It begins at NY 27A (Montauk Highway) in Amityville and heads north to the intersection of Joyce Avenue in East Farmingdale.

On the northwest corner of this interchange is the beginning of St. Johns Catholic Cemetery, where the road widens to accommodate a center left-turn lane.

The northern border of the cemetery is along Edison Avenue, located near the intersection with Little East Neck Road in Wyandanch.

County Route 3 begins as Wellwood Avenue on the Lindenhurst village line at Perry Street in North Lindenhurst, which quickly encounters a cloverleaf interchange with NY 27, where the road briefly becomes a four-lane divided highway until the intersection of Spieglehagen Street and becomes a four-lane undivided highway.

As the road intersects an at-grade crossing with the LIRR Central Branch and becomes a divided highway again north of Gear Avenue as it approaches NY 109.

North of NY 109, CR 3 runs through the center of the Cemetery zone of southwestern Suffolk County along the following burial grounds; Before leaving the cemetery zone, CR 3 becomes a divided highway again and passes by the headquarters of Newsday, which also includes the former right-of-way for the Long Island Motor Parkway.

From there, CR 3 takes a northwesterly turn and crosses over the Long Island Expressway east of exit 49, where it becomes a four-lane highway again approaching NY 110.

Though the first two sites consist of churches listed on the National Register of Historic Places, the rest of the area is standard residential suburbia.

The designation for CR 4 ends at the intersection at Clay Pitts Road in Commack, but the roadway continues northward towards NY 25A in Fort Salonga.

It runs parallel to a former section of the original Long Island Motor Parkway, beginning at an intersection with New York State Route 110 and heading east.

From the 1950s into the 1980s, there were proposals by the New York State Department of Transportation to build the Babylon–Northport Expressway within the vicinity of the west side of Elwood Road, with interchanges at both ends that included ramps utilizing CR 10.

County Route 11, commonly referred to as Pulaski Road, runs west to east between Cold Spring Harbor and Kings Park.

CR 11 provides the closest access to the Port Jefferson Branch of the Long Island Rail Road, which runs roughly parallel to the tracks throughout its span in northwestern Suffolk County.

The Babylon Branch finally runs along the north side of Oak Street between Garfield and Strong Avenues, where it crosses the Copiague–Lindenhurst village line.

However, this name is also shared by a street running parallel to it on the north side of the Babylon Branch maintained by the village.

The roadway east of the terminus reaches NY 109, where it becomes Trolley Line Road, named for the former Babylon Railroad Company, which used the street as part of the route for its streetcar.

[15] When the nearby Long Island Expressway was constructed, the southbound off-ramp was eliminated, but the on-ramp was left intact.

[14] The route continues on, descending Crooked Hill to reach the Long Island Expressway and its service roads.

At the intersection of Reil Place all southbound traffic along CR  13 shifts to a parallel street called Clinton Avenue.

Lake Shore Drive and Portion Road used to be part of CR 19 from the Smithtown–Brookhaven town line to Waverly Avenue in Farmingville.

This section kept its given name while NY 111 was renamed "Wheeler Road" in order to distinguish the state route from CR 17.

Still with four lanes, the road passes near Saint Joseph's College's Patchogue branch campus before crossing NY 27 (Sunrise Highway).

CR 20 was shared with New York State Bicycle Route 25 east of Belle Terre Road in Port Jefferson,[31] and ran through most of the Miller Place Historic District.

Sheep Pasture Road is an alternate route in East Setauket connecting Stony Brook to Port Jefferson Station.

Its name at one time meant that there were sheep and pastures around in abundance but currently it is a heavily travelled artery and is almost all residential.

This project included a bridge over the Long Island Rail Road Main Line as well as some local streets, one of which leads directly to Yaphank Station.

CR 21 was added to the Suffolk County highway system on January 27, 1930, on various roads dating back to colonial times.

County Route 21A was a suffixed alternate of CR 21 created during the construction of exit 57 along Sunrise Highway, which interrupted original sections of both Horse Block Road and Yaphank Avenue, The new section was officially part of Horse Block Road, but was eventually deleted, when it was integrated into part of CR 16.

CR 24, designated Longwood Road, also served Cathedral Pines County Park at its western terminus.

The projects were to stretch from exit 69 on the Long Island Expressway (I-495) southward to the Montauk Highway and the western terminus of the Moriches Bypass, designated CR 98.