I believe that not only should the vehicles be at walking speed, but the airplanes should taxi slow as well. There are times that I see an airplane taxi speed in the 30mph range, way to fast. Many times when I taxi my airplane there are vehicles along the path. I generally stop and let them go by, most the time I have to motion them to go. I'd rather take a little extra time getting to the runway than kissing a vehicle. I'm a tail dragger and everything on the right side is basically unseen and weaving back and forth is not an option either in some areas, so a slow taxi is the ticket.
I remember just a couple weeks ago there was an aircraft not being recieved on radio, it's possible that he was hearing. I was catching on to this because the tower heard the scratching on the radio, so did I, but this guy never got clearance to enter classD airspace. We have two parrallel runways and at the time the right runway was closed with a truck on the runway, possibly replacing some lights, the truck was about 1000 ft down the runway. I was cleared for the left runway and at the time in the downwind. I looked over like I always do and check for traffic on the final approach. To my supprise I saw an airplane on final, there was no traffic cleared to land but me. Then I realised that he was landing on the right runway. I got on the radio and told the controller that there is a plane about to land on the runway and I see a truck on that runway. The controlled completely freaked and tried contacting the aircraft, no response. I noticed just before he touched down that he did a go around, so me making a radio call did prevent an accident, apparently he could hear. He was told to do a left turn and land on the left runway, which he did. Everything worked out on this one, but it was close, even in a controlled field things like this can happen.