I was reading an article the other day about how the current system gets easily overloaded, yet airspace is underutilized due to the use of standard restrictions. Someone unfamiliar with the day to day workings of the NAS concluded that a simple way to increase capacity and efficiency would be to simply allow URET (and other software not yet brainstormed into reality) to allow us to separate the actual airplanes, not just use a standard restriction which is designed to miss chunks of airspace where traffic tends to be most often.
Let me back up. Each sector is designed to do certain jobs (descend planes into other sectors, sequence traffic flows, provide approach services to small airports not within approach controls, etc). Each sector is then run by one to three controllers at a time, based on traffic load. In order to successfully accomplish the mission of the sector, limits are put upon it to decrease its complexity so that safety isn't compromised. There is only so much a controller can focus on at once. Most sectors don't have more than 5 or 6 main flows. Once a sector is deemed too complex or busy, a restriction is placed on the sector that feeds traffic to the sector in distress. Many of these restrictions are in place from 6am-11pm every day to establish a standardized flow so controllers and pilots can get used to them. Certain traffic landing at certain airports are forced out of some sectors and into others, so the complexity level in the busier sectors remains manageable. While it may not be the most efficient use of airspace, it IS the most orderly and the safer way to run things. The FAA has been having a hard time grasping that concept lately, but I'll let that discussion continue elsewhere.
Allow me to refer back to my analogy of working air traffic to not getting into an accident on the Interstate. Each lane is an available altitude, with speeds independent from the other lanes around it. Often times the best way to miss other cars is simply to change lanes. However, lets say you're in the right lane driving 50 miles per hour. The cars in the lane immediately to your left are driving 60 miles per hour, and to the left of that lane the cars are driving 75 miles per hour. Up ahead, the traffic is almost stopped due to an exit. The exit leads to a congested road that can't handle the traffic, and its backing up into your lane. So you move over one lane to the left. All of a sudden, you're the slow one. The cars behind you want to keep driving 60, so they move over one lane to the left, and now everyone is driving slower than they'd like, all because of some congestion that should only be affecting the right lane.
Adding another lane of through traffic may be a solution to the problem, but the traffic in the right hand lane still will affect all the others in a negative way. Often times, busy exits will exit sooner and create a frontage type road that is separate from the main highway. This separate exit road is similar to splitting off the sectors by altitude. You may exit a little sooner than you'd like, and likewise, descend a little sooner than desired for fuel efficiency, but it eliminates the traffic backups effecting the main flow of traffic.
The air traffic controller at the lower sector can worry about all that congestion running into the major airports, and the higher sector can worry about all the fast movers up at attitude.
If the system was controlled solely on the basis of efficiency, planes would do what their on board navigational computers told them to do. This mainly includes remaining at cruise altitude for as long as possible, and then dive bombing using idle thrust into the airport. While we cannot allow straight descent directly onto final approach, we do allow pilots to descend at their discretion to meet crossing restrictions at certain points along their route. These crossing restrictions are generally at the boundary with the next sector, or at the outer edge of the approach control. As controllers, we don't care how the aircraft descend, only they are level at the altitude issued at the location specified. Often times, aircraft will remain level until the last moment, and then descend at a quite impressive rate to make the restriction. Rates of around 4000 feet per minute are not uncommon. This is ironic, because pilots are SUPPOSED to descend at a "normal rate" in order to meet a restriction. A normal rate would be the rate if we simply said "descend and maintain xxxx". This is generally 1000-1500 feet per minute for jets. However, it is become expected that pilots will hang it up at altitude and then dive it in to the restriction.
I believe that in the future this desire to use high rates of descent into airports will serve as a basis for new airspace design. There is only one problem with this. It doesn't bode well for the lower and slower aircraft that will get in the way. Imagine a sports car in the far left lane suddenly cutting across four lanes of traffic to make the exit. Ok, now imagine a bus doing it! The only way that can be safe is if there is no other traffic around, but that would defeat the goal of increasing capacity, now, wouldn't it!?
We can't make the NAS efficient for everyone all the time, but we can slightly inconvenience a few planes here and there to make sure its SAFE and ORDERLY for everyone all the time. No sense saving gas if you never make it there.