I made a rough measurement off the map awhile ago and it came out to be around 700 feet.
Can the towers hit folks on the bridges?
I assume you mean "hit folks with lightning on the bridges". There is not a definitive answer, but the general consensus of the forum is that that the bridges are beyond the range of the lightning towers. I did some calculations long ago and determined that if the lightning can originate from any part of the tower then each bridge is within the range of at least one towers. However, if you assume the lightning originates from the tip of the tower then no bridge is within the area of effect.
There are pros and cons to having the lightning originate at the tip of each tower as opposed to having it come out from wherever is needed. Specifics will depend on the individual DM.
At what distance from the Outer Fane do the people in the towers see those on the bridges?
This depends on many factors including but not limited to: weather, time of day, and use of concealment. This also depends on whether you judge that the bridges are covered bridges or open to the sky. My own interpretation is that the bridges are mostly open to the sky with archways located at the supports at regular intervals along its length. There has been other feeling that the bridges are covered.
On a clear day with an open bridge then I'd say that the Spider Eater Riders see anyone on the bridge from terminus to terminus. The visibility would be reduced by weather (fog, rain, snow, etc, ...) or whether bridge walkers attempt a crossing during the day, moonless night or moonlit night. The towers grant the Spider Eater Riders (SER) an enchancement bonus of +10 to spot checks, as well as see invisible and darkvision out to 120 feet. However, if you rule that the bridges are out of range of the lightning (150 feet) then bridge walkers are out of the range of the SER's see invisible and darkvision. So, magical concealment for a bridge walker is a possibility to avoid detection by the SER.
What's the normal routine for intruders on the bridges? Do the towers not care, figuring the ground forces can take care of it if need be?
All in all, the general thought is that the towers and its SER are a defensive measure to deal with flyers. SER who spot people on the bridge alert the sorcerer who then alerts Hedrack. Depending on how notorious your players have become, the SER may have descriptions of your PCs and report that the enemy is at the gate. Even if they are not recognized, Hedrack would be extremely suspicious of a group of strangers coming unbeckoned to the Outer Fane and put his forces on alert.
By the time your players are high enough level to attempt an attack on the Outer Fanes the SER are likely not a real threat anymore, more like an annoyance. The SER are likely ordered to maintain vigilance against flyers, or at best try to draw bridge walkers into flying into the range of the tower's lightning, or attack anyone retreating from the Outer Fane.
Eltern, Trithereon, and Infiniti2000 contributed to this post.
