From being in the business for a long time, I can safely say that tailgating IS the biggest problem in the industry. As a security consultant, I am very mindful of the fact that access control is a process.... not a system. It makes no difference if the means to access the door is a key or card reader or fingerprint; if an authorized person lets someone else in, there is a problem.
During assessments, I try to give clients some idea of the extent of the problem, by actually staking outdoors and watching "access behavior". I've done this many times (probably should write a paper on it.... don't think many people study this behavior). I count the number of times I see the behavior and the number of people permitted access on a single authorized entry. The record (are you ready?).... is 63. Yes, 63 people on a single card read which occurred during the noon hour in a university res hall. Lack of policy, lack of enforcement, lack of buy-in and a major flaw in the building design (a central campus food service was located in the same building) were all seen as problems.
Now this problem can be addressed by "positive access controls" such as turnstiles, interlocked doors and revolvers with tailgate/piggybacking sensors, but the cost is significant (i.e. $30-40K per lane for optical turnstiles, $60-$100K per door for revolvers) and though proper design of the access scheme; but if you don't do something at other doors into the protected space as well, you may be wasting money. In a large building, to implement such controls and do it right, you could easily be talking about $500K or even $1M. So, I am very clear to my clients as to what these systems can offer.... lowering expectations if you will.
One place where I am seeing positive change, is in the K-12 schools. Teachers and the younger students are being convinced that letting someone in could be dangerous (the signs help as well). And the practice is tested (i.e. having a "stranger" employed by the security department attempt to get someone to let them in) in many schools. I don't think the same thing can be said for high school students however, who tend to be a bit more difficult to control and dictate policy to.
I would love to know how effective the fine process is working. I have seen fines used in colleges and universities, but its really difficult to judge whether or not the practice is effective (i.e. Does the practice actually prevent crime). I would also like to know how long the fine policy has been in effect.