Roundabouts can be a great improvement to stop lights or four-way stop signs, eliminating the need to stop and greatly reducing the chances of a head-on collision. Roundabouts typically replace two lanes crossing each other with one shared lane. They are most successful when:
- Traffic from all directions has clear visibility to “the competition”, cars entering from either side;
- There is little “mixed” traffic (either mostly cars or mostly bikes);
- Pedestrian traffic is discouraged.
Here are a couple notable examples:
This is Highway 89, just south of Highway 80. Cars have plenty of space to turn in the tighter inner circle, where it is clear that they have the right of way. Traffic separators force cars entering to come in at an obliqueangle, ensuring that at worst they will be pushed to the outside.
This is Castro Street in Mountain View. Note the very wide islands, where a pedestrian can actually feel protected from traffic. Note how small the actual roundabout is, in relation to the width of the streets. The sidewalks do NOT force bikes into the street, and indeed are lower and rounder. Bikes are NOT encouraged to join the flow of traffic, but are indeed safe when they continue along the road because there is room for a bike and car to share the space.
Here is East Meadow at Ross before the changes. Wide lanes, bikes could go straight through, pedestrians had clearly marked (straight) crosswalks.
Here is Ross Road after … akin to three rounds of bad plastic surgery.
- First the roundabout, so wide that a fire truck has to slow to one mile per hour to get around it;
- Then the “bulb-out” extensions to the sidewalks, to ensure that bikes cannot simply avoid traffic; and
- Last, the lane separators to ensure that cars CANNOT make room for the bike, since the driver can barely fit the car itself through the chute at an angle.
And sadly, the predictable result is that bikes are getting squeezed into the bulbouts. Video of a little girl getting squeezed into traffic is in another post on this blog.
Here are two pictures from March 11th. The bulbouts are already dinged up where vehicles have struck the raised curbs. There is a growing pile of hubcap debris on the southeast corner of the intersection where cars cannot navigate through the fifteen-foot wide lanes without striking the curb.
A proponent of the bike boulevard shared this link, which is informative.
In my effort to continue to learn more about roundabouts, and about what is being proposed in many more Palo Alto intersections, I welcome this type of education. Here’s what I just posted on NextDoor based on this article:
Below are direct quotes from the article. Clearly others know more than I do about roundabouts, but the analyses cited are based on much bigger intersections without foliage (and not a single example given in the 26 pages is in a residential neighborhood). Quotes:
- “mini-roundabouts are typically not suitable for use on higher-volume (greater than 6,000 AADT) state routes.”
- “a compact roundabout …has a completely mountable center island”
- “All curbing within a roundabout should be rolled”
- “The length of the splitter island will vary (typical lengths: 30 ft to 350 ft)”
- “Try to maximize the splitter island width adjacent to the circulating roadway”
- “The Inscribed Circle Diameter (ICD), that is, the overall outside diameter of a roundabout”
- “The offset left alignment is preferred.”
- “angle values … usually between 20 and 40 degrees.”
- intersection sight distance should be 100 feet for 15mph traffic (p 19)
- “Design the splitter island pass through a minimum of 5 feet wide, or the width of the sidewalk, whichever is greater.” (ours are 3 feet)
Taken together, here are a few conclusions (based on THESE FACTS alone):
- What was built is the smallest option, a “mini-roundabout” with an inscribed circle diameter of 64 feet in the narrow direction (and 15-foot lanes on the narrow axis);
- What was built is NOT suitable in this intersection, which clearly has more than 6,000 average annual daily traffic (with a peak of close to 1,000 in each of two twenty-minute windows).
- The raised curbs violate any guidelines I can find anywhere (watch the video of a young girl on a bike forced by the raised curb into traffic here): https://wordpress.com/view/stoprossroadchanges.wordpress.com
- The “entry width” is N/A for a mini-roundabout because they should not have islands! The recommended speed for a roundabout with a radius of 32 feet is somewhere under 10 mph (chart, page 14).
- The intersection sight distance is something closer to 10 feet due to foliage on three of the four corners, when it should be 100 feet even for 15 mph traffic.
Stop signs are the right traffic calming measure for this intersection, without question. Here is another good summary of the tradeoffs to a roundabout; note that LANDSCAPING is a real issue for us:
One-lane slow point.
One-lane slow points restrict traffic flow to one lane. This lane must accommodate motor traffic in both travel directions. Passage through the slow point can be either straight through or angled.
- Vehicle speed reduced.
- Most effective when used in a series.
- Imposes minimal inconvenience to local traffic.
- Pedestrians have a reduced crossing distance, greater safety.
- Reduced sight distances if landscaping is not low and trimmed.
- Contrary to driver expectations of unobstructed flow.
- Can be hazardous for drivers and bicyclists if not designed and maintained properly. Opposing drivers arriving simultaneously can create confrontation.