The northwest end of 400 is near the west town line of the Town of West Seneca at the Thruway. The southeast end is north of the hamlet of South Wales, which is located in the town of Wales. At this end, the route connects to Olean Road and continues south as NY 16. The highway crosses Cazenovia Creek just before ending.
Remaining rural, NY 400 crosses just to the east of East Aurora, encountering US 20A/NY 78 (Big Tree Road) at a northbound entrance/southbound exit. North of here, entering the town of Elma, it turns to the west to meet with Maple Street at the final single-direction junction, a northbound entrance and southbound exit. The expressway then turns northwest to intersect with Jamison Road (formerly NY 422) by way of a full diamond interchange. NY 400 also passes a small industrial area containing manufacturing plants for Moog, Inc, Motorola, Servotronics, and Steuben Foods located on Maple Street and Jamison Road.
NY 400 continues northwest, paralleling NY 16/NY 78 on Seneca Street and becoming more suburban and heavily trafficked. On the Elma/West Seneca town line, a full cloverleaf interchange occurs with US 20/NY 78 on Transit Road, just north of the Seneca Street/Transit Road intersection. The expressway turns in a more western direction here, as well, though there is still a slight deflection to the north.
The expressway continues to parallel NY 16 (Seneca Street), now running between NY 16 to the south and the North America Center industrial park to the north. The Norfolk Southern railroad separates NY 400 from the industrial park. It intersects with NY 277 (Union Road), again just north of the NY 16 (Seneca Street) intersection with the road. After a final exit for NY 16, it encounters the New York State Thruway (Interstate 90) at the beginning of I-90's six-lane section through Buffalo, signifying its high use during rush hour.
The intersection where NY 400 terminates at NY 16 in South Wales has been reconfigured over time. When the expressway was first constructed, the flow of traffic along Olean Road was unimpeded while traffic from NY 400 was forced to merge with NY 16 at a sharp angled intersection. The junction has since been altered so that southbound traffic on NY 400 continues onto NY 16 south with no merging required while commuters on NY 16 north are guided onto the expressway rather than onto Olean Road. Olean Road, however, is still accessible by way of a four-way intersection with Richardson Road just west of the former junction.
|Erie||Aurora||0.00||Continuation beyond Olean Road|
|Olean Road (NY 951V)||At-grade intersection|
|East Aurora||1.99||Northbound exit and southbound entrance; NY 16 leaves northbound and joins southbound|
|4.05||Southbound exit and northbound entrance|
|Elma||7.12||CR 241 (Maple Road)||Southbound exit and northbound entrance|
|8.39||Former routing of NY 422|
|14.98||NY 277 (Union Road)|
|16.91||, Erie||Exit 54 (I-90/Thruway)|