The phrase "are a frequent sight" is correct and usable in written English.
It can be used to describe something that is commonly observed or seen in a particular context or location.
Example: "In the spring, blooming cherry blossom trees are a frequent sight in the park."
Alternatives: "are commonly seen" or "are often observed".
Lorries carrying troops are a frequent sight in Natal.
White plates are a frequent sight throughout: performers spin them, balanced on their edges, like tops.
Russians are a frequent sight in Paris, on the C d'Azur and at high-end ski resorts.
We saw them in the open fields, skulking beneath the hedges, and of course by the roadsides: squashed pheasants are a frequent sight all over East Anglia.
Jerseys bearing the number of Ramirez, who grew up in the neighborhood, hang in its sports shops, and Red Sox caps are a frequent sight in its restaurants and cafes.
Solar cells adorn the roofs of many homes and warehouses across Germany, while the bright white blades of wind turbines are a frequent sight against the sky in Spain.
It has been a frequent sight and sound at training camp, which has been a place of revival for Maynard after an indifferent season a year ago.
Santiago Peralta, 48, who works at Apollo Radio Dispatch, a car-service business at that corner, said that the prostitutes were a frequent sight, day and night.
Those lunches usually include friends like Helen Hartley du Pont, whose long platinum hair is a frequent sight at the seat of honor at many a charitable event.
The image of longing, in the form of reaching arms, was a frequent sight, but the dead-serious faces of the dancers made such actions mundane.
Cars bearing the banners of Fatah, the Popular Front for the Liberation of Palestine and other groups were a frequent sight around the city center, organizing rallies and mobilizing their supporters.
