Are Beliefs and Attitudes a proxy for Behavior?

Beliefs and Attitudes are a proxy for Behavior
Ferguson, UK
Yes. the above theory of planned behavior is really all about the link between attitudes and behavior. It was proposed as an extension of the (previous) theory of reasoned action, also by Ajzen.
According to the Theory of Reasoned Action, if people evaluate the suggested behavior as positive (Attitude), and if they think their significant others wanted them to perform the behavior (Subjective Norm), this results in a higher intention (motivation) and they are more likely to do so. A high correlation (!) of attitudes and subjective norms to behavioral intention, and subsequently to behavior has been confirmed in many studies.
But a counter argument against the high relationship between behavioral intention and actual behavior was proposed as results of some studies show that behavioral intention not always (!) leads to actual behavior because of circumstantial limitations. Behavioral intention cannot be the exclusive determinant of behavior when an individual’s control over the behavior is incomplete. For this situation, Ajzen introduced the Theory of Planned Behavior by adding a new component, “Perceived Behavioral Control”, an individual's perceived ease or difficulty of performing the particular behavior.

Attitudes, Subjective, Perceived!
Jaap de Jonge, Editor, Netherlands
Karla, a policy change alone in this type of circumstances (allowing visitors to come and go as they please with no restrictions of time per visit) is unlikely to change the behavior. What matters (according to Ajzen) is:
- if the attitude of the person towards the behavior changed, or
- if the subjective (the individual’s perception) norms changed, or
- if the perceived behavioral control (perceived ease or difficulty of performing the particular behavior) changed.

Beliefs Precede the Behavior
Clinton, USA
By using "proxy" I assume you mean "a replacement for..." If this is the case, then no. Beliefs and attitudes are not a proxy but actually precede a behavior.
People bring a combination of values, beliefs, objectives, principals, and perspectives to the actions and decisions that they perform. The actions and decisions are based on these underlying principles, etc.
The behavior is the perception of the actions and decisions made over some period of time or some number of action/decision points. This separation is one reason why you can feel powerfully strong about about a subject, but unfortunately do nothing about it.
In this example, your beliefs and attitudes exist, but perhaps a stronger belief or attitude (or fear) prevents a desired behavior.

Thought Begets Behavior
Tallman, USA
This sounds like psycho-babble.
We behave like we think. If we want to behave differently, we have to think differently. That's the reality of behavior.

Are Beliefs and Attitudes a proxy for Behavior??
Phuong Nguyen, Vietnam
Very brief view: The direct antecedents of behaviors are attitude toward the behavior, subjective norms about the behavior and the perceived behavioral control on the behavior. The corresponding believes are the antecedents of the above 3 constructs.
The behavior under research needs to meet certain criteria (Target, Action, Context, and Time). Unless you define the behavior you want to study clearly, using the TPB is useless.
Furthermore, what is the objective of your planned study? Then you can use the TPB to study the antecedents of your defined behavior.
The next step is to investigate the intervention using the TPB. We need to intensively review the theory and set up very clear objectives of the study. No "imply" can be concluded in general.
Useful ref: Jillian J Francis et al, Constructing Questionnaires Based On The Theory Of Planned Behaviour, A Manual For Health Services Researchers, Centre For Health Services Research, University Of Newcastle, United Kingdom, 2004

Are Beliefs a Proxy for Behaviour
John Jenkins, UK
No, measuring beliefs and attitudes, by whatever means, may give you a measure of those 'abstract' constructs.
Actual behaviour is a very different thing: what a person actually does in the real world will be influenced by many other variables than simply belief and attitude.
The TPB theory does not represent a 'closed' system; there are many possible unrepresented variables that might affect the actual behaviour.