Joined: Nov 2011
Posts: 162
pros and cons
12/10/2015 at 9:31 PM
Let's be clear. There is a purpose for zipper merging. This just isn't the right place for it. It's not a 'Brandon' thing, it's the specifics of this situation. The zipper merge, as already said, may make movement safer if EVERYONE follows it, but it doesn't move cars through faster. Imagine a zipper (I know, don't need to say it): If all the cars in both lanes were spaced safely for normal traffic flow, then the spaces between would be sufficient for cars to merge - like a zipper. BUT just like a zipper, to maintain safe following distance, everyone in the new single lane would have to slow a bit to open up safe following distance again. Just like how the teeth on a zipper are all very close together after the zipper is closed...
As someone said, this works when traffic is already moving slower approaching the merge (so distances can be closer) then speeds up after the merge, so vehicles naturally space out a little as flow gets going. But in this situation, everyone is either already going at normal traffic speed (if the Rosser light is green) or accelerating from the light at Rosser AND merging at the same time. Then slowing down to keep safe following distance. Sure, it will work, but it categorically will not get more vehicles over the bridge any faster. As someone said, 20 cars in one lane would go over in about the same time as 10 cars each merging from 2 lanes. Delay for lights notwithstanding. So as the City press release says, all they're addressing is queue length, not overall volume or rate of throughput.
Oh, and to whoever asked if I'm a traffic engineer - no, but the City is full of examples of bad traffic management that does not improve efficiency of flow. The eastbound lane designation on Rosser at 18th can only have one possible outcome: some cars have to wait at the light longer than necessary. No flow is improved with the current lane designation. Left turning traffic could not possibly be in conflict with oncoming traffic because east of 18th is one way. So nobody going straight could possibly be delayed by left turning traffic in front of them. But right turning traffic might get stuck behind straight through traffic waiting at the red, even if the southbound lane of 18th is clear. So for reasons like this, I suggest that our traffic gurus could do better. 2 lanes northbound and 1 southbound for Daly Bridge would go a very long way to reducing the problem we all recognize here - whatever we think of zipper merging.