What the AASHTO Guide says
and does not say
about bicycle sidepaths
A bicycle sidepath is a path next to a road, like a sidewalk, except that signs are
posted designating it for bicycle use. Sidepaths are easier to build than paths in most
other locations, because the government usually has the right to build on the land next to
The belief that sidewalks are safer than roads for bicycling is widespread -- though
incorrect. Designation and construction of sidepaths has many problems.
The AASHTO sidepath warnings
The 1999 AASHTO Guide for the Development of Bicycle Facilities (ordering
information for print version with
CD-ROM; CD-ROM only) is
the standard reference work for design of bicycle facilities in the United States of
America. It strongly warns against sidepaths, and gives a list of problems with them which
is damning, but incomplete. I critique that list here (in the full-width paragraphs which
follow citations from the AASHTO guide, which are indented). On page 33 of the AASHTO
guide, we read:
Separation Between Shared Use Paths and Roadways When two-way shared
use paths are located immediately adjacent to a roadway, some operational problems are
likely to occur. In some cases, paths along highways for short sections are permissible,
given an appropriate level of separation between facilities, as in Figure 16.
Note that the facility shown here is separated from the highway by a barrier, and there
is no cross traffic. These provisions avoid hazards of car-bike collisions. In urban or
suburban areas, a path like this may be justifiable if it provides a shortcut, or access
that is not possible via roads. However, in rural areas, accommodating bicycle traffic on
the shoulder of a limited-access highway is practical and avoids the cost of a separate
facility where use may be very low: see my
comments on an installation in New Hampshire. Continuing now with the list of problems
in the AASHTO guide:
Some problems with paths located immediately adjacent to roadways are as follows:
1. Unless separated, they require one direction of bicycle traffic to ride against
motor vehicle traffic, contrary to normal rules of the road.
If the path is truly separate as shown in the picture, this is not a problem except as
it creates other problems, described in the next few items on the list and in my comments
2. When the path ends, bicyclists going against traffic will tend to continue to travel
on the wrong side of the street. Likewise, bicyclists approaching a shared use path often
travel on the wrong side of the street in getting to the path. Wrong-way travel by
bicyclists is a major cause of bicycle/automobile crashes and should be discouraged at
3. At intersections, motorists entering or crossing the roadway often will not notice
bicyclists approaching from their right, as they are not expecting contra-flow vehicles.
Motorists turning to exit the roadway may likewise fail to notice the bicyclist. Even
bicyclists coming from the left often go unnoticed, especially when sight distances are
4. Signs posted for roadway users are backwards for contra-flow bike traffic; therefore
these cyclists are unable to read the information without stopping and turning around.
The problem with signs also applies to traffic signals, and is even more serious in
connection with them because inability to see a traffic signal makes it impossible to
determine when cross traffic may start. Special "wrong-way" traffic signals
(usually, pedestrian "walk/don't walk" signals) must then be installed for the
sidepath, but often no such signal is installed.
AASHTO does not mention that during hours of darkness, contraflow sidepath users are in
the bright side of motorists' headlamp beam pattern, another very significant hazard.
Continuing with the list from the AASHTO guide:
5. When the available right-of-way is too narrow to accommodate all highway and shared
use path features, it may be prudent to consider a reduction of the existing or proposed
widths of the various highway (and bikeway) cross-sectional elements (i.e., lane and
shoulder widths, etc.). However, any reduction to less than AASHTO Green Book (or other
applicable) design criteria must be supported by a documented engineering analysis.
Narrowing cross-sectional elements is not the only solution. Segregating bicyclists
uses road space inefficiently. If the right-of-way is too narrow to accommodate a sidepath
without compromise, that is one more reason not to build a sidepath at all.
6. Many bicyclists will use the roadway instead of the shared use path because they
have found the roadway to be more convenient, better maintained, or safer. Bicyclists
using the roadway may be harassed by some motorists who feel that in all cases bicyclists
should be on the adjacent path.
It may also be necessary to use the roadway to get to or from an entry point to the
sidepath, or to travel to or from from a location which the sidepath does not provide
access -- for example, one on the opposite side of the roadway. Nonetheless, harassment
may be codified into law requiring use of a sidepath, as is the case in several states.
See for example the New York law described in my Web page
about a bicycle facility in Herald Square in New York City. Continuing with the list
from the AASHTO Guide:
7. Although the shared use path should be given the same priority through intersections
as the parallel highway, motorists falsely expect bicyclists to stop or yield at all
cross-streets and driveways. Efforts to require or encourage bicyclists to yield or stop
at each cross-street and driveway are inappropriate and frequently ignored by bicyclists.
See photo illustrating the
problem of priority at intersections. The usual practice in the United States is to
install stop signs on the sidepath. In practice, due to the increased sensory burden
imposed by the sidepath, bicyclists on a sidepath can not safely expect that motorists
will yield to them unless the path has an exclusive signal phase. Such a phase results in
reduced throughput, and is not necessary when bicycles travel on the roadway.
8. Stopped cross-street motor vehicle traffic or vehicles exiting side streets or
driveways may block the path crossing.
You may view a photograph
illustrating this problem on another Web page describing a sidepath in Hyannis,
And continuing with the final item on AASHTO's list:
9. Because of the proximity of motor vehicle traffic to opposing bicycle traffic,
barriers are often necessary to keep motor vehicles out of shared use paths and bicyclists
out of traffic lanes. These barriers can represent an obstruction to bicyclists and
motorists, can complicate maintenance of the facility, and can cause other problems as
Barriers can obstruct travel, but if travel is facilitated by leaving openings in the
barriers, then two other problems occur:
Crossing and turning maneuvers are made more difficult and complicated,
as merging is no longer possible.
The barriers can become sight obstructions.
Deliberately installed barriers are not the only ones which cause these problems.
Parked cars on the roadway pose a hazard by blocking motorists' and bicyclists' view
of each other. Pedestrians, vegetation, street furniture and other obstructions can also
create the same problem.
The AASHTO guide concludes its section on sidepaths as follows:
For the above reasons, other types of bikeways are likely to be better suited to
accommodate bicycle traffic along highway corridors, depending upon traffic conditions.
Shared use paths should not be considered a substitute for street improvements even when
the path is located adjacent to the highway, because many bicyclists will find it less
convenient to ride on these paths compared with the streets, particularly for utility
AASHTO warnings about sidewalks
The section of the AASHTO guide cited above refers to paths specifically intended for
bicycle use. The AASHTO guide also warns against sidewalks designated as bicycle routes:
pages 8-9: general warning against designating sidewalks as bicycle routes:
Sidewalks generally are not acceptable for bicycling. However, in a few limited
situations, such as on long and narrow bridges and where bicyclists are incidental or
infrequent users, the sidewalk can serve as an alternate facility...
page 20: another, similar warning:
In general, the designated use of sidewalks (as a signed shared facility) for bicycle
travel is unsatisfactory. (See Undesirability of Sidewalks as Shared Use Paths, page 58.)
It is important to recognize that the development of extremely wide sidewalks does not
necessarily add to the safety of sidewalk bicycle travel, since wide sidewalks encourage
higher speed bicycle use and increase potential for conflicts with motor vehicles at
intersections, as well as with pedestrians and fixed objects....
page 58: And yet another similar warning:
Utilizing or providing a sidewalk as a shared use path is unsatisfactory for a variety
of reasons. Sidewalks are typically designed for pedestrian speeds and maneuverability and
are not safe for higher speed bicycle use. Conflicts are common between pedestrians
traveling at low speeds (exiting stores, parked cars, etc.) and bicyclists, as are
conflicts with fixed objects (e.g., parking meters, utility poles, sign posts, bus
benches, trees, fire hydrants, mail boxes, etc.)
Some points the AASHTO guide does not make
The AASHTO Guide, with its repeated warnings, is damning about designation of sidepaths
and sidewalks, but the Guide is skittish about raising the issue of liability. The Guide
does not come right out and say that these facilities have an elevated crash rate, which
And there are additional points which AASHTO does not make:
If the sidepath is too close to the road to allow a storage area for
motor vehicles between the two, a motor vehicle turning across the sidepath and yielding
to a bicycle must block the road. But if the sidepath is far from the road, motorists are
less likely to see and yield to through-traveling bicyclists.
Bicyclists and motorists must turn by crossing the adjacent roadway
which carries two-way traffic, greatly increasing the difficulty of scanning for
approaching traffic, and reducing the number of opportunities for turning.
If a sidepath is close to a road, snow clearance from the road
frequently results in snow's being dumped on a sidepath, preventing the sidepath from
being used in the winter even as a cross-country ski trail. Piles of snow typically
obstruct the sidepath for a month or more after snow has cleared from the road; when the
snow finally melts, remaining sand and debris pose a hazard to bicyclists. You may view a photo showing this problem on
Mopeds are generally slower than motor traffic, but faster than
bicycles, and in tourist areas in the United States where mopeds are most common, most are
rented to people who have little or no previous experience with them. Moped riders may use
a sidepath even if such use is forbidden, endangering bicyclists, pedestrians and
Sidepaths are located where sidewalks usually are, and are used as
sidewalks. The distinction which the Guide makes between sidewalks and sidepaths is
One goal of building trails is to provide a pleasant, quiet and scenic
recreational bicycling experience. This goal is much better served by trails away from