CE 372 - Fundamentals of Transportation Engineering
(2 April 2007)

Questions and Answers
1. Referring to the gap acceptance model, are vehicle movements included in the usefulness of gaps parameter. For example is the gap for a left hand turn requires a larger gap than a right hand turn? How is the usefulness of gaps determined?

2. Can you explain lag again? My understanding is that lag is when a vehicle arrives at a stop bar the lag time starts and the lag time ends when the next oncoming car passes. Is this a correct understanding?

3. The capacity model is measured on driver behavior for the critical gap time and follow up time. How are the street observations included in the model?

4. Can you provide a more clear definition of follow up time?

5. Why don't our DOT's use more roundabouts?

6. How is model applied to an actual intersection?

7. How are the Numbers used to make decisions on actual systems?

8. What happens if the follow up time varies among a group of vehicles entering a main corridor together?

9.I still don't understand the lag concept

10. Statistically, how much safer is a round-about than a 4-way stop? Which one is more expensive to construct?

11. If Poisson's relations are used for random variables, can the equations for traffic analysis derived from them be used in situations where most of the intersections are signalized, and traffic is not really random?

12. Why are roundabouts not used more often in the U.S. ?

13. How many lanes should a roundabout be in relation to car capacity?

14. How much should statistics and probability utilized in roundabout design? (i.e. versus other factors)

15. If the mainstream traffic is turning right, does it still affect the critical gap?

16. What are the typical ranges of capacities for minor streets and when do they become a problem?

17. Is the area, a round about, takes into consideration when choosing what type of intersection to put in?

18. Are round abouts being instaled more often in the US, after studies showed how accidents decreased?

19. How big of a circle diameter is required to implement more than one lane in the roundabout?

20. How is the size of a roundabout determined?

21. Does the follow-up time take into account the time it takes for the next vehicle to approach the stop bar and come to a complete stop and then make their decision?

22. How are the hierarchy of streams determined?

23. How do we establish an appropriate range for the capacity when analyzing an intersection?

24. How can engineers determine how effective a round-a-bout would be if it were to be installed?

25. In a situation where the major road has two lanes in each and the minor road traffic is a vehicle turning right, does traffic in the lanes the right turning vehicle contribute to gap analysis?

26. In general, does the decrease in gap availability during high flow offset the decrease in gap acceptance which would accompany a high volume flowing through a 2 way stop intersection? Or would flow through the intersection actually improve (or decrease) in a high flow situation?

27. How does one determine the follow-up time most accurately? Is it simply based off of observation? Would you average these observations or take the lowest or highest?

28. How does one go about finding the critical gap? I know that it is based on ratios of those accepted to those rejected, but I would need a little more guidance to actually perform this.

29. Roundabouts? I'm just generally curious. How can one justify putting one in as opposed to other types of unsignalized intersections?

30. I wasn't completely clear on the hierarchy of the streams. How was it prioritized?

31. What did all the terms mean in the Poisson's equation again? I didn't get all of them down.

32. . How much time or how many readings are needed to have a complete and accurate gap acceptance analysis?

33. Does the critical gap time vary for every kind of driving condition (ie. patient drivers after a long wait) or do you use the critical gap time of the standard wait and use it even when you know the drivers will be impatient?

34. When performing calculations based on having a signal at the intersection, do we need to adjust the base saturation flow for things such as grade, heavy vehicles, etc, or should we just neglect them and use 1900 for the two phases (including the EB left turn movement)? Also, do we assume that there is no left turns made off the major street onto the minor street during 2-way stop control heading northbound? Thanks a lot, hopefully these questions make sense.

35. 1.) For the 20 sec gap, with tc= 6sec and tf= 3.3 sec, you drew it up on the board with four cars being able to make it through the gap. I was wondering if it should really be five or if I'm making a mistake. 2.) What is the lamda in the equation you gave us with the statistics.

36. 1. What is follow up time? (Missed the definition when it was given) 2. When calculating critical gap how would you take into account traffic coming from both the left and right directions when turning left?

37. Do people use different gaps for different speeds? Why is the follow up gap less than the critical gap? Jared Hopkins

38. 1.) When getting the critical gap value from the fireld data do you take the average critical gap or the minimum or max? 2.) How do you know from the values you calculate whether it would be a good idea to switch to a signalized intersection?

39. How long is the average critical time for young? Old people? How accurate is the model, even though it assumes random flow, and flow isn't really random?

40. - Roundabout seem like they are productive, but are they as easy to put into production as a light signal? 2- What value do you use for lamda in the capacity equation? Thanks,

41. 1) How much field data do you need to collect in order to have reliable values for Tc and Tf? 2) Why have we, as Americans, chosen not to employ roundabouts?