 Route 28 crosses into Plainfield , Union County , where it becomes county @-@ maintained Front Street , resuming northeast through residential and industrial areas before entering a more residential environment . The road encounters a mix of residences and businesses before intersecting Plainfield Avenue , where County Route 620 continues northeast on West Front Street and Route 28 turns to the southeast onto Plainfield Avenue . The route crosses the Raritan Valley Line before turning northeast on a one @-@ way pair , with eastbound Route 28 following Fifth Street and westbound Route 28 following Fourth Street . County Route 603 continues southeast on Plainfield Avenue from this point . The one @-@ way pair , which has two lanes in each direction , passes residences before crossing County Route 531 ( Park Avenue ) in the commercial downtown area of Plainfield , coinciding with the directional change of Fourth Street and Fifth Street from west to east . At the intersection with Roosevelt Avenue , Route 28 becomes 25 mph ( 40 km / h ) two @-@ way East Fifth Street , with the name becoming South Avenue at the intersection with Richmond Street . The route passes through urban residential and industrial areas , with the Raritan Valley Line located a short distance to the northwest . The road extends through more suburban commercial areas before crossing County Route 611 ( Terrill Road ) , where Route 28 enters Fanwood as a 35 mph ( 56 km / h ) road . A short distance later , at the second intersection with Old South Avenue , the route becomes state @-@ maintained again and continues past businesses , intersecting County Route 655 ( Martine Avenue ) before it heads into residential neighborhoods and meets County Route 601 ( Laurel Place ) . 
