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Highway 280 and Broadway Interchange Comparison of Alternates January 10, 2002 SEH No. A-ROSEV0201 .00 Three separate alternate concepts have been developed for a potential interchange at Highway 280 and Broadway. These include the Central Alternate which is a concept which retains the Highway 280 lanes in their e�sting location. The East Alternative moves the lanes to the east and essentially takes most needed right-of-way from the northeast and southeast quadrants. The West Alternate moves the roadway to the west and takes significant amounts for right-of-way in the northwest and southwest quadrants. Each of the alternates has advantages and disadvantages and a comparison of the three against some of the design and operating considerations presented in the following paragraphs. Copies of the three alternates are attached. TH 280 Alignment The existing alignment of Highway 280 through the Broadway area consists of a reverse curve. The e�sting alignment has a design speed of between 55 and 60 mph. Moving the roadway to the east reduces the degree of curvature and improves the smoothness of the flow of traffic. Shifting the roadway to the west requires introduction of a third curve for the roadway which means, as an example, that northbound traffic would first curve slightly to the left, then, to the right and ultimately to the left again. However, the degree of curvature would a11ow retention of a 55 mph speed limit, but the operation would be less desirable. Ramp Alignment A design consideration is how the exit from a freeway occurs in terms of the impact on through motorists. As an example, an exit on the outside of a curve on a freeway tends to draw through traffic towards the e�t because of the additional pavement. Curves or reverse curves on ramps sometimes create operational problems. The entrance from an on ramp also needs to have a smooth flow since merging traffic must pay attention not to its roadway path but to the approaching traffic. The Central Alternate and the East Alternate both have the e�ts on either a tangent or an inside curve section which will ha�e no problems for through traffic. The West Alternate does place the northbound e�t on the outside of a curve. The Central Alternate has very sma11 curves on its ramps. The East Alternate has 4 to 5 degree curves but there is a significant tangent section between them. The West Alternate has 6 to 8 degree curves with a relatively short tangent section. The Central and East Alternates ha�e the entrance ramp entering the main line on either a tangent or the outside of a curve. The West Alternate has the northbound entrance ramp on the outside of a curve but the southbound entrance ramp on the inside of a curve which is less desirable. Broadway Street Design A major concern for design of Broadway is the distance between the railroad track and the ramp intersection. The railroad tracks will ha�e a fixed elevation and require a flat grade across them