Summary

The City of Hilliard commissioned a final study by Burgess & Niple and Kittelson & Associates to identify potential long-term intersection improvements for the Hilliard Triangle. The Hilliard Triangle is an area comprised of three closely spaced intersections: Scioto-Darby Road / Cemetery Road (unsignalized), Scioto-Darby Road / Main Street (signalized), and Cemetery Road / Main Street (signalized). The Hilliard Triangle has been fully developed with a variety of land uses that include schools, medical offices, and small-scale commercial uses. As a system, the Hilliard Triangle currently operates near capacity during the weekday a.m. and p.m. peak hours. Approximately 65 crashes have occurred at the study intersections within the past three years. Based on regional planning model estimates, the study area is expected to undergo significant growth over the next 25 years.

Two intersection improvement alternatives (signalization enhancement and roundabout conversion) were evaluated to mitigate existing safety concerns and to accommodate future traffic projections. The results of the evaluation showed that signalization of the Scioto-Darby Road / Cemetery Road intersection and conversion of the two signalized Main Street intersections to double-lane roundabouts provided the best combination of system intersection operations, safety enhancement, right-of-way impact minimization, and property access.

Analysis of Alternatives

Traffic counts were taken for the study intersections on a mid-week day in June 2005 to establish current traffic volumes. These counts were conducted during the weekday morning (7:00 am – 9:00 am), midday (11:00 am – 1:00 pm), afternoon (2:00 am – 4:00 am), and evening (4:00 am – 6:00 am) hours. A review of the traffic counts found that the weekday morning and evening are the critical peak periods of the system. The weekday morning peak hour was found to occur between 7:00 and 8:00 am, while the evening peak hour was found to occur between 5:00 and 6:00 pm.

Year 2030 traffic volume projections were developed to account for regional growth in the study area. The 2030 year traffic volumes were based primarily on the regional planning model and take into account planned transportation improvements in the study area.

For year 2030, calculation of volume to capacity ratios, average delay, and queue lengths indicate that, with signalized intersections, Main Street, Cemetery Road, and Scioto Darby Road will all need to be expanded to provide two travel lanes per direction with widening at the intersections to accommodate additional turn lanes (up to six lanes of total pavement). Even with these improvements, significant queues and delays are anticipated at all of the study intersections. As a result, queue spillback from the Scioto Darby Road / Main Street and Cemetery Road / Main Street intersections would effectively limit left-turn ingress and egress to all driveways located within 400 feet of the respective intersections.

Roundabout operations were evaluated for the study intersections using the same year 2030 peak hour traffic volume projections. Acceptable intersection operations were achieved under year 2030 traffic conditions with the installation of a two-lane roundabout at the Scioto-Darby Road / Cemetery Road intersection and installation of a three-lane roundabouts at the Cemetery Road / Main Street and Scioto Darby / Main Street intersections. The roundabout alternative provides greater capacity, less delay, and shorter queues than the signalized alternative with fewer traffic lanes required. However, issues involving right-of-way constraints and the need to safely accommodate pedestrian activity call into question the feasibility of this alternative. It is also unlikely that the traffic volumes projected by the regional model for 2030 are reasonable without significant system transportation improvements such as widening Main Street to four lanes through downtown Hilliard.

Given the low probability of widening Main Street to the north, a two-lane roundabout scenario was explored as a viable interim solution, and potentially a viable long-term solution until such time as significant transportation improvements are made system-wide, enabling full realization of the regional model’s projected demand. The two-lane roundabouts would be much more pedestrian friendly and require less right-of-way.

Assuming linear growth rates from 2005 traffic volumes to 2030 projections, and estimating that southbound Main Street (north of Cemetery) will reach it’s capacity at year 2020 projected volumes, two-lane roundabouts at the Scioto Darby Road / Main Street and Cemetery Road / Main Street intersections would operate acceptably until at least 2020 or beyond until such time as significant regional transportation improvements are made. The corresponding signalized alternative at the Scioto Darby Road / Main Street and Cemetery Road / Main Street intersections for year 2020 would still require two travel lanes per direction with widening for turn lanes and significant queues and delays still anticipated.

Recommendation

From an operational and safety perspective, the roundabout alternative provides superior performance over the signal alternative. Additionally, roundabouts can accommodate u-turn maneuvers which enhance access to properties located in their immediate vicinity. The drawback for roundabouts is the amount of right-of-way needed at each intersection. Right-of-way constraints at the Scioto Darby Road / Cemetery Road intersection effectively eliminate a double-lane roundabout as a potential alternative at that location. Balancing the needs of the system against reasonable right-of-way requirements, the final recommendation is for two-lane roundabouts at the intersections of Scioto Darby Road / Main Street and Cemetery Road / Main Street, and a traffic signal at the Scioto-Darby Road / Cemetery Road intersection.

Conceptual Design

Four conceptual layouts of the recommended improvement alternative were developed for the Hilliard Triangle. The layouts are identical in nature except for the design of the Scioto-Darby Road / Main Street roundabout. The skew angle of intersecting roadways at this intersection leads to design trade-offs involving right-of-way impacts, speed management, and design vehicle accommodation. The layouts were prepared in design sketch form, drawn over scaled photogrammetric and survey base mapping. Although in sketch form, the layouts were developed in sufficient detail to ensure adequate speed reduction objectives are met and appropriate natural vehicle paths can be provided. The sketches are also sufficient to identify local access implications, property impacts, and the approximate extent of roadway widening and realignment. These sketches are the basis for evaluation so that the most feasible and appealing options can be selected and advanced to a more detailed level of design.

Design Alternative #1 – “Centered”

View Alternative #1

The first conceptual alternative, shown in Figure 8, provides two-lane roundabouts at both Main Street intersections with outside circle diameters of 160 feet that are centered on the existing intersections. Advantages of this alternative include minimizing right-of-way impacts to surrounding properties and consistency with driver expectations. Disadvantages of this alternative include the need to redevelopment the Fifth Third Bank property, potential high-speed eastbound and westbound right-turn movements at the Scioto Darby intersection, and the need for truck aprons on entry to accommodate northbound and southbound right-turn maneuvers at Scioto Darby.

Design Alternative #2 – “Flattened Oval”

View Alternative #2

The second conceptual alternative, shown in Figure 9, provides an oblong two-lane roundabout at the Scioto Darby intersection that is centered on the existing with the long side parallel to Main Street. Advantages of this alternative include desirable entry speeds and the elimination of truck aprons on entry to accommodate northbound and southbound right-turn maneuvers. A major disadvantage of this alternative is the potential for exit-circulating crashes due to the increased likelihood of vehicles entering next to circulating vehicles that are intending to exit at the next exit. This condition, created by the geometric configuration of this alternative, cannot be completely mitigated by striping. Other disadvantages include the need to redevelopment the Fifth Third Bank property, modification of Donatos Pizza on-site circulation, and potential high-speed circulating movements and speed differentials.

Design Alternative #3 – “Ellipse”

View Alternative #3

The third conceptual alternative, shown in Figure 10, provides an ellipse roundabout at the Scioto Darby intersection that is 190 feet long by 160 feet wide. The ellipse is centered on the existing intersection and rotated with the long side oriented northeast/southwest. Advantages of this alternative include adequate entry and circulating speeds and reduction in size of truck aprons on entry to accommodate northbound and southbound right-turn maneuvers. Disadvantages of this alternative include the need to redevelop the Fifth Third Bank property, and impacts to the Hilliard Methodist Church property.

Design Alternative #4 – “Offset”

View Alternative #4

The fourth conceptual alternative, shown in Figure 11, provides a two-lane roundabout with an outside diameter of 175 feet (slightly larger than Alternative #1) that is centered on the existing intersection. Advantages of this alternative include second fewest right-of-way impacts to surrounding properties, adequate entry and circulating speeds, and consistency with driver expectations. Disadvantages of this alternative include the need to redevelopment the Fifth Third Bank property and the need for truck aprons on entry to accommodate northbound and southbound right-turn maneuvers.

Design Recommendation

View Assessment Matrix of the Four Design Concepts

Overall, Option #4 provides the best balance between right-of-way impacts, speed management characteristics, and consistency with driver expectations. Option #1 had the fewest overall impacts; however, it still resulted in the need for site redevelopment of the Fifth-Third Bank property (same as Option #4). As such, speed became the deciding characteristic between the two alternatives and Option #4 provides better speed management. In conclusion, Option #4 is the preferred alternative for final design.