National Academies Press: OpenBook

Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report (2021)

Chapter: Section 3. Observational Field Study

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Suggested Citation:"Section 3. Observational Field Study." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
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Suggested Citation:"Section 3. Observational Field Study." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
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Suggested Citation:"Section 3. Observational Field Study." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
×
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Suggested Citation:"Section 3. Observational Field Study." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
×
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Page 64
Suggested Citation:"Section 3. Observational Field Study." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
×
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Page 65
Suggested Citation:"Section 3. Observational Field Study." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
×
Page 65
Page 66
Suggested Citation:"Section 3. Observational Field Study." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
×
Page 66
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Suggested Citation:"Section 3. Observational Field Study." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
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Page 67

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60 Section 3. Observational Field Study This section describes an observational field study of vehicle speeds on interchange ramps. The purpose of the study was to gather supplemental speed data along interchange ramps that could be used in combination with speed data collected along interchange ramps as part of previous research to further develop and refine speed prediction models for use in a spreadsheet-based RSPM tool that designers and traffic engineers may use to estimate vehicle speeds along interchange ramps. The research team had access to speed data collected along ramps as part of previous research (Torbic et al., 2012) and collected additional speed data at ramps to fill in gaps in these previous data sets to develop speed prediction models for interchange ramps. 3.1 Study Site Selection As part of previous research (Torbic et al., 2012; and Torbic et al., 2016), speed data had been collected at service interchanges at both entrance and exit ramps and diagonal and loop ramps. To supplement these datasets, priority was given to collecting speed data at ramps of system interchanges and ramps along freeways with posted speeds limits of 75 mph or greater. The goal was to collect speed data at a one diagonal ramp, one loop ramp, one semidirect connection ramp, and one direct connection ramp for both entrance and exit ramps, for a total of four entrance ramps and four exit ramps. Vehicle speeds operating in the normal traffic stream were collected at selected study locations in the metropolitan and surrounding areas of Kerrville, Texas, and Kansas City, Missouri. These areas provided a suitable variety of urban and rural ramps, high-speed ramps, and ramp configurations to satisfy the priorities and goals identified by the research team and project panel. Speeds were collected at three exit ramps and four entrance ramps. To select study sites, research team members used aerial mapping tools to identify potential candidate locations based on ramp configuration and freeway posted speed limit, then they contacted colleagues at state DOTs to gain additional information as necessary on ramp design criteria and supplemental site characteristics. 3.2 Study Locations The speed data along ramps were collected at selected study locations in the metropolitan and surrounding areas of Kerrville, Texas, and Kansas City, Missouri. Speed data were collected along four ramps in Texas and three ramps in Missouri. The study sites in Texas were located along Interstate I-10 between the towns of Sonora and Kerrville. The study sites in Missouri were all located near Kansas City along Interstates I-29 and I-635. Aerial images of the ramps where speed data were collected are provided in Figures 8 through 14.

61 Figure 8. Loop Ramp – Westbound Exit at I-10 and Harper Rd (Image Credit: Google EarthTM Mapping Service) Figure 9. Loop Ramp – Eastbound Entrance at I-10 and Harper Rd (Image Credit: Google EarthTM Mapping Service)

62 Figure 10. Diagonal Ramp – Eastbound Entrance at I-10 and Martinez St (Image credit: Google Earth™ Mapping Service) Figure 11. Diagonal Ramp – Eastbound Exit at I-10 and US-277 (Image credit: Google Earth™ Mapping Service)

63 Figure 12. Outer Connection Ramp – Ramp from Westbound Missouri Route 152 to Northbound I-29 (Image credit: Google Earth™ Mapping Service)

64 Figure 13. Outer Connection Ramp – Ramp from Southbound I-29 to Westbound Missouri Route 152 (Image credit: Google Earth™ Mapping Service)

65 Figure 14. Direct Connection Ramp – Ramp from Northbound Missouri Route 9 and Southbound US-69 to Northbound I-635 (Image credit: Google Earth™ Mapping Service)

66 3.3 Data Collection Site characteristic data for the ramps were collected through a combination of reviewing plans, profile sheets, and aerial images, while other data were collected in the field or estimated based on available information. The following site characteristics were collected for the ramp proper and freeway mainline ramp terminals: • Ramp configuration. • Area type (dense urban, urban mixed-use, rural). • Type of ramp (entrance, exit). • Design speed, posted speed, and advisory speed of ramp. • Curve radius. • Curve length. • Superelevation. • Tangent length. • Number of lanes. • Lane and shoulder widths. • Presence of barriers. • Presence of compound curves. • Grade (upgrade / downgrade). • Presence of ramp meters. • Type of diverge/merge. • Taper length. • Length from end of taper to gore nose. • Length from gore nose to end of acceleration lane. Speed data were collected at study sites on weekdays during non-peak hours. Data were collected using Pro Laser 3 laser guns. The data collected from the laser gun were the speeds and distances (i.e., locations) of subject vehicles along the ramps. Laser guns were hand-held and operated by a data collector on the roadside, located in a position chosen based on several criteria, including: • Safety of data collectors and equipment. • Minimal impact of presence of data collectors and equipment on driver behavior or desired speeds. • View of as much of the ramp as possible. • Minimum angle between the laser gun and the vehicles being tracked. Speeds of free-flow vehicles were collected. Two laser guns were used to collect speeds and distances of vehicles along the ramps. The operator of the first laser gun tracked a vehicle along the ramp and then communicate by radio to the operator of the second laser gun the description (make, model, and color) of the vehicle being tracked. The operator of the second laser gun then tracked the subject vehicle when it came into view along the ramp. The operators then confirmed that the speeds and distances of the subject vehicle were successfully recorded. Speeds of vehicles were collected at critical locations along the ramp, such as the beginning (PC) and end (PT) of the controlling curve in the direction of travel on the ramp. Vehicles were classified as passenger vehicles (including light trucks and sport-utility vehicles) or trucks (including single- unit and tractor-semitrailer trucks). Table 16 indicates the posted speed limit of the freeway at each of the study ramps and the number of vehicles for which speed data were collected.

67 Table 16. Summary of data collected Freeway Freeway/Crossroad Ramp Configuration Entrance or Exit Ramp Freeway Posted Speed Limit (mph) Number of Vehicles Observed I-10 Harper Rd. Loop WB exit 75 130 I-10 Harper Rd. Loop EB entrance 75 128 I-10 Martinez St. Diamond EB entrance 80 21 I-10 US-277 Diamond EB exit 80 94 I-29 Missouri Rt 152 Outer connection WB MO-152 to NB I-29 70/65 125 I-29 Missouri Rt 152 Outer connection SB I-29 to WB MO-152 70/65 110 I-635 Missouri Rt 9/US-69 Direct connection NB MO-9/SB US-69 to NB I-635 65/55 115

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Designing extended sections of highway based on the design speed process is relatively straightforward. However, when applied to interchange ramps where high-speed facilities meet low-speed facilities and drivers are expected to accelerate or decelerate over short distances, application of the design speed process is more complex.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 313: Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report provides enhanced design guidelines for selecting appropriate ramp design speeds in a consistent manner, accounting for sequential speed transitions from one component or section to the next, consistent with performance capabilities of vehicles and driver expectations.

Supplemental to the document are NCHRP Web-Only Document 313: Selecting Ramp Design Speeds, Volume 1: Guide and Ramp Speed Profile Model figures.

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