Sub-problem 2c: What is the level of service at the ramp junction at the northbound on-ramp to I-87?

Step 2. Results

The input data for this problem is gathered from several sources. We can use the aerial photographs in Exhibit 4-17 and the map in Exhibit 4-18 that were presented earlier in this problem to determine the number of lanes on the I-87 mainline and on the ramp itself. The length of the acceleration lane was also determined from the map and aerial photos. We determined the free flow speeds on the freeway and on the ramp using data collected previously by the New York State DOT.

The volume data is a topic that is worthy of additional discussion. While we could use ground counts collected by the DOT, we must be careful. Why? These counts represent service volumes, or the actual volumes using the facility during a given time interval. However, to have a true estimate of the facility performance, we must instead consider demand volume, or the number of users of a given segment during a specific period of time. Particularly when congestion exists, it is often difficult to estimate demand volume. Remember that the distinction between demand and volume is this: demand is the number of system users desiring service over the course of a given time interval, whereas volume is the number of system users actually served during that same time interval. With this distinction in mind, you can see that demand and volume are equivalent to one another only when the system is operating in an undersaturated mode.

The data used for this problem are summarized in Exhibit 4-30.

Continue to the next page to see the results of applying the HCM ramp junction analysis to the conditions found in these two time periods.