It is obvious that TAMC and Caltrans have only listed what they believe to be the positives of roundabouts and have not listed any of the recognized negatives of them. I can only assume that they do not want the public to know their limitations so they continue to believe that roundabouts are perfect and fix everything. Some people believe that the roundabout at Pebble Beach/Holman Hwy has improved the traffic flow, so putting 9 more of them on Hwy 68 would solve the congestion there. The roundabout at Pebble Beach is a hybrid design with a significant amount, if not the majority, of the traffic bypassing the 2-lane roundabout without even entering it. This is not the same as the 8 single lane roundabouts and 1 double lane roundabout being proposed for the 8 miles on 68 where all of the traffic must enter the roundabout on the main line. As was mentioned earlier, there are no examples anywhere of a successful reduction of peak commute congestion by putting 9, single lane roundabouts in an 8-mile stretch of a busy highway. It is a bit ironic that Caltrans proposed the 4 roundabouts on Hwy 126 in Ventura County to “slow the traffic down” and now they are proposing adding 9 roundabouts to Hwy 68 to “make the traffic flow faster.”
Those of us who live along this stretch understand that the AM peak commute congestion is mainly cause by Westbound 68 merging from 2 lanes to 1 lane just after Portola Rd and not primarily due to the signal lights along 68. Putting roundabouts in will not improve things. This was even recognized in the August 2013 TOAR (Traffic Operations Analysis Report) that Caltrans produced that stated on page 5 “the AM Peak Hour performance of the proposed Alternative 1 (Roundabouts) is marginally better than the No-Build” which means they admit that spending millions will not significantly improve the current AM peak commute.
There are a number of limitations or negatives for roundabouts according to Mike Spack, a nationally recognized expert on roundabouts, a Professional Transportation Operations Engineer and past president of the North Central Section of the Institute of Transportation Engineers. He described them in a recent Webinar “Are Roundabouts a Silver Bullet to Traffic Issues?”
*Roundabouts cannot be customized like signals or upgraded after being built
*Roundabouts are not as efficient as signals for emergency response vehicles.
*Roundabouts are not recommended for situations where the traffic in not balanced in flow as when the majority of traffic is flowing along one main line.
*Roundabouts cannot give priority to the mainline traffic like signals can.
*When roundabouts get to gridlock as capacity is reached, it takes significant time to undo it.
*Minor traffic from the left can take over the mainline movement in a roundabout.
*Traffic is slow going into and coming out of roundabouts.
*Roundabouts are a problem for sight impaired pedestrians.”
It is clear that a number of these negative conditions are present in this stretch of Hwy 68, yet TAMC and Caltrans just ignore them and hide them from the public. Why?
Roundabout Capacity
All roundabouts have a capacity limit and when that is reached, the flow essentially stops. When that happens, traffic signals are typically installed to regulate access to the roundabout as they did in Portland, Oregon. 8 of the 9 proposed roundabouts are single lane and thus have a capacity of 1800 vehicles per hour according to the US Department of Transportation Operational Guide, page 87. “Circulating flow should not exceed 1800 veh/hr at any point in a single-lane roundabout.” Caltrans has not really addressed this fact for current and future peak commute numbers. In calculating the flow, trucks count as 1 ½ and semi trucks as 2 vehicles. As roundabouts start to reach capacity, the gaps between vehicles get smaller and smaller, making it more difficult to enter, especially for the timid or inexperienced driver, which results in the backup of traffic trying to enter the roundabout. Once constructed, roundabouts cannot modify their operation, like adaptive signal controls can, since they are a physical structure.
Brilliant points and position, I have driven that stretch and can’t imagine the torture of building such a labyrinth, ridiculous. Your AI solution is the only sensible path forward. I hope everyone does their best to prevent a roundabout nightmare.
Good points. The AI solution seems a very reasonable alternative – without even considering that it will cost much less and can be altered if needed. Roundabouts are extremely expensive with no option to make corrections after the fact. Plus AI could be installed in much less time with much less disruption to traffic during the installation process. I can’t even imagine the delays and disruptions to traffic flow that roundabout construction would cause during the construction period. Roundabouts should NOT be constructed on Hwy 68.