395 - GSA Development of St. Elizabeths Campus

2.4.3.1 Interchange of Malcolm X Avenue with I-295The Alternative 2 Modified of the interchange of Malcolm X Avenue with I-295 is a variant orhybrid of the alternatives evaluated in the earlier processes. While similar in nature, it hascomponents that address issues raised concerning the alternatives in the DEIS. ThisAlternative 2 Modified has a single bridge structure crossing I-295 with a pair of traffic signalsto control traffic operations to and from SB I-295. Alternative 2 Modified has a circular loopramp and a direct access ramp over Malcolm X Avenue and the loop ramp. There is a revisedramp configuration at the NB I-295/ South Capitol Street Interchange over the currentconfiguration.DDOT prefers Alternative 2 Modified because it does not use broken-back curves in the loopramp, and thus does not have the issue with design-exceptions that other alternatives had. Thedesign also resulted in less impact on adjacent Shepherd Parkway over other designs. It hassimilar, or better in some cases, transportation system characteristics in relation to other designsconsidered. Details of these considerations are described below.Since the development of the initial interchange concepts in the DEIS, the direction receivedfrom both National Park Service and GSA is to minimize direct impacts on parkland andhistoric resources. The project team went through multiple iterations to minimize the parkimpacts. DDOT has allowed the access ramps south of Malcolm X Avenue to encroach intoDDOT ROW, which has further reduced the impacts. NPS Parkland impacts have been reducedto 9.2 acres for the Alternative 2 Modified. The fly over ramp configuration which spans theloop ramps has the least impact area among alternatives devised because it reduces the overallwidth of the interchange near the Malcolm X Avenue intersection.Cost Analysis:A rough order of magnitude cost estimate has been prepared. The costs estimate includes theconstruction costs of the interchange improvements, engineering and contingencies. The costestimate does not include right-of-way or mitigation costs, stormwater management facilities orescalation to year of construction.This construction cost estimate is higher than earlier alternatives because of the additionalbridge and retaining walls required for the direct connection ramp through the loop ramp.From a long-term maintenance standpoint, all the alternatives reviewed have similar costs.Constructability:From a constructability standpoint, the Alternative 2 Modified is feasible but does create somedifficulties associated with maintaining traffic flow during construction operations. A phasedapproach leads to additional difficulties since the previously constructed roadways will havetraffic in service while the next phase is under construction.The greatest challenges lie in the construction of the bridges over SB and NB I-295. Constructionof the bridge superstructures will require periodic lane closures of the freeway to installstructural steel, set forms and place concrete for the bridge decks. The Alternative 2 Modifiedwill be less complicated to construct than other possible designs because it uses a singlestructure which is shorter and normal to the freeway leading to easier installation ofsuperstructure elements. Similarly, the bridges over both Malcolm X Avenue and the new NBramp over South Capitol Street will require periodic closures for the superstructureconstruction.2-12