway, the area where the driveway intersects the public highway or street. Since these connections form the link or interface between public streets and highways and the activities they serve, driveways are an integral part of the roadway transportation system. There has been relatively little comprehensive research on or national guidance for the geometric design of driveways in recent decades. The objective of this project was to develop recommendations for geometric design of driveways that will be useful to state departments of transportation, local governments, and consultants in preparing driveway design standards and practices. To accomplish this, the project had been structured as follows. 1. The contractor reviewed research literature, obtained examples of transportation agency design documents, and conducted a survey of transportation agencies, in order to document the current state-of- practice and highlight research needs, and to identify topics and collect source materials for inclusion in the design guide. 2. After reviewing and considering a synthesis of the literature and documents, the project oversight panel selected topics for research. 3. The contractor conducted research related to the geometric design of driveways. 4. The contractor prepared a driveway design guide. The recommendations were based on research findings, standard engineering practices, and engineering judgement. The intent of the recommendations is to provide safe and efficient travel by motorists, pedestrians, bicyclists, and transit users on and in proximity of the affected roadway. The contractor reviewed almost 100 documents, received survey responses from one city and 16 state departments of transportation, and received input from 13 other entities. From this, a list of almost 100 factors that may affect the operation of a driveway was prepared. The contractor prepared a preliminary list of 14 driveway design-related topics that may warrant additional research, and then offered five of them to the project oversight panel for consideration. The project panel selected the following three areas for research. 1. Determine the crest and sag grade changes at which the underside of a static vehicle drags. 2. Determine what actual driveway profiles cause the undersides of vehicles to drag.

ES-2 3. Assess the effects of angle changes (roadway cross slope – driveway grade) at the roadway- driveway interface and driveway grades on the speed and elapsed time of vehicles turning left and turning right into a driveway. To determine the crest and sag grade changes at which the underside of a static vehicle drags, the contractor measured or obtained the profile dimensions of two automobiles, a pickup truck with a trailer, a Class A motor home (i.e., “diesel pusher”), and a beverage delivery truck. A geometric analysis was performed to determine at what crest and sag grade change the underside of the vehicle would drag the pavement surface. Since these analyses do not account for the effect of static load (weight of passengers or cargo) or dynamic load (vehicle bounce), maximum desirable grade changes will be less that those indicated by the calculations. To determine what actual driveway profiles cause the undersides of vehicles to drag, the contractor surveyed the profiles of 31 driveways that displayed scrape marks near vertical crests and sags. From this, it was concluded that for driveways at which the passenger car design vehicle governed, the maximum vertical profile breakover without a vertical curve should be 10% at crests, and 9% at sags. To assess the effects of angle changes (roadway cross slope – driveway grade) at the roadway- driveway interface and driveway grades on the speed and elapsed time of vehicles turning left and turning right into a driveway, the contractor made measurements at 12 commercial driveways on non-fringe suburban arterial multilane roadways with posted speeds of 40 and 45 mph. All of the roadways had either a raised median or a TWLTL. The data were collected at driveways with right turn entry radii ranging from 13 to 19.5 ft, and an entry lane width of about 13 feet. Over 1500 vehicle movements were recorded. Very few vehicles about to enter a driveway exceeded 20 mph at the locations at which speeds were measured. After the fronts of vehicles crossed the driveway threshold and were approaching a typical sidewalk location, average speeds for vehicles turning left into the driveway were around 10 mph. Vehicles that had turned right into the driveways were slightly slower, with average speeds around 7 mph. At the driveways studied, little differences in speed were found between driveways with flatter and with moderate grades (up to 9% grade, 10.5% breakover at the gutter). However, at the steeper sites (12.5% to 15.5% grade, 13.5% to 19% breakover at the gutter), speeds were slower and elapsed travel times greater. An analysis of the effects on motorists and pedestrians indicated that the greatest negative impact at the steeper sites would involve conflicts between vehicles turning into a driveway and oncoming through vehicles. The quality of a design is determined by how well the design works after it is placed into operation. Therefore, the objective of geometric design is to identify the factors that affect the outcome, then choose component elements and combine them into a design in such a way that a desirable outcome is achieved, and avoidable undesirable outcomes are avoided, all the while being cognizant of economic constraints.

ES-3 The design guide prepared reflected concerns of various groups that use the driveway, including bicyclists, motorists, pedestrians, and pedestrians with disabilities. The project report also included a number of suggested changes for the AASHTO Green Book.

1 CHAPTER 1 Introduction 1.1 BACKGROUND AND CONTEXT Driveways are private roads that provide access (both ingress and egress) between a public way and abutting properties, and any facilities on those properties. Since they form the link or interface between public streets and highways and the activities they serve, driveways are an integral part of the roadway transportation system. Driveways can be found along rural highways, suburban arterials, city streets, and alleys. They vary in size, activity, types of vehicles served, roadways accessed, development density, proximity to intersections, and pedestrian exposure. Where they are located and how well they are designed affect the safety and mobility of vehicles and pedestrians, and may impact the quality of roadside development. Driveways, especially busy commercial drives, can have a significant impact on the flow of traffic. As Exhibit 1-1 shows, in the area where the roadway, the sidewalk and the driveway intersect, there are three distinct user groups with different and sometimes conflicting needs. Although members of all three groups typically want to make their trips as expeditiously as possible, the roadway user usually moves at a greater speed and, therefore, is often focused some distance ahead on the roadway. The sidewalk users (a heterogeneous group – such as pedestrians, pedestrians with disabilities, and those waiting for a bus or taxi – with different needs) move at a much slower pace, and are unprotected and vulnerable to vehicles. The driveway user typically has a speed and a path that can create conflicts with the other two user groups. Vehicles entering or leaving the driveway impact other motorists, as well as pedestrians and bicyclists crossing the driveway. Sometimes they affect traffic within the private development. EXHIBIT 1-1 Driveway interactions Sidewalk users Driveway users Roadway users border area border area

2 Interactions among the various user groups often occur within or near the border, the area between the roadway edge and the right-of-way line. (Roadway engineers often use the term driveway to denote that part of the driveway within or near the public right-of-way, the border area; that meaning was adopted for this study.) Therefore, the design of driveways in and near this area of interaction should consider the needs of each group of users. The designer should attempt to: 1. minimize impacts on other roadway users; 2. provide safe and convenient access for vehicles; 3. provide safe accessibility for pedestrians, including those that are disabled; 4. where bicyclists are present, accommodate interactions with bicycles; and 5. not adversely affect access to or the operation of public transit stops. There has been relatively little comprehensive research on or national guidance for the geometric design of driveways since the American Association of State Highway Officials (AASHO) publication, An Informational Guide for Preparing Private Driveway Regulations for Major Highways, was published in 1959 (AASHO, 1959) . Since that time, roadway design, function, and volumes have changed as have vehicle design and many other aspects of the roadway environment. In addition, there has been a growing emphasis placed on managing access and on accommodating pedestrians. The U.S. Architectural and Transportation Barriers Compliance Board’s Draft Guidelines for Accessible Public Rights-of-Way (Access Board, 2005) contain specific guidelines pertaining to pedestrian needs. There remains, however, an important need to better integrate vehicle and pedestrian design criteria. 1.2 RESEARCH OBJECTIVES AND SCOPE The objective listed in the research problem statement was to develop recommendations for geometric design of driveways. The research problem statement went on to say that such recommendations will be useful to state departments of transportation and local governments in preparing driveway design standards and practices that consider standard engineering practice and accessibility needs and provide for safe and efficient travel by motorists, pedestrians, and bicyclists on the affected roadway. To achieve the objective, the project was structured as follows. 1. The contractor reviewed research literature, obtained examples of transportation agency design documents, and conducted a survey of transportation agencies, in order to document the current state-of- practice and highlight research needs, and to identify topics and collect source materials for inclusion in the design guide.

3 2. After reviewing and considering a synthesis of the literature and documents, the project oversight panel selected topics for research. 3. The contractor conducted research related to the geometric design of driveways. 4. The contractor prepared a driveway design guide. When roadway designers use the term “driveway”, they are often referring to just a part of a driveway, the area where the driveway intersects the public highway or street. For the most part, this project reflects the roadway designer definition of driveway, and does not consider the design of a driveway well within a private site, except as it affects the driveway intersection with the public roadway. During the initial stage of the project, the decision was made to limit the project scope to driveways that “look like driveways,” and exclude driveways “that look like streets.” 1.3 RESEARCH PLAN The research project was structured into two basic phases. Phase 1 included Tasks 1 through 5, while Phase 2 included Tasks 6A, 6B, and 7. In Task 1, the research team reviewed research literature and transportation agency documents that address the geometric design of driveways. Also, a survey instrument addressing the geometric design of driveways was prepared and sent to state and local transportation agencies. The responses were reviewed and summarized. In Task 2, the team identified geometric elements and developed performance measures, based on the information from Task 1. Two detailed tables showing almost 100 factors that may need consideration during the geometric design of driveways were created. A list of 14 elements was prepared as preliminary candidates for additional research, along with associated design objectives and possible performance measures. Task 3 involved an evaluation of the knowledge and practices associated with listed elements. These evaluations identified current practices, discussed the degree to which certain aspects had been studied and addressed, and stated outstanding questions. In Task 4, the contractor suggested that for Phase 2 research activity, the project oversight panel consider and select from among a short list of five topics selected from among the preliminary list developed during Tasks 2 and 3. During Task 5, the contractor submitted a draft report, and the project oversight panel discussed and selected the topic for Task 6A research. Task 6A was devoted to conducting the selected research activities. Task 6B involved the preparation of a separate document, a guide for the geometric design of roadways.

4 Task 7 was the completion of the report, along with developing suggested revisions to the AASHTO (American Association of State Highway and Transportation Officials) Green Book (AASHTO, 2004).