ITE Journal March 2018 - 48

example of calibration for local conditions in the final report of its
Roundabout Geometric Design Guidance.4
There are many more examples like these. Roundabouts are
inherently flexible, and experienced, creative designers can fit them
around quite tricky constraints while keeping them safe, efficient,
and delightful to drive through.

Skagit County

The Tipping Point

Figure 2. Example of providing cuts for drains at Best Road & McLean
Road, Skagit County, WA, USA.
Curbs are used to control speeds at roundabouts. The outer
curb can often mean adding a drainage basin to the cost of a rural
roundabout. One option that doesn't compromise speed control is
to provide cuts for drains every so often along the outer curb line.
The intersection of Best Road and McLean Road in Skagit County,
WA, USA is a great example of this (see Figure 2).
While a reverse curve is used to slow a high-speed approach,
this feature is expensive. Without a reverse curve, designers can
still slow the approach using a conspicuous center island profile
(daytime visibility) combined with illumination, reflective stripes
and signs (nighttime visibility), and a well-designed entry curve.
Extra-wide lanes are used at two-lane roundabouts to
allow semi-trailers to stay in their lane. This is also expensive.
Narrower lanes reduce speeds, further improving the safety of
two-lane roundabouts, especially for pedestrians and cyclists.
Using narrower lanes that require semi-trailers to straddle both
entry lanes can significantly reduce cost while also improving
performance for all users including semi-trailers. Turning
semi-trailers don't stay in their lane at traffic signals, and they
prefer to straddle both lanes at roundabouts as well.3
Some roundabouts have more lanes than they need. This
is expensive-and tricky. Estimating how long a single lane
roundabout could last and designing it to expand later can help.
Bypass lanes or metering during rush hour can also extend the life
of a single lane roundabout. Just because the volume-to-capacity
ratio (v/c) reaches some magic number during a future hypothetical
rush hour doesn't mean another lane is needed right away. How
many traffic signals exceed that magic number today? The v in v/c
can be quite fuzzy. A lot depends on the planning model. The c
can also be fuzzy. To start it depends on v, but it also needs to be
calibrated to local conditions. This is true whether a micro-simulation or a deterministic equation is used. Caltrans offers a great
48

Ma rch 2018

i te j o urn al

When roundabouts reliably cost less than traffic signals, the decision
to implement them becomes simple. It is far easier to go with the
cheapest option than it is to ask for more money and defend it,
especially when that defense is complicated, includes assumptions
and estimates, and the majority of the added value benefits users and
doesn't show up on the agency balance sheet. Roundabouts will soon
reliably compete on price. The goal is for all roundabout designers to
achieve this cost parity without sacrificing performance.

Accelerating the Implementation of Roundabouts
The trend toward lower cost is primarily driven by roundabout
designers' continuous improvement and lifelong learning from
experience, but we can all support and accelerate this trend, leading to
broader roundabout implementation. Here are a few ways the following
groups can help save lives through implementing roundabouts.

Planners:
ƒ Include specific roundabout locations and layouts in community
and master plans. Even if they are only potential locations,
agencies can leverage developers to set aside the right land early,
greatly reducing cost when it is time to build the roundabout.
The Ada County Highway District in Idaho offers a great
example of right-of-way preservation in its Roundabout Preservation Map, Final Report.5

Public Agencies:
ƒ Integrate the roundabout option fairly into your existing
intersection alternative selection process using some form of
intersection control evaluation (ICE). Caltrans offers a great
example of this on its ICE website.6 Evaluate the roundabout
option fairly before deciding whether or not to rule it out.
ƒ Create a roundabout program or policy that ensures roundabouts
are a healthy part of your capital improvement program.
ƒ Designate a single person to be the subject matter expert for the
agency. Invest in their roundabout education, and take their advice.
ƒ Enlist the help of experienced roundabout designers during
the earliest conceptual stages to ensure the roundabout option
meets your performance objectives with an accurate cost
estimate prior to making a comparison/decision.
ƒ Keep standards flexible. Roundabouts can be molded to fit the
needs of many unique sites, and don't even need to be perfect



Table of Contents for the Digital Edition of ITE Journal March 2018

ITE Journal March 2018 - 1
ITE Journal March 2018 - 2
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http://www.nxtbook.com/ygsreprints/ITE/G110939_ITE_November2019
http://www.nxtbook.com/ygsreprints/ITE/G110110_ITE_October2019
http://www.nxtbook.com/ygsreprints/ITE/G110109_ITE_September2019
http://www.nxtbook.com/ygsreprints/ITE/G108559_ITE_August2019
http://www.nxtbook.com/ygsreprints/ITE/G108250_ITE_July2019
http://www.nxtbook.com/ygsreprints/ITE/G107225_ITE_June2019
http://www.nxtbook.com/ygsreprints/ITE/G104039_ITE_May2019
http://www.nxtbook.com/ygsreprints/ITE/G104038_ITE_April2019
http://www.nxtbook.com/ygsreprints/ITE/G104036_ITE_March2019
http://www.nxtbook.com/ygsreprints/ITE/G103582_ITE_February2019
http://www.nxtbook.com/ygsreprints/ITE/G102868_ITE_January2019
http://www.nxtbook.com/ygsreprints/ITE/G100155_ITE_December2018
http://www.nxtbook.com/ygsreprints/ITE/G100154_ITE_November2018
http://www.nxtbook.com/ygsreprints/ITE/G99495_ITE_October2018
http://www.nxtbook.com/ygsreprints/ITE/G98028_ITE_September2018
http://www.nxtbook.com/ygsreprints/ITE/G97366_ITE_August2018
http://www.nxtbook.com/ygsreprints/ITE/G96287_ITE_July2018
http://www.nxtbook.com/ygsreprints/ITE/G94315_ITE_June2018
http://www.nxtbook.com/ygsreprints/ITE/G93877_ITE_May2018
http://www.nxtbook.com/ygsreprints/ITE/G93065_ITE_Apr2018
http://www.nxtbook.com/ygsreprints/ITE/G91484_ITE_Mar2018
http://www.nxtbook.com/ygsreprints/ITE/G89434_ITE_Feb2018
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