We’ve all seen the graph: a person hit by a car going 40 miles per hour (mph) has an 85% chance of being killed.  Reducing the speed to 30 mph cuts the odds of death in half; reducing speed to 20 mph drops the fatality rate by an astounding 94%.  Even more dramatically, at 5 mph cars (and very cautious trucks), bikes, and pedestrians can all safely share the same street space.  According to the US Department of Transportation, about 33% of vehicle-related deaths are speeding-related.  Of those, around 40% occur in urban areas.


Almost every neighborhood feels that too many cars (and trucks) are driving through too fast making too much noise and endangering everyone.   One impact is to severely limit the number of people who are willing to walk, bike, or even hang around outside with their neighbors.   Especially affected:  the elderly, the infirm, the very young, the slow moving.  Especially impaired:  public health, social connectivity, local businesses, and (because it is almost always worse in low-income communities) social equity.

The most effective way to slow traffic is through physical changes in the road and its surroundings.  But despite progressive policy statements at the state and federal levels about the need to promote walking, bicycling, and transit as much as – or even more than – car traffic, current planning and operational procedures and practices continue to generally lead to car-centric results.   Changing this requires elevating the concept of “desired” or “target” speed to a central place in all road transportation analyses and decisions, while broadening its interpretation to incorporate criteria relating to the promotion of non-car modes.  As a step towards that goal, we also need to support two bills currently in the Massachusetts Legislature that would allow the local creation of additional 20 mph Safety Zones and reduce the Legislatively-set Statutory (default) speed limits on streets without Posted Limits.



After (or in fear of) an accident, or just to make their neighborhood’s public space more inviting for walking, bicycling, or simply being outside with others, the first impulse of many concerned citizens is to demand a traffic light – which unfortunately often ends up having little effect on car speeds before or after the intersection and may even cause some motorists to speed up in order to beat the light or make up for lost time.  The second impulse is to demand that the local authorities find a way to reduce car speed.

Cars’ actual operating speed is affected by many factors.  Some are specific to the individual motorist, car, or moment of time: personal attitudes and skills, vehicle capabilities, police presence, level of congestion, and the weather. However, because these vary widely across the population and times they don’t change the overall average operating speed.

Other factors are more external and permanent and affect almost every driver and car:  the road’s physical structure, the signage and pavement markings, the surrounding environment, and the speed limit.  Because they affect everyone in generally similar ways, these structural factors are considered the primary influence on overall operating speed.  Of the physical factors, the infrastructural components — the physical and painted layout and signage – are mainly preset, based on the Design Speed chosen to guide planning when the road was constructed or upgraded.

But Design Speed is a conceptual tool, not a speed limit.  Partly because straight wide roads invite it and partly because engineers understandably try to include a car-focused margin of safety to compensate for inevitable “driver errors” Federal Highway Administration researchers have shown that when roads are put into use their layout “often appear adequate for speeds far above the designated design speed.”  Making matters worse,  the higher manufacturing standards used for cars these days make going faster feel more comfortable, so motorists are likely to go even faster in a wider variety of road conditions.  And we do go fast.  Because the stress of our lives rushes us through the day, because our culture encourages us to push forward, and because it is so easy to press down on the gas pedal, most people (myself included) tend to drive as fast as the road comfortably allows.


In recognition of this reality, MassDOT’s  Speed Zone Regulations state:  “Numerous studies have indicated that drivers will not significantly alter what they consider to be a safe operating speed, regardless of the posted speed limit, unless there is constant heavy enforcement…[To be] both acceptable to the prudent driver and enforceable by police…the ideal speed limit is…[based on the] prevailing speeds of motorists on a particular section of a roadway under ideal conditions.”

Obviously, the best way to reduce car speed is to change road conditions.  But, in the absence of a history of car accidents or a recent headlined tragedy, when local activists demand structural changes that would dampen traffic flow they are often rebuffed when the current legal speed limit (and local practice) allows higher speeds.   And when they try to get around this by pushing for a Speed Zone Study to justify their desire for lower speed limits they run into the “prevailing speeds” issue – current transportation policy is implemented through formulas that  speed limits can’t be lowered if as few as 15% of motorists are already going faster.  And this is almost always the case:  studies show that “the majority of speed limits are posted below the average speed of traffic.”  Even worse: if a Speed Zone Study finds that the current threshold operating speed is high than the existing Speed Limit, the Transportation Agency is required to raise the speed limit — which is why most Speed Zone Studies get cancelled before their “official” end.

Catch 22 – it was exactly because local people felt that too many cars were going too fast that led to the desire to slow things down in the first place!


The source of the problem is the car-prioritizing values and procedures that remain at the core of transportation planning and analytic methods and expressed through current practices.   According to Massachusetts’ Procedures for Speed Zoning on State and Municipal Roadways manual, “The goal of our Speed Limit Traffic Control Program has always been… in the best interest of the motoring public’s right to use a roadway in a reasonable and proper manner….”  It is theoretically possible to stretch the words “reasonable and proper” to include all the other factors relating to the impact of motorized traffic on our social environment and public health as well as the rights of non-motorists to use public space.  But, in procedural reality, the “motoring public’s right to use a roadway” to go as fast as “ideal conditions” allow takes priority.  (For another procedural tool that leads to the same car-centric results, check out the way Level of Service analysis is usually done.)

In effect, the rules and procedures for setting speed limits are entirely focused on accommodating car drivers.  Period.  There is no method to incorporate any other factor besides simplifying enforcement.   Safety of pedestrians?  Encouragement of bicycling?  Impact on the social environment in public spaces?  No mention; no formal way to include; and certainly not prioritized.

The result is that despite widespread desire and many efforts to lower car speeds, it seldom occurs.


There is a solution.  It starts by affirming the core principal that motorist comfort should not be the primary determinant of Design Speed, road structure and markings, or the Speed Limit.  This is not actually a new or off-the-chart idea.  In fact, at both the federal and state levels, official policy now insists that the safety, comfort, and mobility of people traveling by foot, bike, bus, trolley, and train be given equal consideration as those in cars. Former US Transportation Secretary LaHood issued a new multi-modal-favoring policy and blogged that “This is the end of favoring motorized transportation at the expense of nonmotorized.”  In Massachusetts, because of its aggressive “mode shift” goals to triple the number of walking, bicycling, and transit trips, official policy even can be interpreted as prioritizing the needs of non-car users – an interpretation reinforced by the recent Healthy Transportation Policy Directive.

However, there remains a huge gap between policy and practice.  One way to begin operationalizing these policies is to put the concept of Target Speed – the “desired operating speed” – at the core of not only the Design Speed selection and Speed Zone Study processes but also every routine restriping, maintenance, and repair job.  This relatively new piece of Transportation jargon needs to be understood, and if necessary redefined, to include everyone’s safety; and the criteria used to calculate it must include the impact of car (and truck) movement on people’s willingness to use other modes, as well as on the large influence of transportation on the quality of life in the surrounding neighborhood.  And we need to go beyond statements of policy and principle to have the new approach incorporated in the guidelines, formulas, and evaluations of every Traffic project.

While MassDOT adjusts its internal operations, the process can be moved forward through passage of several bills now under Legislative consideration.  H.550, sponsored by  State Representative Denise Provost, will authorize locally-designated 20 mph Senior Citizen Safety Zones  in addition to the current School Zones – a first step towards allowing the creation, at local discretion, of Safety Zones near hospitals, playgrounds, railroad stations, senior housing, even business areas.  H.3129, also sponsored by Representative Provost, will set 25 mph, instead of the current 30, as the default speed limit for local roads in urban districts without explicit Speed Limit posting.   Call your Representative and let them know what you think!




Thanks to Tom DiPaolo and Peter Furth for extensive feedback on previous drafts; the remaining errors are entirely my own responsibility.


Related previous blog posts:

THE RIGHT TO BE ON THE ROAD: When Bicyclists Have To Pull Over, When Cars Can Pass

INTERSECTION CAMERAS, TERRORISM, AND TRUST: Fears and Memories Across the Generational Divide

CIRCLING THE BLOCK: Saving Money, Time, and Aggravation Through Parking Reform

ACTIVE TRANSPORTATION CREATES HEALTHY COMMUNITIES: How To Use Your Roads To Lower Your Doctor (and Insurance) Bills

MODE SHIFT AMPLIFIERS: The Importance of the Out-of-Vehicle Experience

VULNERABLE ROAD USERS (VRU) PROTECTION LAWS: “Whoever Can Do The Most Damage Has To Be The Most Careful”



One way to increase safety for non-motorists in the midst of large traffic volumes and speeds is to separate the various modes – wide sidewalks for pedestrians, cycle-tracks for cyclists, busways (with easy-access stops) for transit users, and lanes for motorized vehicles.  But building separate “ways” for each mode requires space and money which isn’t always available.  And separation isn’t always appropriate: many of us have lived in quiet residential neighborhoods without sidewalks where the kids all played in the street but traffic was so light and slow and drivers so aware of being in a shared space that even the most nervous mothers were unconcerned.

Another response is to slow things down to some desired target speed.  Summarizing the evidence, researchers point out that in urban or densely populated areas “the mean speed of traffic…has a more powerful effect on road accident fatalities than any other known risk factor, including the overall amount of travel.”  On any given road, the faster drivers go the more likely they are to have an accident and the more severe the resulting injuries: “with each 5 kph [3.1 mph] increase in travel speed, the risk of involvement in a crash involving injury doubles.”  Conversely, in urban areas it has been found that for every 1mph reduction in average speed there is a resulting 6% decrease in collision frequency as well as a decrease in injury severity.


A Speed Zone is a stretch of road where speeds are governed by a posted speed limit rather than by the default, statutory limit established by the state.  While speed limits were originally a local matter, policies long ago became centralized as part of a national movement to facilitate motorized travel (and commerce) against local efforts to force traffic to go slow.   When cars were first introduced to our streets in the early 1900s many local leaders tried to control this dangerous new entrant to the public right of way by imposing very low speed limits.  As the number of cars increased and the power of the automobile industry grew these limits were slowly raised and the concept of “jaywalking” was invented to channel pedestrians to the corners.   (Ironic historical note:  a lot of the original political pressure to put pavement on dirt roads in the early 1900s came from the first wave of bicyclists!)  However, in the pre-Interstate era, some districts figured out that if the main highway passed through their town they could raise a good portion of their annual budget by imposing extremely low speed limits at their border and then wait to arrest non-local, long-distance motorists who entered at regular speed – quick payment of a hefty fine would cancel the necessity of returning for a future court appearance.  (There are still some “speed trap towns” , mostly in the rural South and in Florida.)

It was in the context of these obstacles to regional travel that automobile advocates began demanding that states take over the job of setting speed limit.  And when the federal government became the major source of highway construction money one of the attached conditions was adherence to the standards set by National Association of State Highway Transportation Officials (AASHTO).  The FHWA language is that an appropriate Posted Speed Limit is the speed, under free-flow conditions at which motorists feel comfortable.  MassDOT’s language is similar:  the Speed Limit should be based on motorists’ “prevailing speeds…under ideal conditions.”  In practice, this is defined as the 85th percentile of actual operating speed – meaning that the limit should be set at the speed of the 85th fastest car out of every 100 – rounded up or down to the nearest multiple of 5.   In effect, as noted by Northeastern University Professor Peter Furth, “Cars went from being intruders to being the “normal” users of roads, and speed limit policy changed from protecting other users to protecting motorists from being slowed down.”

Of course, it is impractical to actually measure car speeds on every road.  So state legislatures set standardized default Statutory Speed Limits for various categories of road which don’t have Posted signage.  In Massachusetts they are:  65 mph on Limited Access Highways, 50 mph on a divided highway outside of a thickly settled district or a business district; 40 mph on any other roadway outside of a thickly settled district or a business district; and 30 mph within a thickly settled district or a business district.  (According to Chapter 90 of the Massachusetts General Laws, a “thickly settled district” is an area in which houses or buildings are, on average, less than 200 feet apart for a distance of one-quarter mile or more.”)  Although also set by statute, 20 mph School Zones are posted.

Where the speed limit is posted, police can issue a ticket based on measurements at a specific spot.  On unposted roads, where Statutory Speed Limits govern, the police have to observe the illegal speed over a distance of up to ¼ mile.  Combined with the common practice of giving drivers a 10 mph leeway, actually enforced speed limits are significantly higher than officially set ones.  As a result, it is extremely difficult to stop any but the most extreme violators on urban streets that rely on the default limit.

(New in 2012 is an as-yet-untested provision — Part III, Section b, page 13 of MassDOT’s Speed Zone Manual — that speeds may be set at up to 7 mph below the 85th percentile in “high accident” areas where “consistent enforcement…[will ensure] an acceptable level of conformance.”  This would allow a 25 mph limit to be set if the measured 85-p is as high as 32 mph.  The manual describes this as “more an exception than the rule” but aggressive municipalities with supportive police departments and pushy citizens might find some cover in this provision.)


The most effective speed-reducing strategy is to change the physical infrastructure through engineering and other measures, including programs such as Road or Lane Diets(thinning car lanes or reducing their number often done to widen sidewalks or add bike lanes), Traffic Calming (which uses diets and other techniques such as raised intersections, curb extensions, even speed humps), Complete Streets (which often re-allocates some road space for non-car users), Smart Growth (that makes roads visually seem more constrained through the addition of adjacent activity such as sidewalk cafes, cross-walk zebra markings, or new trees or the presence of busier cross-streets).

These changes have measurable impact.  At three completed traffic calming projects in Cambridge the measured speed (85th percentile, the speed of the 85th fastest car out of every 100 –see below) dropped from 30 to 21, from 31 to 26, and from 28 to 24.   The percent of drivers who yielded to a pedestrian in the crosswalk increased from under 20% to over 50% in the one project in which this was measured.  And making roads more multi-modal doesn’t necessarily reduce throughput.   Road capacity on Massachusetts Avenue in Central Square was reduced from 4-5 lanes down to 2-3, with the acquired space reprioritized for pedestrians, bicyclists, and transit.  Afterwards, while individual drivers occasionally experience a slower progression through the area during peak hours, the intersection still comfortably handled the same vehicular throughput as originally, about 21,000 Average Daily Traffic (ADT) vehicles.

But in some situations it is difficult, expensive, controversial, or simply not clear how to physically change a street to effectively slow car and truck speeds on long stretches of straight road, even in urban areas.   One response to these large traffic volumes and speeds is to separate the various modes – wide sidewalks for pedestrians, cycle-tracks for cyclists, busways (with easy-access stops) for transit users, and lanes for motorized vehicles.  But building separate “ways” for each mode requires space and money which isn’t always available.  And separation isn’t always appropriate: many of us have lived in quiet residential neighborhoods without sidewalks where the kids all played in the street but traffic was so light and slow and motorists so aware of being in a shared space that even the most nervous mothers were unconcerned.


Obviously, there are some roads where high speeds make sense:  long-distance Interstates and some major regional arterials.  But beyond those, there are few urban roads – residential or commercial – where going faster than 20 mph accomplishes very much besides getting you to the next red light.  In fact, both traffic “running speed” (the amount of time it takes to traverse a given distance) and traffic “throughput” (the number of cars that pass through some length of road and its intersections in a given amount of time) are almost never impacted by mid-block speed.

While not nearly as powerful as structural or striping changes, simply lowering the speed limit does have an effect, particularly in urban or densely populated areas – and is much less costly than either new construction or even paint.   Traffic’s overall operating speed usually only goes down a couple miles per hour.  More importantly:  the lower official limit seems to reduce the number of people going significantly faster than the legal speed.  Since speed reduces reaction time, this leads to fewer accidents and injuries.  It also narrows the spread between typical and fastest moving cars, which reduces the amount of “turbulence” on the road as faster drivers spurt and swerve around the slower ones — which is also directly related to the likelihood of accidents.

When several areas of Australia lowered the default urban speed limit by 10 kph (6.2 mph) from 60 kph (37 mph) to 50 kph (31 mph) accident frequency dropped by 13 percent, with nearly no increase in total trip time.  Most importantly, the tiny 10 kph reduction in default speed limits led to “vulnerable road users…gain[ing] the mostfrom…induced speed reductions.”   In the UK, a nation-wide movement has emerged around the slogan 20 is Plenty.   In townships that have adopted a 20 mph speed limit traffic fatalities – including both pedestrians and car occupants – have plummeted by nearly 60% and there has been a statistically significant increase in both walking and bicycling.  It is probably no accident that many of the northern European cities famous for their high rates of bicycling already have 19 mph [30 kph] limits – mostly gained through their ability to create lower-speed-limit “safety zones” wherever they think appropriate, meaning that up to 70% of the street-miles have 30 kph limits in many cities and towns.


When a road is being first planned or segments are being significantly redone, the Design Engineer selects an appropriate Design Speed based on the underlying terrain’s hills and curves and its Functional Class — the road’s status within a car-centric “natural hierarchy” ranging from low-traffic residential or slow-moving commercial streets, to “collectors” that gather and funnel local traffic into cross-town “arterials”, to region-spanning divided highways and Interstates.   While the Federal Highway Administration (HWA) technically sets a road’s status within the hierarchy, there isn’t much debate about the issue and there is no formal appeal process for changing a road’s designated type.   Once set, a road’s functional type influences a cascade of other default assumptions including the appropriate range of Design Speeds, lane widths, treatment of cross-streets, signage and signalization among others.  (Design is also influenced by anticipated future traffic volume in the selected Design Year, which is usually set for 20 or so years ahead, making the estimated volume very dependent on the formulas used to calculate the traffic implications of also uncertain economic development.  A significant percentage of past traffic projections have turned out to be high.)

Massachusetts’ 2006 Highway Design Guide (Section 3.2) was one of the first in the nation to also require that attention be given to the local context or “Area Type” and that replaced Functional Class with a broader set of “Road Type” options.   While this gives Project Managers greater potential flexibility, the process is constrained by so many other car-centric requirements and liability-avoiding-if-anything-goes-wrong standards of practice, that few MassDOT Designers move beyond the more conservative choices among the options theoretically open to them.

Further confusing the choice of Design Speed is the fact that there is little post-construction cross-checking between it and real-life Operating Speeds.   Even the FHWA acknowledges that “many researchers have noted that the design speed process has no systematic check for consistency between operating and design speeds…. [Design Speed] policy-compliant designs often appear adequate for speeds far above the designated design speed….”   So, of course, that’s what drivers do.


In additional to Functional Class or Road Type, there are other categorizations that affect road design decision-making process.  Major roads, arterials and some collectors, tend to be “state owned” roads, which are maintained by MassDOT.  (Just because a road has a state number does not mean it is state owned; some are municipal roads.  And not all state-owned roads have numbers.  The numbered network was created simply to facilitate car travel wayfinding around the state.)

Most smaller roads – residential, secondary commercial streets, some collectors and even some arterials – are owned by municipalities.  Despite the local ownership, MassDOT does a significant amount of the repair and improvement on these roads using federal and state funds, and even the locally-contracted work is paid for with state funds under Chapter 90.   This impacts Design and Operating Speeds because, as part of the complicated negotiations leading to the adoption of Massachusetts’ advanced-for-its-time Highway Design Guide in 2006, projects funded under Chapter 90 were exempted from state road design requirements.  And this allows some recalcitrant municipal officials to continue making car-centric road layout decisions, which invite faster car speeds or less car driver attentiveness, even as MassDOT adopts more multi-modal policies.


An even more significant negotiation was between the Massachusetts Highway Department and the Federal Highway Administration (FHWA) through which flows much of the money states have for road work.   As it does with every state, the FHWA retains oversight, meaning veto power, over projects that met certain thresholds.  In Massachusetts those thresholds are any work done on an Interstate, any work with a budget over $10 million, and any work with a budget over $5 million done on a road that is part of the National Highway System (NHS).  Roads designated as part of the National Highway System are selected by the state and the regional Metropolitan Planning Organizations and approved by Congress.   There are nearly 2,000 miles of NHS highway in Massachusetts.  Although MassDOT is trying to raise these thresholds, inflation has made them so low in current dollars that the FHWA ends up with approval power over roads like Mass Ave in Arlington and Beacon Street in Somerville.

Massachusetts and the FHWA negotiated for three years over the criteria to be used by the federal agency to evaluate road designs.  The results, detailed by MassDOT in a 2009 Engineering Directive, are based more on the American Association of Highway Transportation Officials (AASHTO) “Green Book” than the state’s Highway Design Guide.  These criteria include default assumptions (deviance from which require getting special permission through an “Exception Request”) of 12’-wide travel lanes, 8’-wide shoulders, and other highway-derived features intended to facilitate the safe and fast movement of as many cars as possible.

In fact, numerous studies have proved that even 10’-wide lanes are no more dangerous than 12’ lanes in urban areas.  And even the Green Book allows 11 foot lanes where speed limits are 35 mph or less.  But the reputation of the New England regional FHWA office for car-centric conservatism, and the desire of often overstretched Design Engineers to avoid the time and effort required by the Exception Request process, creates incentives for both MassDOT and local Transportation Officials to stay on the most traditional (e.g. wide lane) side of the range of possibilities rhetorically allowed by both the Green Book and the state’s own Highway Design Guide.  Reinforcing this tendency is the knowledge that projects that do include more aggressive traffic-slowing techniques often suffer time-consuming push-back from the regional FHWA.

In reality, most oversight activity deals with major projects.  There is little oversight of basic maintenance or restriping work, either by the FHWA over MassDOT or by MassDOT over municipal work.  Ironically, it is exactly during routine “resurfacing, rehabilitation, and reclamation (scarifying)” projects that it is possible to most cost-effectively change the layout of a road and make it safer for non-car travel.   However, the possibility of higher-level push-back from FHWA (or even from within MassDOT), combined with lack of experience with less traditional designs and fear of being held liable if anything goes wrong, keeps Project Managers in check even in these situations.


On trains and airplanes “safety is given highest priority and passengers must accept the time spent waiting on the ground [or in a station] for safety reasons.”  In contrast, it appears that a certain level of injury and death is considered and inevitable cost of car travel, at least in the United States.  In Sweden, the Parliament has adopted a “Vision Zero” policy that seeks to eliminate traffic-related death and serious injury by 2020 based on the principle that “Life and health can never be exchanged for other benefits within the society.”   One strategy has been to lower urban speed limits to 30 kph (19 mph).  (Newly elected NYC mayor, Bill de Blasio’s – a Cambridge Rindge & Latin Graduate – has announced his own Vision Zero” plan to eliminate traffic fatalities in the city by 2024.)

The least drastic strategy for slowing traffic is to allow the creation, at local discretion, of various kinds of 20 mph Safety Zones in addition to the current School Zones – near hospitals, playgrounds, railroad stations, senior housing, even business areas.  In Massachusetts, State Representative Denise Provost is sponsoring H.550 to authorize locally-designated Senior Citizen Safety Zones – you can ask your own Representative and Senator to become a co-sponsor!  (Opposition has focused on the possibility that a proliferation of new slow zones would confuse drivers and disrupt traffic flow – or even allow the re-emergence of the old speed-trap problem.)

Another approach is to change the default speed limit in locations without a posted Speed Limit state-wide.  H.3129, also sponsored by Representative Provost, would set 25 mph, instead of the current 30, as the assumed speed limit for local roads in urban districts.  (WalkBoston has acted as the central point for transportation activists pushing for these bills.)   While MassDOT has endorsed this bill, it has taken a long time to develop legal language that would limit the impact to local, urban roads while exempting rural and major roads.   Part of the problem is that if MassDOT’s current definitions of “local” are followed the bill would exempt municipal “collector” and “minor arterial” roads, which are where speeding is often even more of a problem than on minor local streets.

A better, combined approach has been implemented in the Netherlands combining local control over speed limits with federally-financed structural requirements.  Municipalities are allowed to set lower speed limits generally or in special safety zones, but only if they also implement physical changes such as traffic calming – for which the national government provides a 50% subsidy if the target speed is 30 kph (19 mph).


Section 3.6 of MassDOT’s Highway Design Guide states that “the objective in the planning and design of a roadway is to determine a speed that is appropriate for the context and results in a safe facility for all users, is consistent with the community’s goals and objectives for the facility, and meets user’s expectations.”  However, the way this statement gets operationalized by current MassDOT procedures turns this inclusive philosophy into a car-centric reality; and the user’s whose expectations are to be met turns out to primarily be car drivers.

But car drivers aren’t the only people on our roads.  A speed safe for cars may not – and often is not – safe for people trying to cross the road, in a wheelchair, pushing a baby carriage, or running or bicycling alongside it.  If roads are truly “public ways” – a corridor owned by the public and supposed to be managed for the benefit of the entire population – then neither road design nor speed limits should be shaped by traditional car-centric goals.

Furthermore, in recognition of the economic and environmental unsustainability of current our Transportation System, Massachusetts’ policy currently seeks to create a “mode shift” from cars to other forms of travel.  Accomplishing the announced goals requires ensuring that almost the entire growth in trips over the next 20 years will occur via transit, shared vehicles, bike, or foot rather than by single-occupancy cars (aka Single Occupancy Vehicles, SOV).  And accomplishing this requires increasing the relative desirability of non-car modes over SOV cars, which in turn requires prioritizing the alternatives over cars when doing road design, making investment decisions, and stepping up enforcement.   All of which makes slowing cars down in any mode-sharing situation a key part of successful mode-shift strategies.


Among its various innovations, the 2006 Massachusetts Highway Design Guide introduced a relatively new term: Target Speed.  This is the “desired operating speed” which, on an already existing road may be significantly different than the Design Speed used to guide construction or either the Posted or Default Speed Limit or the Operating Speed that most drivers actually do.  Target Speed is an extremely power but currently dangerously underused concept that needs to move to the center of road design and operational practice.

Hopefully, Transportation Secretary Davey’s new Healthy Transportation Policy Directive should make it more acceptable for MassDOT planners and engineers to not only use the full range of traffic-slowing possibilities available to them under current policy but to also begin experimenting with emerging practice in more progressive places.   But such a radical change from past practice will only occur if the public demands it.  One way to express that demand is to join the push for lower default speed limits and for the local ability to create Senior Safety Zones now pending in the state Legislature.

The vision underlying each of these speed-reducing strategies is to create more livable streets, to make our roads more directly support and reflect the type of community space that we want to live in. It has become clear that safe-for-cars speed is often dangerous for everyone else.  Lowering speed is a way to say that there are other users and other types of uses that are just as – and perhaps even more – important  than car traffic.  Once we know what we want our streets to be like, we can then decide on the structure and speeds that would most facilitate those uses.


SPEED GLOSSARY (from MA Highway Design Guide, Section 3.6 – Speed)

Design Speed:  “…the selected speed used to determine various geometric features [such as curves and sight lines] of the roadway [during construction and major upgrades]”

Operating Speed:  “measured speed…in fair weather…during off-peak hours…at discrete points….reported using percentile speeds”

Posted Speed Limit:  “… based on an evaluation of the observed operating speeds… [generally set at] the speed at which 85 percent of vehicles are traveling at or below”

Running Speed:  time a car takes “to traverse a set distance…”

Target Speed:  “the desired operating speed…”


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