Why did they screw up my freeway when they put in a diamond lane?I've seen this referenced in the local RoadShow columns. Highway 280 going south backs up behind Magdelena Road exit in Los Altos Hills, just past Foothill College. This is kind of amazing since 1/4 mile past Magdelena road there is an extra inserted diamond lane? Why would adding a lane back up traffic? The answer it doesn't. The backup occurs because the the last two exits both shave an entire lane off of the freeway.
It's straightforward to do the math that these two exits become the bottlenecks when the freeway is near to capacity load elsewhere. Four lanes of traffic cruise along at 65 mph, bumper to bumper. Unless there was an an entire lane of exiting cars at full speed exiting the freeway (which there isn't, this is pretty much the middle of nowhere) this will obviously be a bottleneck and limit the throughput of the freeway. It's a simple queueing problem. I guess they don't teach queueing to traffic engineers.
Ah, but you say they do teach queueing to traffic engineers. So what are they trying to optimize when they design a freeway system? You've got a couple of choices. You can minimize travel time at a certain time of day or you could maximize carrying capacity. This design does neither. Are they purposely slowing down traffic before the diamond lane starts? Seems misguided, but there doesn't appear to be any other choice.
So Let's look at throughput predicted. One lane goes away to an exit, cutting the flow from four to three lanes. Then the onramp merges into those three lanes, then 1/4 mile down the road you get a fouth diamond lane back. What a nightmare.
- Arrival Rate = = 2200 cars per hour
- Departure Rate = = 1100 cars per hour