My Biggest Question About the Post 9/11 Era
Put simply, why haven't the Al Qaeda backed suicide bombers launched more attacks on the U.S.? In the Wall Street Journal this week, Holman Jenkins answers this question in the context of the underwear bomber:
Considering the ease with which a suicide bomber could stroll into a Starbucks in any American city and kill a dozen people, you have to wonder at al Qaeda's obsession with targeting commercial airliners.
If 19 terrorists (the number who carried out the 9/11 attacks) each blew himself up at one- or two-week intervals in a shopping mall or a movie theater, America likely would become a seething nation of paranoid shut-ins. That it hasn't happened tells you something: Al Qaeda doesn't have a ready supply of competent suicide bombers, domestic or imported, to carry off serious attacks. That it continues to pour what little resources it can command into lame airliner attacks, like shoe bomber Richard Reid's failed attempt to blow himself up in 2001 and Umar Farouk Abdulmutallab's failed attempt on Christmas Day, tells you something else:
Al Qaeda may be incapacitated, but its leaders aren't dumb. So what if their hapless messengers only embarrass themselves and burn their legs? Al Qaeda can still count on the sizeable damage we will inflict on ourselves through an airport security apparatus that specializes in expensive political displays of barn-door closing that seldom have any real security payoff.
I don't think you need to go as far afield as a Starbucks to improve their approach. Just pick other critical transportation infrastructure, like tunnels and bridges, and you would inflict more damage and instill more fear than merely blowing up (as opposed to crashing) airplanes. I am led to the conclusion (like Jenkins) that they cannot yet infiltrate another group that can plan and execute another 9/11-style attack or that they are content to inflict primarily financial or symbolic damage. For some earlier thoughts on this issue, see this post.

"I am led to the conclusion
"I am led to the conclusion (like Jenkins) that they cannot yet infiltrate another group that can plan and execute another 9/11-style attack or that they are content to inflict primarily financial or symbolic damage."
So you have a metric that you use that measures fear? I believe that financial or symbolic damage would strike fear into the hearts of the public.
Simultaneous attacks maximize surprise
According to the 911 commission report, Bin Laden, Mohammed Atef, and Khalid Sheik Mohammed had hatched various ideas for a planes operation, but didn't have the pilots for it and didn't come up with the 3 of the pilots themselves. A lot of this planning was incredibly naive and grandiose, which suggests to me that they weren't going to pull it off without help.
The key to the 9/11 attack was a set of engineering students in Germany who became both westernized and radicalized and only after that presented themselves to Al Queda.
The notion that exposing foreigner to western culture will cause them to accept us better has a dangerous edge to it -- some will become vastly more dangerous agents against us. One of those engineering students was Mohammed Atta, who was essential to getting the muscle into the United States and keeping them off the radar of officials. I think it's significant that, before 9/11, Moussaoui was arrested and another potential hijacker was turned around upon arrival in the US because he had a one-way ticket and couldn't adequately describe what he would be doing here.
Another thing: At 8:25AM, air traffic control heard the first indications of hijackings in progress, and at 8:47AM, the American public at large saw the first crash. At 9:23AM, the pilots of flight 93 were advised to increase security, which didn't work, but by 9:57AM the passengers engaged in an assault for control of the plane. The M.O. of the hijackers worked for 1 hour and 32 minutes, after which the American public adapted.
Persistent attacks, as you suggest, might be pretty hard to accomplish a long way from home, in a strange land, with a public becoming increasingly suspicious of foreigners engaged in things that might lead up to such an attack.
Al-Qaida's problem must be
Al-Qaida's problem must be ideological. If they just caused a lot of freeway accidents, they might cause riots. I think they want people to take them seriously maybe.