Introduction

The overall subarea network that is the focus of this case study is shown in Exhibit 4-3. Alternate Route 7 is in the middle of the figure. I-87 is on the left and I-787 is on the right. The drawing is not to scale and the Alternate Route 7 segment in the middle has been shortened.

While congestion in the Alternate Route 7 subarea network is not bad overall, there are two places where significant queuing takes place: the I-787 interchange (viz., on the right-hand ramp leading from NY-7 East to I-787 South); and the I-87 interchange (viz., the right-hand ramp leading from NY-7 west to I-87 north). In both cases, the queues can be more than a mile long when traffic is heavy. For example, some mornings the line for the right hand ramp at the I-787 interchange extends halfway back to I-87. Similarly, on Fridays, especially in the summer, and on many weekday afternoons, the queue for the right-hand ramp from NY-7 and I-87 extends halfway back to I-787.

Alternate Route 7’s basic freeway section is about four miles long, with two lanes eastbound and three lanes westbound. The third lane westbound is used by trucks that are climbing the grade that starts at the Hudson River and ends near Miller Road. From there to I-87, the third lane becomes an extension of the right-hand ramp leading to I-87 north.

The interchanges at either end of Alternate Route 7 are good facilities to study. The I-787 interchange has short weaving sections, complicated merging geometries with tight geometry, and an auxiliary lane eastbound that separates the ramps from the main traffic lanes. The I-87 interchange has similar problems with short weaving sections and complicated merges and diverges.