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| This is from Don Sims, written to a South Cove
Resident on Sept 9th, 2005. Subject: RE: Sammamish Roundabout I hope this answers your questions. Please contact me directly should you have any further questions or comments. 1) What was the original problem at this intersection that WSDOT needed to address? Answer: The pavement in the roundabout (circulating roadway) had failed so severely that it could not simply be overlaid with another lift of asphalt, but rather needed to be completely dug up and replaced including portions of the sub-base material below it. In addition, one of the quadrants of the circulating roadway was not draining properly causing water to pool on the circulating roadway, which could also be addressed by completely replacing the pavement. 2) So, why didn't WSDOT just replacement the pavement and be done with it - leaving the geometry and the existing right turn bypass lanes in place? Answer: The first roundabout was built in the mid 90's to address a severe delay problem for the east leg of the intersection which at the time was only stop sign controlled. Before the original roundabout was constructed it was becoming nearly impossible to find a gap in the north-south traffic to turn left from the east leg of WLSP to access I-90. The original roundabout was WSDOT's first roundabout in the state. There are now approximately 68 modern roundabouts in the Washington State alone. The first roundabout was designed and constructed with very limited knowledge, guidelines, and standards regarding modern roundabouts because at the time it was a new emerging intersection control device in the United States. Since that time (in the last 10 years) WSDOT and the Federal Highway Administration (FHWA) have gained considerable knowledge and guidance regarding modern roundabout design. During the design process for the pavement replacement many design/geometric features of the roundabout were identified as non-standard (not meeting current guidelines) and furthermore, these particular design elements were identified by recent national design guidance as elements that "rob capacity" from a roundabout. 3) What were these non-standard elements and how did they "rob capacity"? Answer: 1) The old roundabout had a small-inscribed diameter. The small size of the circle caused drivers to hesitate when entering the roundabout when there were any cars circulating in the roundabout. Larger roundabouts have greater capacity and can serve more vehicles on all the legs because speeds in the circulating roadway are faster and because more cars can travel in the circulating roadway at the same time. The old roundabout did not meet the minimum current state and federal size requirement for a single lane roundabout. 2) The old approach legs did not provide proper deflection into the circulating roadway. In other words, the old approach legs did not smoothly direct vehicles into the circulating roadway. Rather vehicles had to make a fairly abrupt right turn as they entered the roundabout vs. the new roundabout, which aligns vehicles directly into the circulating roadway. This old design element also "robbed capacity" for all legs of the roundabout. 4) How did the design proceed? Answer: WSDOT engineers met with the Federal Highway Administration because WSDOT was required by FHWA to document all these non-standard features of the old roundabout and determine whether or not bringing them up to current standards was appropriate. It was decided by both Federal and the State agencies that because WSDOT was investing a sizeable amount of money to completely reconstruct the pavement; it should also (for a relatively minor additional amount of funding) be brought up to current design standards. The major justification for this decision was to improve capacity and traffic operations at this intersection by enlarging the size of the roundabout. Early in the design effort and because there was no funding allocated for additional right-of-way, it was identified that the only way the size of the roundabout could be increased and still keep the whole intersection within public right of way was to eliminate the two right turn slip lanes (or bypass lanes) on the east side of the intersection. WSDOT traffic engineers were reluctant to do this because of the obvious additional capacity these right turn slip lanes provide. However, they also realized a potential increase in capacity that could be created by the larger roundabout. WSDOT traffic engineers then utilized two separate independent traffic modeling software tools to model the two scenarios. The first being the old smaller roundabout configuration with the slip lanes, and the second being the new larger roundabout without the slip lanes. Both of these models were run completely independent of one another. One of these software tools was developed in England and the other in Australia where thousands and thousands of roundabouts are in operation. Much to our surprise, both independent tools confirmed the 2nd alternative with the larger diameter and no-slip lanes provided higher capacity and less delay for sum or average of all vehicles using this intersection. 5) How did the two models predict queuing, delay and overall capacity for the individual legs? Answer: During the evening commute, the model showed increased capacity and reduced delays and queuing on all legs of the intersection - particularly the extremely heavy southbound movement in the p.m. peak that was regularly backing up over a mile every evening. During the morning commute, the model predicted increased capacity and reduced delays for the north and south legs of the intersection. It did show a minor reduction in capacity and increased delay on the east leg of the intersection for about a half hour in the morning between 8 and 8:30a.m., however, the queues were predicted to be less than 2/10 of a miles and cause about 3 minutes of delay. 6) Is WSDOT favoring some legs of the intersection over others? Answer: Absolutely not! WSDOT attempted to make a decision based on the overall capacity, delay, and queuing for all vehicles entering the intersection. We refer to this as average intersection delay. Plain and simple, the model predicted less average delay for all users combined using the intersection in both the morning and the evening commutes. Furthermore, it was hoped that many of the commuters that saw an increase delay on the east leg in the morning would realize a greater decreased delay in the evening on the north leg. For parents taking children to school from the east, the predicted improvement on the north leg in the afternoon would not be realized. 7) It is inconceivable to me how traffic engineers could believe a model that predicts removing the right turn slip lanes would result in better capacity even with the potential modest gains in capacity realized by the larger diameter and the improved entry angles into the roundabout. I am not a traffic engineer and I could have told you that things would be worse without the slip lanes? Answer: Traffic that enters a roundabout and then immediately turns right, creates a gap in the otherwise through movement for traffic attempting to enter the roundabout in the next quadrant. As an example, taking out the northbound to eastbound right turn slip lane forces drivers wanting to make this movement to enter the roundabout. Every time a vehicle makes this right turn from within the roundabout, it creates a gap for westbound traffic in the morning. With the slip lane put back in the project, westbound vehicles would otherwise have to find a gap in a more constant, continual stream of northbound thru traffic traveling from Lakemont to Redmond. 8) Are there any other factors that contribute to congestion that the model did not take into account: 1) There is a pedestrian activated signal approximately 300 feet north of the roundabout. WSDOT engineers confirmed with the City of Bellevue engineers that this signal was malfunctioning on the 1st day of school. Essentially, it was not giving the proper amount of minimum green time to north and southbound vehicles in between pedestrian calls. This problem has since been corrected. Even with the signal working properly, many times during the morning northbound queues from the pedestrian signal will back up into the roundabout and cause complete gridlock in the roundabout until the queues from the signal dissipate. 2) Also during the 1st week of school westbound traffic out on I-90 between SR-900 and the WLSP interchange was practically at a standstill. When I-90 is not flowing smoothly in the morning additional traffic diverts to the westbound leg of WLSP in the morning. Traffic counts reveal that as many as 200 additional trips can divert to WLSP during the morning commute when traffic on I-90 is worse than normal. These additional trips are on top of the average of more than 300 vehicles per hours that use this bypass regularly. 9) Enough explaining and excuses, what is WSDOT planning on doing to address the westbound approach problem in the morning? WSDOT had plans to do follow-up travel time and queue studies for this intersection during the next couple weeks. We still plan to do that to document what improvements if any were realized on the other legs of this intersection during both morning and the evening commute times. However, we have concluded that we don't have to do an after study to know that the westbound movement in the morning is much worse than the models predicted and that the current delay in unacceptable to not only the users of the facility, but also to WSDOT engineers. WSDOT design engineers have already begun plans to reinstate the westbound to northbound right turn slip lane in order to provide relief for the morning congestion. We here at WSDOT also acknowledge that a series of honest mistakes and errors will result in a prolonged construction contract and additional delay for drivers on the west leg of this intersection this fall. However, we do not envision any significant additional costs to this project over what would have been realized had we done the correct thing from the beginning and made the roundabout larger while retaining the right turn slip lanes. At this time we do not have a schedule or a cost estimate for the westbound to northbound right turn slip lane, however, we will provide that information on our website when that information becomes available. At this time we do not plan to reinstate the northbound to eastbound right turn slip lane as - to date - we have not seen worse congestion on this leg. In addition, reinstating that right turn slip lane could have a negative impact on the westbound leg of the intersection. Don Sims, PE WSDOT Traffic Engineer King County/NW Region
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