A map of Fairfax County shows that SR 645 is somewhat circuitous: Wall Road is east-west, Lees Corner Road, Stringfellow Road, and the northern part of Clifton Road are north-south, the rest of Clifton Road is mostly east-west, and Burke Lake Road is northeast-southwest.
Wall Road
SR 645 starts in Oak Hill at a cul-de-sac between SR 28 and SR 657. It goes one block and terminates at SR 657.
Lees Corner Road
At this point SR 645 turns south and is concurrent with SR 657, Centreville Road. SR 645 then turns southeast along Lees Corner Road in Herndon. It terminates at U.S. Route 50, Lee Jackson Memorial Highway, in Chantilly.
Stringfellow Road
At this point SR 645 turns east and is concurrent with US 50. SR 645 then turns south in Fairfax. Once south of U.S. 50, Stringfellow Road serves as the ZIP Code border between Chantilly and Fairfax. The road passes through the Greenbriar development, where it intersects with SR 4831, Poplar Tree Road. It then passes by Fair Lakes' western end where the Fair Lakes Parkway terminates at SR 645. After this, Stringfellow Road passes east of the Stringfellow Road Park & Ride, a park and ride facility, and has an interchange with Interstate 66. From 5:30 to 9:30 a.m. weekdays, HOV-2 traffic is allowed to enter I-66 eastbound from Stringfellow Rd. Westbound traffic can exit onto Stringfellow Road at all other times, but the exit is restricted to HOV-2+ traffic weekdays from 3:00 to 7:00 p.m. South of the I-66 interchange, Stringfellow Road heads to an intersection with U.S. Route 29 at the Fairfax/Centreville/Clifton border. A project was completed to widen Stringfellow Road from SR 7735 to US 50 from two lanes to four. The rationale for this project was that VDOT saw that Stringfellow Road was a major connector between US 29 and US 50, and was a very congested road. The project included:
Wide curb lanes in order to accommodate bicyclists
Clifton Road (north of Clifton)
After crossing U.S. 29, SR 645 becomes Clifton Road and has entered the Clifton ZIP code. The road remains four lanes and the speed limit remains 45 MPH. After about half a mile the road intersects with SR 620, Braddock Road. After crossing SR 620 Clifton Road reduces from four to two lanes, although the speed limit remains 45 MPH. After passing by the Twin Lakes Golf Course, the road intersects with SR 654, Popes Head Road, and enters a rather equestrian area for about before reaching the Clifton corporate limits. During that approach the speed limit is gradually reduced to 25 MPH before intersecting with Newman Road. The road then turns and enters the town. This portion of SR 645 has been designated a Virginia Byway.
Main Street (town of Clifton only)
In the town SR 645 intersects with Chapel and Kincheloe roads before it leaves the town. Also in the town SR 645 crosses the Southern Railway tracks. Infrequently SR 645 is impassable due to flooding from Pope's Head Creek.
Clifton Road (east of Clifton)
Once past the town's corporate limits, the speed limit increases to 35 MPH. It then intersects with SR 643 / SR 612 which draws heavy traffic because it leads to the only crossing between Fairfax County and Prince William County that is not physically located at or near Woodbridge or Manassas. Crossing into Fairfax Station, Clifton Road then intersects with SR 610. After the intersection the road heads east, and then intersects with SR 123. This portion of SR 645 has been designated a Virginia Byway.
Burke Lake Road
After crossing SR 123, SR 645 continues with two lanes, but the speed limit increases to 40 MPH. The road forms the northwestern boundary of Burke Lake Park. It then intersects with the Fairfax County Parkway. SR 645 continues and widens from a two lane undivided road to a four lane divided highway, although the speed limit remains 40 MPH. It intersects with SR 643. SR 645 leaves Fairfax Station and enters Burke. It intersects with SR 652 where the speed limit decreases to 35 MPH. It passes by Lake Braddock Secondary School and then leaves Burke when it intersects with SR 638, and crosses into Springfield. SR 645 ends at SR 620 on the Springfield / Annandale border.
