Paul Plotas, P.E., PTOE
October 24, 2002
Re: Draft Guidelines for Accessible Public Right-of-Way (June 17, 2002)
Roundabout Alternative Design Strategies
Dear Access Board:
Although I certainly would not classify myself as a roundabout expert, I am a
practitioner of traffic engineering with 16 years of experience. During my
review of the “Draft Guidelines for Accessible Public Rights-of Way (June 17,
2002)”, the following passage in particular caught my attention:
To provide safer crossing at roundabouts, the draft guidelines would require
pedestrian activated crossing signals at each roundabout crosswalk, including
those at splitter islands. (The draft guidelines would ensure that such signals
are usable b with vision impairments under requirements in section 1106
discussed below.) Although roundabouts are typically used to avoid
signalization, the Board is not aware of alternatives that would allow safe
passage for pedestrians with disabilities. Aside from accessibility, the use of
roundabouts in areas of high pedestrian use has been questioned 4 some in the
industry. Requiring the signal to be pedestrian activated may help limit the
impact on traffic flow. Signal technologies are available that can further
minimized the impact, such as devices that halt traffic only while a pedestrian
is in the crosswalk. The Board seeks information on alternative design
strategies and available technologies that can improve access at roundabouts for
persons with disabilities, particular those with vision impairments.
I developed the attached summary of my thoughts on this subject and respectfully
submit it for your review.
Please give me a call at [...] if you would like to discuss my thoughts further.
Very truly yours,
Paul Plotas, PE, PTOE
Section 1 — Introduction
All at-grade pedestrian crossings of roadways are handled in one of the
following two ways:
Vehicular traffic is stopped, allowing the pedestrians to cross — example:
Pedestrians judgment determines appropriate crossing opportunities — example:
Typically, in both cases vehicles are already expecting to stop at an
intersection and pedestrian crossings are accommodated. Roundabouts present a
different challenge since vehicular traffic at the egress crosswalks will never
be stopping for other vehicles, see Figure 1. Additionally, roundabouts present
a particular challenge to visually impaired pedestrians since the geometrics of
a roundabout distort audio information from vehicular traffic giving unclear
audio clues. To aid pedestrians at roundabouts, one can either attempt to
control vehicular traffic or improve the information provided to pedestrians.
This report discusses these two alternatives.
Referring to Figure 1, for the purposes of the following discussion, it is
assumed that the pedestrian crosswalk is located approximately I to 2 car
lengths from the edge of the central circle, which is the typical layout.
In this section various roundabout alternatives for both controlling vehicular
traffic and for improving information available to pedestrians will briefly be
Control Vehicular Traffic
There are at least four alternatives available to control vehicular traffic and
give pedestrians the right-of-way at a roundabout egress pedestrian crossing.
a. Stop sign
b. Yield sign
c. Traffic signal
d. No control/Understanding that vehicle driver yields right-of-way to
a. A stop sign alternative breaks the basic principals of roundabouts: all
traffic control of a roundabout is yield controlled. Additionally, with low
pedestrian traffic, stop signs will serve no useful purpose the majority of the
time, will eventually be ignored by drivers, and will eventually function as a
yield sign, and probably as an uncontrolled intersection.
b. As discussed for the sop sign alternative, at low pedestrian volume
locations, vehicle drivers will also eventually ignore the yield sign, resulting
in an uncontrolled intersection.
c. A third method for controlling vehicles at the egress pedestrian crossing is
a traffic signal, however, two issues immediately come to mind. The first is the
issue that there is only storage for up to two vehicles before additional
vehicles in the queue impede vehicular flow around the roundabout. Even with
minimum timings for a signal, a roundabout with relatively low vehicular volumes
will experience some blockage.
Having the call for the signal solely dependent on pedestrian demand without
regard to vehicular flow in the roundabout could have significant impacts to the
capacity of the roundabout. It may be possible to place additional vehicle
detectors strategically about the roundabout to delay the pedestrian call to a
convenient time for vehicles; however, additional study will be required to
determine if a suitable location is actually available.
The second issue that comes to mind with signalized control is driver
expectation, especially with low pedestrian volume roundabouts. If a driver
continuously meets a green indication at a signal, driver expectation is that a
green signal will be met every time. My personal observations are that at low
pedestrian volume, mid-block, stand alone pedestrian signals, it is the
combination of the physical person and the signal that alert drivers to the
signal. My personal observations are that most pedestrians instinctively realize
this and will not begin to cross a road, even with the signal giving the right-
of-way, until they visually receive confirmation that vehicles are stopping.
Sight distance for the mid-block pedestrian signal is very important. Although
the same sight distance may be available at a roundabout, the curvature of the
intersection at times focuses drivers’ attention on negotiating the curves of
the road and away from the distant cross walk. The bottom line of this
discussion is that due to driver expectations, a signalized crossing could
actually give visually impaired pedestrians a false sense of security, and
further study is necessary to determine if this is true.
d. Based on the discussion above, the final alternative, No
control/Understanding that driver yields right-of-way to pedestrian is the
alternative that the unsignalized alternatives default to, particularly at
locations with low pedestrian volumes.
Control Pedestrian Traffic
There are at least two alternatives to control pedestrians while maintaining
virtually free-flow conditions for vehicular traffic.
a. No control, pedestrian judgment determines acceptable gaps
b. “All Clear” Signal to pedestrians
a. Due to distorted sound the no control alternative is unacceptable.
b. The concept for the “All Clear” alternative is that vehicle detectors can be
placed in the pavement upstream of the crosswalk as shown on Figure 2. The
upstream detector could be coordinated with a pair of downstream detectors to
determine when no vehicles are within the clear zone. When no vehicles are
within the clear zone pedestrians would be given an “All Clear” signal.
While it is possible to build such a system, the following issues deserve
1. Do pedestrians have enough time to cross the egress lane in the scenario
where a vehicle is just before the detector just as the pedestrian steps off the
2. Is it prudent to depend on technology to this extent to determine when
conditions are all clear?
3. What liability is involved with this alternative? (Is it really any different
from a standard signalized intersection?)
The following problem with roundabouts has been identified: the circular portion
of the roundabout creates confusing sound patterns. Being able to clearly read
these sound patterns is essential for visually impaired pedestrians to safely
cross the road. While it is possible to control vehicular traffic to enable
pedestrians to cross, it is not obvious, at least to me, that the pedestrians
will be safer. Instead of trying to control the vehicular traffic, I propose to
improve the information available to pedestrians. It appears that by
strategically placing detectors visually impaired pedestrians can have the same
information as all other pedestrians to make informed decisions about when to