With this background, we have been asked to evaluate, along with traffic engineering and transportation consultants, Kittelson and Associates, a range of grade-separated interchange possibilities for this intersection that will provide acceptable levels of service for automobile traffic and improved pedestrian and bicycle access to the park; and to make recommendations for an appropriate grade-separated structure to this highly visible "gateway" to Charlottesville.

We began with a review of the types of grade-separated interchanges available to identify the particular ones that we felt warranted further testing. We then evaluated the traffic efficiency of each, discussed the place of masonry bridge design as a part of the history of park roads and parkways, prepared preliminary comparisons of cost, and made recommendations for further refinement.

Conventional Interchanges:

An interchange is a grade-separated intersection (one road passes over another) with ramps to connect them, as pictured. In the picture, thru traffic on the freeway (red) can Pass through without stopping. Even if traffic signals are installed at the ends of the ramps, traffic on the surface street (blue) also flows smoother without interference from the freeway.

Diamonds and Other 4-Ramp Interchanges:

The basic diamond (1eft) is often the design of choice for lower-traffic interchanges without special constraints. The interchanges at Park Street, Locust Avenue, and Barracks Road, are examples of this type of interchange.

It does not scale up well to heavy traffic on the surface street or ramps, or if there is heavy left-tuning traffic. Traffic signals can be installed at the two points where the ramps meet the surface street (e.g. 250 at Barracks Road), but high enough traffic volumes can cause backups on the street and the ramps -- even resulting in stopped traffic on the freeway. All ramps function to connect the freeway to the surface street, as well as transition traffic from low speeds, or a dead stop, to freeway speeds. If a ramp also has the task of storing queued-up traffic, its length becomes a critical design factor. Another problem: for higher traffic volumes, the surface street will need left turn lanes for the entrance ramps. In a tight diamond, there's not much length between ramps available for turn lanes. Having turn lanes for each direction in parallel forces the roadway to be wide. Because of these limitations, and the level of traffic projected for this interchange, a conventional diamond interchange was not deemed practical at 250/McIntire/Meadowcreek Parkway.

Single-point Urban Interchange (SPUI) The Single-point Urban Interchange is a relatively new variant of the diamond. Other names for this interchange include "urban interchange" and "single-point diamond." The SPUI's advantages include the fact that it requires less right-of-way acquisition than a cloverleaf, and it allows concurrent left turns for greater capacity. Its disadvantages include the fact that it uses complex and unfamiliar turning movements, is not friendly to pedestrians, and multilane ramps or surface streets can lead to very large areas of uncontrolled pavement and very large bridge structures. While the SPUI evaluated as a part of the 1998 Report would be larger than the current configuration (with a two-lane Meadowcreek Parkway), the scale of the bridge and the difficulty of pedestrian access remain major concerns, and a SPUI was not evaluated for these reasons.

The Full Cloverleaf

The classic cloverleaf allows "non-stop" full access between two busy roads. Traffic merges and weaves, but does not cross at-grade; unless the interchange is too congested, no stopping is required.

Typically a cloverleaf is used where a freeway intersects a busy surface street, though many older freeway-freeway interchanges are also cloverleafs. The full cloverleaf is not considered as applicable in some situations now as it might have been a few decades ago; in several places cloverleafs have been replaced with either signalized interchanges or higher-capacity directional interchanges with flyovers. An operational disadvantage to the plain cloverleaf is the "weaving" process, where drivers exiting one loop have to merge and cross other drivers entering the next one. Weaving, which causes bottlenecks and accidents, is the primary reason cloverleafs are now deprecated in designs for new or revamped interchanges. In addition, cloverleafs require a great deal of land, and for this reason alone would not be appropriate to consider for the 250/McIntire/Meadowcreek Parkway interchange.

The Parclo

The interchanges at left are called partial cloverleafs, or "parclos," as they have loop ramps. (A full cloverleaf has eight ramps.) However, they're functionally equivalent to a diamond, with two entrance ramps and two exit ramps. These types of parclos are also known as folded diamonds, as one or more ramps are "folded" into a loop on the opposite side of the surface street.

These variants are useful in different situations. Sometimes the layout of the land, or property on it, creates a constraint. Sometimes traffic is better served if motorists can turn one way instead of the other. Sometimes nearby intersecting streets make a modified diamond a better solution than a conventional one. In the case of the 250/McIntire/Meadowcreek Parkway interchange, the use of parclos, enabled us to extend the left turn lanes and to avoid construction in critical locations. We evaluated two conventional parclo interchanges.

Roundabout Interchanges:

Roundabouts have been used in Western Europe and in most British-influenced countries around the globe since the mid-1950s. Although more than 35,000 roundabouts are in operation around the world, there are fewer than 50 in the United States. The advantages of roundabouts for intersections, and grade-separated interchanges have become increasingly apparent in this country. The specific scale and configuration of roundabouts are essential to their operation. The following diagram illustrates the key elements of a modern roundabout:

Roundabouts may be used in interchanges in two ways. They can be incorporated in the interchange itself, with one of the intersecting roads going through the roundabout, and the other being connected to the roundabout with ramps. If the roundabout is above the passing road, the configuration is called a "Tabletop" roundabout. (In our case Route 250 traffic would go into a roundabout that would be elevated above McIntire/Meadowcreek.) If the roundabout is under the passing road, it may be called a "Double-Bridge" roundabout. This configuration is an older form of interchange roundabout that was widely used in England, but is now usually eschewed in favor of the configuration described below for cost and efficiency reasons.

The other way in which roundabouts art used at interchanges is to substitute for the signaled intersections in diamond or parclo interchanges. This is more cost-effective, and more efficient, from a traffic management perspective, than the Tabletop or Double-Bridge roundabouts. As a result, these are the most often-utilized form of roundabout interchanges today. They can often produce operational and safety improvements over signalized grade-separated interchanges.

We evaluated diamond and parclo interchanges that utilize roundabouts in lieu of signalized intersections at the ramps, as well as tabletop and double-bridge roundabouts for this interchange. While it is certain that this does not exhaust the alternatives possible, we believe it offers a good range of possibilities to test for this important location.

Interchange Alternatives

Alternate 1 - Half Cloverleaf, or Parclo (signal control): Alternate 1 consists of a partial cloverleaf interchange with loop ramps and directional ramps in the southeast and northeast quadrants. This design has two at-grade, signalized intersections at the ramp junctions with Meadowcreek Parkway.

Alternate 2 - Half Cloverleaf, or Parclo, (signal control): Alternate 2 consists of a partial cloverleaf interchange with loop ramps and directional ramps in the southeast and northwest quadrants. This design has two at-grade, signalized intersections at the ramp junctions with Meadowcreek Parkway.

Alternate 3 - Half Cloverleaf; or Parclo, (roundabout control): Alternate 3 consists of a partial cloverleaf interchange with loop ramps and directional ramps in the southeast and northeast quadrants. This design has two at-grade roundabout intersections at the ramp junctions with Meadowcreek Parkway. The interchange form is essentially the same as Alternate 1, except that roundabouts are used instead of signals.

Alternate 4 - Diamond Roundabout: Alternate 4 consists of a standard diamond interchange (directional ramps in each quadrant) with roundabouts as the intersection control at each of the two at-grade intersections.

Alternate 5 - Tabletop Roundabout: Alternate 5. The Mclntire Road/Meadowcreek Parkway approaches would not go through the roundabout, instead they would pass under the roundabout. This would result in two sections of the roundabout on bridges.

Alternate 6 - Overpass with Roundabout: Alternate 6 consists of an interchange form similar to Alternate 5, except that the 250 By-Pass traffic is carried over the roundabout. The major difference is that, rather than using two separate bridges for the grade separation, one longer structure could be used.

The basic configuration and scale of the roundabout intersections are shown These have not been modified to accommodate the entry and exit lane development requirements such as flares, tapers, right turn bypass lanes, etc. The roundabout diameters have been approximately scaled to indicate the intersection roadway footprints.

TRAFFIC ANALYSIS

For purposes of evaluating future traffic operations, forecast volumes for the 250 Bypass/Meadowereek Parkway interchange were derived based on the assumption that Meadowcreek Parkway will operate as a two-lane roadway facility. Using the software analysis program SIDRA 5.20a (ARRB Transport Research Ltd.), each alternate interchange design was analyzed by Kittelson and Associates to determine the following performance measures:

Volume-to-Capacity Ratio V/C);
Average Intersection Delay;
Total Intersection Delay;
Longest Vehicle Queue; and
Level-of-Service (LOS).

The SIDRA 5.20a analysis program was selected based on its capability to model both conventional signalized intersections and modern roundabouts. A summary of the analyses results is provided in Table 1. These results are reflective of the traffic volumes, traffic control, and lane configurations for each alternate interchange design. Table 1 shows the "opening day" traffic volumes based upon 1997 traffic figures, and projections for a two-lane Meadowcreek Parkway operating at capacity. Analysis was also done on 2020 projections, and these will be discussed in the narrative that follows and annotates the Table.

Table 1 SIDRA

Generally, for a signalized intersection to have acceptable operations, the intersection should maintain LOS "D" or better, with a volume-to-capacity ratio of less than 1.0 (for roundabouts a v/c of less than 0.85 is desirable).

SUMMARY

The following conclusions and recommendations are based on a review of the design and traffic impacts for each of the six interchange alternates.

Alternates 1 and 2 (signalized parclo interchanges) will not operate at an acceptable level of service or volume-to-capacity ratio unless the Meadowcreek Parkway/Mclntire Road segment is widened to provide two through lanes in each direction through the intersection. While this would provide a higher level of service than the 20-lane at-grade intersection previously studied, the short and long-term projections show a marginal-at-best level of service.

Alternates 3 and 4 (parclo and diamond with single-roundabouts) will operate at a level-of-service of "B" on opening day with a volume to capacity ratio of less than 0.85. Analysis of 2020 traffic projections indicate that two-lane roundabouts with flared entrances will accommodate the projected increase in traffic should the City choose to make the intersections more efficient as traffic increases.

Alternates 5 (tabletop roundabout) will operate at an acceptable level-of-service and volume to capacity ratio if the roundabout is constructed with three-lanes. This will also function at an acceptable level of service for the longer-term.

Alternate 6 (double bridge roundabout) will friction at an acceptable level-of-service for the near future, and at a marginal volume-to-capacity ratio if the single-lane roundabout is constructed with right bypasses all around. Acceptable level-of-service may be attained for the short and longer term if the roundabout can be expanded to two lanes with double entry and exit lanes.

Alternates 5 and 6 will be substantially more expensive than any of the other alternatives, because it requires the construction of twice as many bridges. It does not offer a benefit in efficiency to offset the initial cost and added maintenance requirements. (See Cost Estimate.)

Recommendations:

Based on quantitative operational performance, qualitative economic feasibility, and environmental impacts, Alternates 3 and 4 are recommended for further evaluation and design refinement. Although Alternate 6 will be more expensive than Alternate 3 and 4, it will have the least impact on the land, and we believe also warrants further study.

A fuctional design review should be further conducted for Alternates 3, 4 and 6. In addition, the designs should be reviewed to ensure adequate pedestrian facilities are provided.

Alternates 3, 4 and 6 should be analyzed to determine the required adjustments to make the designs function for future year traffic volumes, should the City decide that meeting such an increase in this corridor is warranted.

Components of an Interchange:

Grade-separated intersections are made up of two key elements: the roads and ramps and the bridge structure. While the road and ramp configuration will determine the operational capacity and function of the interchange, it is the bridge that will give it its architectural character. The bridge component of the McIntire/Meadowcreek interchange should clearly indicate that, while this location can handle a considerable capacity of vehicular, pedestrian, and automobile traffic, it remains an important piece of civic architecture in a PARK. Its form, scale and materials should reflect this essential fact. Bridges are one of the defining elements of the American parkway.

The masonry arch bridge has played an important part in the early country's transportation development, and later survived in its parks, not for its utility as much as for its beauty. Built in 1813, the Casselman Arch, near Grantsville, Maryland, was at that time, the largest masonry arch bridge in America. It is now the focal point of a Maryland State Park.

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