[Table of Contents]
[Previous: Alternate proposal]
[Next: Meeting notes]

Vassar Street project: technical issues

document prepared by John S. Allen, March 5, 2002
and delivered to MIT and City officials.
Some of this same material is duplicated in
more generic form at another location.

Crash risks
Operational issues
Liability issues
What the AASHTO guide says

The bicycle accommodation in the proposed Vassarscape project does not reflect the state of the art in civil engineering and traffic engineering knowledge. It appears that reference has not been made to applicable standards for bicycle facilities, or to the research literature on such facilities.

Sidewalk-type facilities were first built in Europe beginning before the Second World War, when no research base existed. Research dating as far back as 1938 has shown beyond any reasonable doubt that the sidewalk-type facility has serious civil engineering, safety, operational and liability problems, and that it is far preferable for bicyclists to travel either on the roadway, or on separated paths which are entirely away from roadways. A major backlash against sidepath facilities has been occurring in Europe since the research results became widely known.

The proposed facility would be expensive to construct and expensive to revise once it is constructed.

A park-like sidewalk environment could be achieved on Vassar Street in conjunction with bicycle accommodation which reflects national design standards and avoids these problems, and which is much less expensive to construct. The money which would be saved could then be turned toward projects which have a positive, rather than negative, effect on bicyclists' mobility and safety.

Elevated risk of accidents on sidewalk-type facilities


Sidepaths have a higher rate of bicycle-motor vehicle collisions than streets have. This may seem surprising at first to people who have are not familiar with accident studies. However, the overwhelming majority of car-bicycle collisions-- approximately 90% -- involve turning and crossing movements. Sidepaths complicate these movements by placing the bicyclist and motorist out of sight of each other and on conflicting paths at junctions (The term "junctions" as used in the research literature refers not only intersections with streets, but also places where driveways cross the sidewalk.)

Bicycle accidents which do not involve motor vehicles are rarely reported to police. However, as shown in all studies which are population-based, bicycle/motor vehicle collisions are only 10% to 20% of injury-producing bicycle crashes. The comparison between sidepaths and roads for crashes which do not involve motor vehicles is extremely unfavorable to sidepaths. In particular, the more restrictive geometrics of sidepaths lead to more single-bike crashes; also, bicyclists can not travel safely at normal speeds in the presence of pedestrians, because pedestrians suddenly and unpredictably change direction. A collision between a bicyclist and a pedestrian is as likely to injure the pedestrian as the bicyclist, and so pedestrian injury rates also increase greatly from the placing of bicycles on sidewalks.

The false belief that car-overtaking-bicycle collisions are the predominant car-bicycle collision type is in large part responsible for the disproportionate fear of that type of collisions, and the false idea that placing biyclists on sidewalks will increase safety. As first demonstrated in the Cross and Fisher study conducted in the 1970s, and as confirmed for the Boston area in the 1984 Metropolitan Area Planning Commission study posted on my Web site, only a small percentage of car-bicycle collisions on roadways is overtaking collisions.

Would any traffic engineer design a divided highway, with two-way traffic on each of three parallel roadways? That is what is proposed here. The problem of conflicts at junctions with sidepaths can be reduced only by measures that substantially slow or delay bicyclists, motorists or both. Even the most advanced measures of this type do not succeed in reducing the crash rate to that for riding on streets.

Evidence from the research literature

The evidence that bicycling on sidewalks and similar facilities is more hazardous than bicycling on streets is overwhelming.

A survey of research on the accident rates of streets, bicycle paths and sidewalks is available on the Internet at http://www.lesberries.co.uk/cycling/cy_pathr.htm. And here are some links to specific studies posted on the Internet:

Adult Bicyclists in the U.S. by Dr. William Moritz. Relative danger index 25 times as high for sidewalk riding as for major street without bicycle facilities. (Data include all crashes, not just car-bike collisions).

A Survey of North American Bicycle Commuters by Dr. William Moritz   Relative danger index 5.32 times as high on "other" facilities (mostly sidewalks) as on average of all facilities (mostly streets). Data include all crashes, not just car-bike collisions. Lower ratio than in previous study probably related to typically lower speed and overall higher crash rate of average commuters compared with avid adult cyclists.

The Risks of Cycling by Dr. Eero Pasanen, Helsinki, Finland. Higher car-bike collision rate for one-way sidepaths compared with streets, even though pedestrians are prohibited from the sidepaths. Extremely high rate of car-bike collisions with bicyclists crossing intersection on left sidepath.

Alan Wachtel and Diana Lewiston, Risk Factors for Bicycle-Motor Vehicle Collisions at Intersections (ITE Journal, September 1994). Car-bike collision rate 1.8 times as high for sidewalk riding as for streets.

The following two studies are are privately posted in connection with this document, as I do not have reprint rights:

Sidewalk Bicycle Safety Issues, by Lisa Aultman-Hall and Michael F. Adams Jr. Bicycle accident rate 6 to 10 times as high as sidewalks as on streets in Toronto. (PDF document. See page 4.)

Toronto Bicycle Commuter Safety Rates, by Lisa Aultman-Hall and M. Georgina Kaltenecker. 4 times as high injury accident rate on sidewalks as on streets (PDF document. See table 5, page 19).

A summary of the topic of bicycle accident types and prevention is to be found in the books Effective Cycling and Bicycle Transportation, by John Forester, published by our own MIT Press.

German bicyclist Berndt Sluka has surveyed the results from several European studies; see my translation of his Web page.

Europe has many more sidewalk-type facilities than the U.S., having gotten into the habit of building them before there were research results to warn against them. A strong backlash against them has occurred in Europe in recent years. A press release by the German Bicycle Federation ADFC (the national German bicyclists' organization) gives an example of the tone of the discussion occurring at present. An ADFC report on a conference held in Austria in 1990 includes additional data on crash rates for sidepaths.

Civil engineering issues

Root damage

Much of the proposed sidepath is adjacent to tree plantings. The roots of the trees will heave up the pavement as they grow. This problem is especially serious because trees will find water more abundant beyond the sidepath than on the street side.

Root damage is an endemic problem on bicycle paths but only rarely a problem in streets. That is because streets are wide enough to shield the soil beneath them from water percolation, and are bordered by deep curbstones which serve as root barriers.

Pedestrians can tolerate pavement heaved up by roots, but for bicyclists it is a serious hazard. A root across the bicyclist's line of travel causes a jolt and possible loss of control. A root that is more nearly parallel to the bicyclist's line of travel can sweep the bicyclist's front wheel to the side, causing a "diversion-type fall" which typically results in injury to the hip, shoulder or head.

An example of root damage on a bicycle path is shown in the photographs below.

Root damage on Paul Dudley White bicycle path near MIT's Pierce Boathouse

021_18Aroot damage.jpg (35959 bytes)


Streets are crowned to facilitate water runoff, and are provided with storm grates, catchbasins and storm sewers. The plans for the project indicate no drainage provisions for the sidepath segments other than for rainwater to run off into the street. Even if the path is sloped to facilitate drainage toward the street, puddling will occur increasingly over the years as the soil settles and as roots heave up the path. When plowed snow blocks drainage to the street, drainage will be completely blocked over long segments of the path.

Snow and ice.

Snow removal from a sidewalk is never a successful as from a street. On a street, motor vehicle tires wear away snow, and quickly open up patches of pavement, which facilitate heating by sunlight to complete the melting.

In addition, a more aggressive use of snow-melting chemicals is practical on streets, where they are carried away in the storm drain system and do not pose a substantial risk of damage to roadside vegetation.

Particularly when drainage is blocked by plowed snow and by heaving up due to tree roots, puddles on the path refreeze into ice, making the path unsafe for bicycle travel. Ice puddles are a very minor problem in a street, because it is crowned and because of its drainage provisions.

Plowed snow from the driveways will block the path until cleared from the path. No such problem occurs with streets.

All in all: Pedestrians can tolerate imperfect snow clearance, and can even beat a path for themselves where there is no snow clearance. Bicyclists cannot. After a snowfall, sidepaths remain unusable, and then unsafe, for bicycle travel far longer than streets.

Operational issues

Inevitability of two-way bicycle travel on Vassar Street sidepaths

The sidepaths have been designed based on the faulty assumption that they will carry only one-way traffic on each side of the street. This assumption is highly unrealistic because, with paths behind curbs, it is inconvenient to cross the street to enter the path on the other side.

Easy crossing can be accomplished only where there is a curb cut or driveway. Getting to the nearest entry point for the opposite sidepath requires either riding away from the intended destination and then making a U-turn, or riding the wrong way on the sidepath -- the more likely choice. Many of the entry points for the two sides are not opposite one another, and so it will often be necessary to lift the bicycle over a curb and sometimes, thread the bicycle between parked cars -- or else ride along the narrowed street -- the more likely choice, to get from one sidepath to the other.

Crossing to a street-level bike lane or the opposite travel lane of a normal street poses no such problems.

Most trip generators on Vassar Street to the east of Massachusetts Avenue are on the south side, and so use of the south sidepath will be heavy in both directions, and use of the north sidepath will be light. The opposite is the case west of Massachusetts Avenue and will be more so following completion of the Simmons Dormitory, now under construction.

Where the sidepath transitions to a bike lane as it approaches the intersection with Massachusetts Avenue or Main Street, bicyclists who are riding opposite the traffic flow in the sidepath will most likely continue on the sidewalk, rather than to approach the intersection traveling opposite traffic in the bike lane. They will be subject to the very high risk associated with crossing intersections on the left crosswalk, as described in the studies cited above.

Are pedestrians separated from bicyclists?

The proposed sidepaths connect with one-way bicycle lanes at Massachusetts Avenue and Main Street, and are unrealistically intended for one-way bicycle travel.The sidepaths are to be 5 feet wide, and separated from pedestrian traffic on the adjoining 8 foot sidewalks by a painted stripe 1 foot wide.

5 feet does not meet the current AASHTO standards for width of even a one-way bicycle path. When one bicyclist is overtaking another traveling in the intended direction, the tendency will be for the slower bicyclist to remain in the bikeway, while the faster bicyclist merges to the right -- not to the left as with normal overtaking -- and into the sidewalk area. This confusion puts both of the bicyclists at risk, as well as pedestrians.

A very confusing situation will occur when a "wrong-way" and "right-way" bicyclist approach one another. One or the other bicyclist will have to merge into the area designated for pedestrians. Which bicyclist? The "right-way" bicyclist may feel justified in keeping a straight course on the designated bikeway, but then the "wrong-way" bicyclist will have to pass to the right of the "right-way" bicyclist, contrary to the usual traffic rules. Suppose that the "right-way" bicyclist yields to the right as is normal, and the "wrong-way" bicyclist yields to the left, assuming that the "right-way" bicyclist will stay in the designated bikeway? Then they collide head-on. Confusions become far more complicated when bicycle and pedestrian traffic is heavy, and they lead to bicycle-pedestrian collisions as well as bicycle-bicycle-collisions.

By way of comparison, the standard overtaking rules on the roadway are simple and clear: slower traffic keeps right and faster traffic overtakes on the left after looking back to check that there is not another vehicle (bicycle or motor vehicle) already overtaking.

Confused yielding rules, sight obstructions

On the street, through traffic has the right of way over turning traffic. On the sidepath, because of sight line problems, both through bicycle traffic and crossing motor traffic must slow and be prepared to yield, since neither the bicyclist nor the motorist can be sure to see the other in time. The problem occurs because the bicyclist is hidden behind parked cars, or in the right rear blind spot of a motor vehicle which has slowed to turn. It is typical in this situation to install stop signs for the path as well as the road. The example below is from the Route 28 bicycle path in Hyannis. Bicyclists on the path must yield at the stop sign, and turning vehicles must yield at the crosswalk marking.

Sight obstruction by tree, bicyclist in camera position would be in motorist's
right rear blind spot; also right-of-way confusion, Route 28 bike path, Hyannis.

027_24A phinney's lane.jpg (64533 bytes)

Blocking of street by vehicles backed up to exit driveways, and blocking of path by vehicles backed up to enter driveways.

Placing multiple two-way rights of ways adjacent to one another creates crossing and turning conflicts like the one illustrated below. To yield to traffic in the roadway, the motorist must block the bikeway. To yield to traffic in the bikeway, the motorist must block the roadway. The standard traffic pattern, with slower traffic on the right and faster traffic in the middle, avoids this problem.

On streets, bicyclists can usually safely travel at the speed their legs can propel them, except when required to yield at intersections or under congested conditions. On a sidepath, every driveway and cross street becomes an intersection and bicyclists must slow or stop much more often. The sidepath promotes a false sense of security and bicyclists often do not use appropriate caution.

Car blocking path, Route 28 bike path, Hyannis

8505N10R31Rte28.jpg (45296 bytes)

Bicyclists will still use the street.

Most of Vassar Street presently has ample width for bicycle/motor vehicle lane sharing. The project proposes to eliminate this, reducing travel lane width to 11 feet. However, the project will not eliminate the need or desire of bicyclists to ride in the roadway, for several reasons, some already stated

  • the sidepath will not be usable at all times;

  • It will not allow fast, uninterrupted travel

  • because of the limited number of entry points, bicyclists will often have to ride along the roadway to cross over to the opposite sidepath.

  • In many cases, bicyclists preparing turns onto cross streets and driveways will be able to do so much more quickly and with fewer conflicts in the normal traffic flow than from the sidepath.

The only part of Vassar Street where lane sharing width is presently an issue is the part which currently has angle parking. In this part, the eastbound lane is narrowed to allow more room for vehicles to back out of the angle parking spaces on the far side of the street. However, the angle parking has a low turnover rate and  motorists have no problem overtaking bicyclists.

Adult riders do not belong on sidewalks

The MIT campus is separated from other areas of Cambridge by the Grand Trunk railroad line. Almost all bicyclists using the Vassar Street corridor are affiliated with MIT and are young adults who are generally in good physical condition and riding efficient, multi-geared bicycles, with typical cruising speeds between 15 and 25 miles per hour. These speeds are unsafe on the proposed facility, and are appropriate for street travel, not for sidewalk travel. MIT students also are sufficiently mature and intelligent to operate a bicycle according to the rules of the road.

Walking a bicycle in order to avoid challenging traffic conditions is sometimes appropriate for children and novice bicyclists. However, it is much slower than riding, and adults do not put up with it except under unusually difficult conditions. Traffic on Vassar Street is relatively light, and all of its intersections with streets are controlled by traffic signals.

The proposed pedestrian-type facilities are a hindrance rather than an aid to bicyclists who expect to be able to get to their destinations quickly, and create a chaotic, disorganized traffic pattern and increased risk of accidents. Without the proposed sidepaths, bicyclists still have the option to ride slowly on sidewalks and cross intersections as pedestrians if they wish to do so.

Arguments that are made for sidepaths and why they do not apply here:

Sidepaths make sense where traffic conditions are extreme and there is no cross traffic: for example on a highway bridge with high-speed traffic, narrow lanes and no shoulders. That is not the case here.

The two most common arguments that have been made for sidepaths are that they are safer, which is false except under unusual conditions, and that people think they are safer. The latter argument, sometimes naively and sometimes not, frequently becomes a basis for political action, on grounds that people should be encouraged to abandon motorized transportation for bicycling and walking. But when the facilities constructed make bicycling more dangerous and slower, this amounts to shooting oneself in the foot on the encouragement issue, and is unethical.

In any case, such encouragement is irrelevant at MIT. Short distances, and an extreme   shortage of parking spaces, assure that even people who drive to campus park their car for the day. The Vassar Streetscape project, as proposed, would make both walking and bicycling more hazardous, and would make bicycling slower, but would do nothing to effect a modal shift away from motor vehicles toward non-motorized modes.

Liability issues

MIT exposes itself to a serious liability risk if it funds the construction of the proposed sidepaths, as described in the paper Liability Aspects of Bicycle Environments: Bicycle Facilities and Roads, by three notable experts on bicycle transportation, Alex Sorton, P.E,.Tom Walsh, P.E and John Williams. This paper was presented at the Institute of Traffic Engineers 1990 Annual Meeting. The complete paper is included with this document for reference (but not for reprinting; I do not have reprint rights) [Not included with version published on the Web due to copyright issues]. Here are extracts:

A commuter bicyclist was riding on a designated bikeway. This bikeway had been created on the curbside half of an existing 10-foot sidewalk. Signs and markings were used to designate the facility and parking was allowed adjacent to the curb. A driver, while making a right turn into an alley, failed to see a bicyclist on the bikeway, primarily because a parked vehicle blocked the view, and ran over him. The bicyclist was severely injured.

The bicyclist sued the operating agency claiming negligent design. He pointed out that the AASHTO Guide strongly suggests not placing bikeways on a sidewalk adjacent to a street because of the sight obstruction created by parked cars.


It is all too common to find bicycle accommodations, especially bike paths, whose principal designers have been landscape architects and environmental planners. The only criticism of this design team is the absence of input from the highway and traffic engineering professions. The adequacy of critical design considerations such as surface, grades, curvature, sight distance, and traffic control devices is best determined by a knowledgeable transportation professional.


Regardless of the designer's expertise, special bicycle accommodations must always reflect current guidelines and sound judgment. Some states have promulgated their own design guidelines, which should be used where applicable. The courts and practitioners have also recognized the 1981 AASHTO Guide [current version is the 1999 AASHTO Guide] and the MUTCD (part 9, which applies to bicycles). Although there are other available references pertaining to the design, maintenance, and control of bicycle accommodations, these two should be considered "required reading" for transportation professionals responsible for bicyclists' safety and mobility.


The extent to which bicycles can be integrated with traffic and uncommon features minimized or mitigated without compromising bicycle mobility will determine the success of bicycle accommodation projects.


Don't put two-way bikeways on one side of a street. Such facilities cause serious conflicts at intersections and driveways. Two-way bike lane use has led to a number of fatal head-on collisions . And such facilities encourage wrong-way riding.

Don't designate sidewalk bikeways. These also cause serious car-bike conflicts at intersections and driveways, as well as conflicts between bicyclists and pedestrians. Eugene, Oregon, and other cities have found that sidewalk bikeways have extremely high accident rates.

What the AASHTO bicycle facilities guide says about sidewalk vs. on-road accommodation of bicycles

[The 1999 AASHTO Guide for the Development of Bicycle Facilities is available online and also from the AASHTO bookstore: Print version; CD-ROM version.  I have published a critique of the Guide's recommendations on sidewalk and sidewalk riding on another Web page. The quotes on that page convey the intent of the Guide. In this public posting of the Vassar Street critique, I am omitting lengthy quotes from the AASHTO Guide which would infringe on AASHTO's copyright. Rather, I am indicating only the beginnings and endings of the sections I quoted, so readers may look them up in the Guide. I suggest opening up the online version in a separate window to read the sections to which I refer below.

Violations of the guidelines in the Vassar Street project are indicated in red; other sections, for reference, are in black. The longer version delivered to the project planners included several illustrations from the Guide.]

pages 8-9:

In selecting the proper facility, an overriding concern is to assure that the proposed facility will not encourage or require bicyclists or motorists to operate in a manner that is inconsistent with the rules of the road...

the sidewalk can serve as an alternate facility, provided any significant difference in height from the roadway is protected by a suitable barrier between the sidewalk and roadway.

pages 11-13:

  • Crash Reduction ...

  • Stops...

  • Conflicts...

  • Maintenance....

  • Pavement surface quality...

  • Intersection Conditions...

  • Costs/Funding...

page 16:

Width ...

page 17:

Increased Lane Width Wide curb lanes for bicycle use are usually preferred where shoulders are not provided...

... supported by a documented engineering analysis based on applicable design criteria.

pages 17-18

On-street parking ...

page 20:

Designating Sidewalks as Signed Bikeways In general, the designated use of sidewalks (as a signed shared facility) for bicycle travel is unsatisfactory...

...bicyclists should not be encouraged through signing to ride facilities that are not designed to accommodate bicycle travel.

page 25

Bike lanes at intersections Bike lane striping should not be installed across any pedestrian crosswalks...

...The bike lane striping should resume at the outside line of the crosswalk on the far side of the intersection. (See Figure 7.)

pages 25-27:

Bike Lanes and Turning Lanes Bike lanes sometimes complicate bicycle and motor vehicle turning movements at intersections...

...(See Figure 10.)

pages 33-35:

Shared use paths should be thought of as a complementary system of off-road transportation routes for bicyclists and others that serves as a necessary extension to the roadway network...

...A barrier between a shared use path and adjacent highway should not impair sight distance at intersections, and should be designed to not be a hazard to errant motorists.

page 36:

Design Speed The speed a bicyclist travels is dependent on several factors...

...To account for vehicle movement E, it may require an all-red phase to protect the path users.

pages 49-51

Assigning Right of Way Volume, speed and highway classification should not be the only criteria to consider when assigning right of way at a path crossing...

...A 1.5m (5-foot) radius or flare may be considered to facilitate right turns for bicycles. This same consideration could also be applied to intersections of two shared use paths.

page 58

Undesirability of Sidewalks as Shared Use Paths Utilizing or providing a sidewalk as a shared use path is unsatisfactory for a variety of reasons...

...Wide sidewalks might encourage higher speed bicycle use and can increase potential for conflicts with motor vehicles at intersections, as well as with pedestrians and fixed objects. For guidance on when and how to designate sidewalks as signed bikeways, see page 20.

pages 64-67

Traffic Signals At signalized intersections where bicycle traffic exists or is anticipated, the timing of the traffic signal cycle, as well as the method of detecting the presence of the bicyclists, should be considered. ...

...Where programmed visibility signal heads are used, they should be checked to ensure that they are visible to bicyclists who are properly positioned on the road.

[Table of Contents]
[Previous: Alternate proposal]
[Next: Meeting notes]