Reaching Mode Share Goals Through Speed Reductions: Allow Local Jurisdictions in California Some Latitude in Setting Local Speed Limits

Posted: 20 February 2013 in Riding

NOTE: A slightly different version of this article was published in a local newsblog in my hometown.  It is one of those rare postings in which I actually talk about bikes! It is important to note that the “end” I am trying to reach through this recommendation is NOT, as the article seems to imply, a certain transportation mode share shift to more walking and biking.  Those are merely means to other ends including better health, cleaner air and fewer carbon emissions.  And, for those who care about such things, I think this provides a nice example of the kinds of decisions that need to be devolved to local entities so that local goals can more easily be met.

I recently had the opportunity to take a course offered by the League of American Bicyclists entitled “Traffic Safety 101”. I am an experienced cyclist but I took this introductory course to better equip me to work with children in Davis on bicycling safety. Part of the course involved practicing various skills on an active roadway in Roseville, CA. I do not know Roseville well and had never cycled there. Parts of Roseville are “poster children” for the concept of sprawl. The city features wide, multi-lane boulevards, a large and spread out set of shopping destinations and, because it sits astride Interstate 80, several large on/off-ramp “cloverleaf” areas. Our practice cycling took place in one of the shopping districts around I-80.Roseville

We practiced things like “taking the lane”, merging with traffic to make turns, and avoiding the dangers of poor road design (yes, most non-cyclists don’t understand that a road designed for cars can be terribly dangerous for a cyclist—and not just because of speed). As an experienced cyclist I don’t get nervous in too many riding situations. But the practice ride through Roseville was very disquieting for me. It was not only no fun, it was at turns challenging, nerve wracking and downright scary. Merging to make turns was daunting and there were several times when I nearly needed to stop my bike because I was approaching my turn but could not get a car driver to allow me to merge into the traffic flow.

In analyzing the experience afterward it became clear that a key problem was that the speed differential between the cars and bicycles was simply too great. But it was not just the speed differentials. I routinely cycle a local county road (CR99) and the speed differentials on that road are much greater than those in Roseville. It is not uncommon, for example, to be pedaling along at 15-17 MPH and have cars whizzing by at 55-65 MPH. While not the most enjoyable riding, the differentials are not too troubling. The reason is that on a rural road a cyclist does not need to make many bicycling maneuvers.

In contrast, to get around Roseville (or any suburban or urban environment) to do the normal kinds of activities cyclists need to engage in—shopping, running errands, commuting, going to meetings—requires using an array of bicycling maneuvers: turning frequently and changing lanes a considerable amount. In such environments speed differentials matter much more because of the need to interact with traffic much more. A cyclist needs to “negotiate” with auto drivers on a regular basis in order to get where they need to go.

Now imagine you are NOT an experienced cyclist. How much more daunting might speed differentials be to you? Even if traffic levels are not as high as in Roseville, might you not be discouraged from riding if you were not comfortable with your ability to make a turn in front of and across traffic because of the speed differentials? Recent research [1] indicates that cyclists avoid roads with high traffic intensity (a combination of volume and speed) and routes that require them to make frequent left turns. In correspondence with one of the authors about the research I raised the question about whether speed differentials discourage people form choosing the routes that require left turns. While the author acknowledged that the research field is too recent to demonstrate that conclusively, she did note that my “hypothesis about left turns is accurate.”

In Davis we have mode share goals that suggest we will want to increase bicycle ridership as a proportion of all trips up to 30-35% (or higher) in the coming decades (we are probably at 25%). We have already reached the “low hanging fruit” in terms of ridership, though that “fruit” must be viewed through the lens that we are one of the most “bike friendly” cities in the US. We have achieved this mostly through investment in bike lanes and bike paths, which provide riders with a separate space in which to ride. We have also done it by designing our cul de sacs with access to greenbelts, which makes most neighborhoods accessible to cyclists, by developing high cost bits of critical bike infrastructure such as bike bridges and tunnels, and by providing ample bike parking at key destinations.

But the low hanging fruit harvested with these important measures is all gone. To go further we must address the potential riders who refuse to use the infrastructure we have. Laying down more infrastructure is going to get us only marginal gains. To increase ridership (again, as a proportion of all trips) we are going to have to coax new riders or more hesitant riders onto the road for more reasons. And to do THAT, we need to deal with the issue of speed.

The problem is that Davis does not fully control the enforceable speeds it sets in the city. The City can post any speed limit it wants (with a few exceptions like around schools) but it does not do so because it cannot enforce the speed limit if the street’s “critical speed” is too high. In other words, if we wanted to we could post a speed limit of 15 MPH on every street. However, people ticketed on many such streets could not be compelled to pay their fines if they contested them. This is because the state requires the City to assess the “critical speed” on streets (or, more correctly street segments—not including neighborhood streets).

The critical speed is the speed that 85% percent of drivers actually drive on the street. Cities (and counties) have a bit of leeway to set speed limits below that—5 mph—without justification based on the latest California law. What this means is that if the critical speed is 30 mph on a street, the City can post and enforce a speed limit of 25 but could not enforce one of 20 mph.[2]

Actions to reduce speeds to a lower enforceable level are called “traffic calming” and there are many methods used to achieve calming. The problem is most are costly and some are onerous to cyclists and cars (think speed bumps or tables). The problem with many of Davis’ streets is that they are simply far too wide. I am not talking here about neighborhood streets but our “feeder” and “arterial” streets. Wide streets, laid out in a grid with few stops, invite higher speeds. This pushes up up “critical speeds” and creates a situation in which greater differentials exist between bikes and cars.

City-wide traffic calming is prohibitively expensive and it may make much more sense to focus on changing car driver behavior to achieve lower speeds. We could do this through public education campaigns or through placing temporary speed estimators on streets to remind drivers of their speeds. Or, if we were allowed, we could use speed limits to change behavior.

Currently, of course, we have no power to use this particular approach for reasons noted above. It is for that reason that I would like to launch a legislative initiative in the state of California that would return a limited amount of control to California’s cities to set enforceable speed limits that enable them to achieve critical community goals. I know nothing about writing legislation but my proposal would look something like this:

Each local government jurisdiction (city or county) in the state will be permitted to unilaterally set enforceable speed limits on select “safety corridors” not to exceed XX% of total street/road miles. Such corridors must be signed so as to alert drivers to their special status with special speed limits posted with special status signs. The purpose of these “safety corridors” is to enable jurisdictions to promote alternative transportation uses that will be more effectively employed if traffic speeds are lowered. Such corridors are expected to have the effect of promoting increased walking and biking for all street users and reduce the incidence of injury or mortality caused by car/pedestrian or car/bicyclist collisions.

The exact proportion indicated by the XX% above will need to be developed. The issue of signage is critical to this proposal because creating new “signage” is no small feat given that the federal and California-specific Manual of Uniform Traffic Control Devices (CA MUTCD) specifically mandates signs that can be used on state roads and streets. It is not feasible to go outside the manual and create new signs. Fortunately, the CA MUTCD has a set of signs that could be adapted for use in these new safety corridors:


These numbers refer to the CA MUTCD and, combined with standard speed limit signs, could provide the special signage necessary to alert motorists that they are in a special zone. These signs were developed to accompany a federal program that permits states (in California through the California Highway Patrol) to adopt special safety rules such as the requirement to use headlights in the daytime in areas with an elevated level of accidents—especially deadly ones. It would seem uncontroversial to use such signage as a “preventive” measure.

Keep in mind that reducing speed limits where there is much mixing of pedestrians and cyclists with cars makes sense from an injury (and mortality) reduction perspective. The following chart [3] shows the relationship between car speed and injury and death when cars hit pedestrians.

Speed and Pedestrian Death

Another study [4] summarizes this in its abstract:

This research explores the factors contributing to the injury severity of bicyclists in bicycle–motor vehicle accidents using a multinomial logit model. The model predicts the probability of four injury severity outcomes: fatal, incapacitating, non-incapacitating, and possible or no injury. The analysis is based on police-reported accident data between 1997 and 2002 from North Carolina, USA. The results show several factors which more than double the probability of a bicyclist suffering a fatal injury in an accident, all other things being kept constant. Notably, inclement weather, darkness with no streetlights, a.m. peak (06:00 a.m. to 09:59 a.m.), head-on collision, speeding-involved, vehicle speeds above 48.3 km/h (30 mph), truck involved, intoxicated driver, bicyclist age 55 or over, and intoxicated bicyclist. The largest effect is caused when estimated vehicle speed prior to impact is greater than 80.5 km/h (50 mph), where the probability of fatal injury increases more than 16-fold. Speed also shows a threshold effect at 32.2 km/h (20 mph), which supports the commonly used 30 km/h speed limit in residential neighborhoods. The results also imply that bicyclist fault is more closely correlated with greater bicyclist injury severity than driver fault. (emphasis added)

The point here is that if we can reduce speeds on certain key streets—those leading to schools or to key activity centers —to 20 mph or below we not only encourage more riding but we also reduce the probability of injury and death to very low levels. At this time, neighborhood streets have speed limits of 25 mph but many culs de sac and short streets keep speeds down, and in the downtown core frequent stops keep speeds down. However there are no feeder or arterial streets with critical speeds that would permit the posting of an enforceable 20 mph speed limit.

If California law allowed us to reduce speed limits on a limited number of streets we could reduce bike/car speed differentials, encourage more cycling by less experienced or confident cyclists while reducing the potential for injury in the case of collisions.

[1] Broach et al (2012) “Where do cyclists ride? A route choice model developed with revealed preference GPS data” Transportation Research Part A 46:1730–1740.

[2] There is a bit more to the story than given here and I would be happy to share more specifics for those interested.

[3]Tefft, Brian C. (2011) Impact Speed and Pedestrian’s Risk of Severe Injury or Death. Washtington: AAA Foundation for Traffic Safety.

[4] Kim et al (2007) “Bicyclist injury severities in bicycle-motor vehicle accidents” Accident Analysis and Prevention. 39:238-251


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