How Not To Design A Roundabout

Nassau County Fails Roundabout 101.

Advertisements

Not far from my house is a prime example of how not to design a roundabout.  The roundabout is on Heron Isle Parkway, between Blackrock Rd. and Chester Rd.

Let’s start with the ideal roundabout, according to Florida Department of Transportation (FDOT).

idealra

Nice symetrical design, typical of most of the roundabouts on Amelia Island, such as Bailey Rd. & Amelia  Island Parkway.

bailey

FDOT also recommends signage, as seen at 14th and Amelia Island Parkway.

14_aip

The Federal Highway Administration provides guidance on the radial alignment of entries.

align
Exibit 6 – 18, FHWA “Roundabouts: An Informational Guide”

.With the admonishment “Approach alignment should not be offset to the right of the roundabout’s center point.” and this elaboration “It is almost never acceptable for an approach alignment to be offset to the right of the roundabout’s center point. This alignment brings the approach in at a more tangential angle and reduces the opportunity to provide sufficient entry curvature. Vehicles will be able to enter the roundabout too fast, resulting in more loss-of control crashes and higher crash rates between entering and circulating vehicles.  Exhibit 6-18 illustrates the preferred radial alignment of entries.” [emphasis added by old curmudgeon]

Now we get to the Heron Isle Parkway roundabout.

Heron.jpg

The Northbound approach centerline is not only offset to the right, contrary to FHWA guidelines, it is tangent.  Entering from any other approach you are forced to slow down.  Not so for the Northbound approach.  There is signage reducing the speed from 35 mph to 25 mph, but it is not in accordance with the FDOT recommendation, no circle graphic on the sign.

heronsign

There is also a new approach being built, to service the new homes being built to the East of the Parkway.  The curmudgeon’s Jeep is parked in the center of the new approach.  This new approach will make the intersection more dangerous than it already is.  The old curmudgeon has witnessed two near misses at the circle, where the northbound vehicle failed to yield to a vehicle entering the circle from Gaylon Dr.  Because of the fence, the sight line of the northbound driver of traffic entering from Gaylon Drive is obstructed as is the sightline of the vehicle entering from Gaylon.

yield

Sightline

The blue line in the figure below represents the sightline, given the fence.  The brown line is the distance from the site line to the entrance of the roundabout, 160 ft.  At 25 mph you have 4 seconds from the time you can see a car on Gaylor until you enter the circle.  The breaking distance from 25 mph is 30 ft, leaving you less than 3 seconds reaction time.  The published perception/reaction distance for 25 mph is 55 ft.  Note: According to Appraiser’s map (figure below) the new entrance will be Furtherview Ct.

sightline

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s