Studies show zipper merging shortens traffic delays

With the return of the summer driving season, the zipper merge debate is back.

Anti-zipper mergers say those who do it are rude, while pro-zippers argue it makes traffic delays shorter when lanes are being reduced because of highway construction.

Zipper merging is when motorists use both merging lanes as long as possible, or at least to a defined merge area, and then alternate (like a zipper closing) into the single lane.

Some states’ departments of transportation have begun actively promoting zipper merging, including Nebraska, Minnesota, and Colorado.

The Colorado DOT (CDOT) has a Facebook post reading in part, “think you're being polite by merging early? Think again! When highways are congested, it actually pays to wait - and merge late!”

The CDOT says zipper merging is “safer, more courteous, and more efficient,” and cites a study it conducted showing the length of the merging line was reduced 50 percent by zipper merging.

However, the Iowa DOT is still undecided, tweeting late last month, “Our safety engineers are still studying the effectiveness. There isn't a consensus at this point on the zipper merge and when to use it and when not to, so we're not promoting it as a standard practice.”

The Federal Highway Administration (FHWA) website indicates that agency is also still on the fence, but notes studies conducted by states including Virginia and North Carolina show zipper merging eases traffic congestion: “While there is no definitive guidance for applying the zipper merge, these studies do offer insights into the potential benefits and applications of the zipper merge.”

The FHWA website says North Carolina researchers “have highlighted anecdotal findings such as a zipper merge site in Michigan where the congestion area was reduced from six miles to three miles.”