I think this little ditty stems from a need to feel that you are in control. I€™ll even go as far to say that all panic attacks are rooted in this idea one way or another.
A bridge will only offer forward progress which is typically dictated by traffic conditions, and once a decision has been made to commit, you are obliged to proceed along with the rest. When the realization takes hold that parking or turning around from whence you came is not possible, you experience a loss of control and panic sets in. A realization, I might add, that may occur at the onset of your journey before you set foot in your car.
Possible helpful tips:
1) Don€™t drink coffee, tea, soda, or any other chemical containing caffeine before you drive. I have observed that caffeine can trigger panic attacks. This applies to nicotine, too.
2) Don€™t drive during heavy commute hours€”that starting and stopping business exacerbates the afore mentioned supposition.
3) Drive with a passenger. Many a time I€™ve felt secure in the idea that if I can€™t drive the other half of the bridge then my passenger can. Impractical perhaps, but knowing that it€™s possible helps.
Regarding the less than panicky return trip as mentioned in shedav6's thread, I have had a similar experience. One explanation might be that the first leg of the trip offers a destination wrought with unknown quantities. Again, events beyond your control. Whereas the return trip represents a more familiar, safer outcome€”home.
Just one guy€™s opinion.
To the mind that is still the whole universe surrenders. --Taoism