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How Should they be Determined?


By the late Professor Patricia F. Waller, Ph.D., (Emerita)


Dept. of Social and Behavioral Analysis, University of Michigan Transportation Research Institute


The following paper was given as an address to

the Sydney Chapter of the Australian College of Road Safety

September 6, 2001.  



The primary purpose of our highway system is mobility, not safety. If it were the latter, we would set speed limits much lower. A 20 mph speed limit, with governors on all motor vehicles to prevent exceeding that speed, would certainly lower highway casualties. Of course, no one would consider such a measure, Clearly, we tacitly agree to accept a certain level of carnage in order to use the highways in ways we value.  At the present time in the US, this tacit agreement says that it is acceptable to sacrifice between 40,000 and 42,000 lives annually.


In seeking an acceptable balance between mobility and safety, speed limits are a critical factor. To what extent is speed a factor in the likelihood of a crash occurring in the first place? An early and widely cited study (Solomon, 1964) found that as speed deviated from the average speed of the traffic stream, crash probability increased. He reported that speeds significantly below that of the traffic stream were at least as likely to cause crashes as those above the average speed. Solomon's data were from the 1950s and were based on rural roads, most of which were 2-lane. Much has changed since then, and, not surprisingly, more recent studies report somewhat different findings.  

In a review of speed and crash probability, Shinar (1999) cites extensive evidence that speed variance increases crash probability. Yet in 2000, 55 percent of fatal crashes in the US were single vehicle. Almost one‑fourth of these involved a pedestrian or bicyclist, where speed could have been a factor, but 42 percent of all fatal crashes involved only one vehicle, with no pedestrian or cyclist (National Highway Traffic Safety Administration, 2001). In these crashes, speed variance per se is less likely to be a factor. Shinar reports that, while there is some evidence that on a given road, crash involvement rates for individual vehicles rise with their travel speed there is no convincing evidence that across all road types, crash rates increase with overall average speed. Of course, overall average speed is highly correlated with road design, and higher speed limits are found on roadways with safer design. While the role of speed deviation per se may not be settled, there is unequivocal evidence that probability of injury and fatality increases with increasing speed. Shinar concludes that: "The overall cost of speed‑related crashes is much greater than the relationship between speed and crash probability indicates. This is because high‑speed crashes are associated with greater injury levels than are low‑speed crashes." (p 271)  


The 55 mph National Maximum Speed Limit  

In 1973 the US experienced a fuel crisis that had repercussions throughout the economy. Congress enacted legislation establishing a National Maximum Speed Limit (NMSL) of 55 mph. Although enacted for purposes of fuel economy, it resulted in a reduction in highway fatalities of over 9000 in the first year. When the immediate fuel crisis had passed, and cars were no longed lined up at the gasoline stations, an effort was made to retain the NMSL for safety reasons. There was strong opposition. Congress requested an independent review of the evidence, and in 1984 the Transportation Research Board issued their report (Committee for the Study of the Benefits and Costs of the 55 mph National Maximum Speed Limit, 1984), making the following points:  

Fuel Economy. The legislation was initially enacted for purposes of fuel economy. Gasoline consumption declined in 1974, for a savings of about 255,000 barrels per day (BPD) of petroleum, and it was estimated that in 1983, the lower speed limits accounted for savings of about 167.000 BPD. Although additional oil reserves have been discovered since 1974, there has been no absolute increase in the total amount of fossil fuel on the planet Earth. Consequently, the long‑term importance of conserving fuel has not diminished.  

Safety. The review concluded that the unprecedented decrease in highway fatalities, over 9000 lives, immediately following the enactment of the NMSL resulted from many factors including reduced exposure and reduced discretionary driving. However, taking into account other variables that may have contributed to the safety benefits, the report concluded that the NMSL continued to save lives. They estimated that, in the early years of the 55 mph NMSL the lower speeds saved about 3000 to 5000 lives annually. In 1984, there continued to be between 2000 and 4000 fewer fatalities, between 2500 and 4500 serious, severe, and critical injuries, and between 34,000 and 61,000 fewer minor and moderate injuries as a result of the lower speed limits (pp 166‑67).  

At a change in velocity of 50 mph, the probability of the driver being killed is twice what it is at 40 mph. Incremental increases in speed add disproportionately to the probability of serious or fatal injury,  

Motor Vehicle Injury Costs.  In the US, injury, both intentional and unintentional, is the leading cause of death from age 1 to age 45. Because it so disproportionately strikes the young, it is also the leading cause of lost years of life prior to age 75, more than either cancer or heart disease. Motor vehicle injury is the largest single component of these losses (Committee on Injury Prevention and Control, 1999). A driver license from one state enables one to drive in other states, and emergency care and medical services must be provided wherever an injury occurs Even if one is not injured, the costs incurred through taxes and health and auto insurance premiums reflect any increases in injury‑associated costs, The cost of motor vehicle injuries crosses state lines - it is a national problem, not just a state or local one.  

Time Savings. The 55 NMSL increased travel time, and it is this issue that generated the greatest opposition. Interestingly, the report found that, "On the basis of average trip lengths and average speeds, most personal vehicle travel slowed down by the 55 mph speed limit involved time losses of less than 3 minute? (p. 115). One committee member noted that, "Over a year's time it may amount to about a day and a half for me, but if my day were broken up into 2 and 3‑minute pieces, 1 don't think I could get much done." Small units of time do not combine to create useable time segments.  

It was further found that it cost "roughly 40 years of driving per life saved and serious injury averted." However, "the average life expectancy of motor vehicle accident victims in 1982 was about 41 year” (p 120). This comparison does not take into consideration the enormous time costs incurred by family and friends of the killed and seriously injured, or the time costs associated with the many more injuries that are less serious, or crashes involving property damage only. Furthermore, crashes are the primary cause of traffic delays, creating enormous time costs to travellers. Thus, when all time costs are considered, the total societal time costs are less at 55 mph than they are at higher speeds.  

Equity. There is also the issue of how time costs are distributed across the population At the lower speed limit whatever time costs are incurred are distributed fairly evenly across the entire travelling public. At the higher speed limits, time costs are borne disproportionately by those who are involved in crashes, especially those who are seriously or fatally injured. Because for most trips, at the lower speed the additional time involved is three minutes or less, the trade‑off between arriving at a destination a few minutes later versus sacrificing a young life is not one that should be difficult to choose.  

Other issues either not covered or touched on only briefly in the report include commercial trucking, composition of the vehicle fleet, consumer expenditures, national security, balance of payments, environmental impact, and our changing demographics. Each of these is discussed below.  

Commercial Trucking. In response to the enactment of the 55 mph NMSL the trucking industry requested, and received, increases in allowable truck size and weight. It was argued that additional cargo was needed to offset the lower speeds, It was found, however, that the lower speeds led to savings in fuel, maintenance, and safety, offsetting the time costs incurred. Most of the trucking industry (excepting the independent operators) supported the 55 mph NMSL. It should also be noted that as speeds have increased, there have been no corresponding decreases in truck size and weight limits.  

Vehicle Fleet. One argument for increasing speed limits on some highways is that they were designed for higher speeds. There is some truth to this, but the design standards were for a vehicle fleet different from what we have today. Passenger cars averaged around 4000 pounds and trucks were smaller and lighter. In response to the fuel crisis, car size was reduced, and trucks got larger and heavier. Although passenger vehicles are much safer today than they were 30 or 40 years ago, the safety gains realized are compromised by the higher speeds.  

Consumer Costs. Even though, in the US, fuel prices are low compared to the rest of the industrial world, in 1999 the average household spent over $1000 on gasoline (Bureau of Labor Statistics, 2001). With today's higher prices, it would be considerably more. Lower speeds translate into greater fuel economy­. They also reduce costs of vehicle maintenance and repair.  

National Security. Reduced fuel consumption reduces our dependence on foreign oil supplies. In 1973, in the US, our net imports of foreign oil comprised 35 percent of petroleum consumption. In 2000 this share had increased to over 50 percent and is expected to reach 64 percent by 2020 (US Department of Energy, 2001a). In 1972, 43.5 percent of our net imported petroleum came from OPEC countries. In 1999, it was 46 percent (US Department of Energy, 2001b). The costs of the Persian Gulf War were directly attributable to our dependence on petroleum imports from that region, and we continue to maintain a strong military presence there, as well as provide substantial support to allies in that part of the world.  

Balance of Payments. Our dependence on foreign oil is a major source of our negative balance of payments. In 1974, imported petroleum exceeded exported petroleum by almost $24 billion. In 2000, the excess came to almost $109 billion and accounted for 28 percent of our balance of payments deficit (US Department of Energy, 2001 c; US Department of Commerce, 2001). Reducing fuel consumption would reduce this deficit.  

Environmental Impacts. Motor vehicle fuel consumption is a major contributor to environmental pollution, including global warming. In the US, of fuel consumed in transportation of all kinds, passenger vehicles and small trucks account for the largest single component. Transportation was also the largest single source of carbon dioxide emissions in the US in 1999, and motor gasoline was the largest contributor to these emissions. Petroleum spills continue to be a source of water pollution, and traffic is an increasing source of noise pollution in neighbourhoods (Bureau of Transportation Statistics, 2001).  

Changing Demographics. Perhaps the most significant issue in setting speed limits, as well as the one most often overlooked, is the changing demographics of our population. Global life expectancy has grown more in the last 50 years than in the previous 5000. Prior to the Industrial Revolution, people aged 65 and over accounted for no more than 2 or 3 percent of the population in the developed world. Today they account for about 14 percent, and are estimated to reach 25 percent by 2030 (Peterson, 1999). In the US, Florida presently has the highest proportion of older citizens, about 19%. However, by 2025, 39 of our 50 states will have similar proportions (Committee for Economic Development, 1999). Italy will achieve this status in 2003; Japan and Germany in 2005 and 2006, respectively; and France and Britain by 2016. Canada will be there in 2023 (Peterson, 1999).  

These changes in our population are not simply the result of greater longevity. In the developed nations, fertility rates have fallen far below replacement level (Commission on Global Aging, 2000). The younger population is decreasing in proportion, and, in many countries, in absolute numbers as well. The US is somewhat better off than most other industrialised nations because of our high immigration.  

This increase in life expectancy, with a corresponding decrease in birth rate, heralds a much larger elderly population driving and using our roadways. Older people are by far the fastest growing segment of our driving population. They are increasing their numbers and proportions in the licensed population, and both their trips and their total mileage are increasing. Unfortunately, age takes its toll and there is some loss of driving proficiency with increasing age. While some of this loss is undoubtedly attributable to medical conditions that occur with increasing frequency with increasing age, some of the loss accompanies "normal" aging. For most people, vision changes occur by the mid‑40s and continue thereafter, and divided attention becomes more difficult with increasing age.  

Overall, there is an increase in crash risk per mile driven, beginning in the late 50s and early 60s, with more marked and accelerating increases in the 70s and older (National Highway Traffic Safety Administration, 2000; Peck and Romanowicz, 1993/94). This increase occurs even though everything we know about older drivers tells us that, as a group, they try very hard to limit their driving to the safest times and places. Older drivers are underrepresented in crashes involving speed, alcohol, or reckless driving. Because age is also correlated with increasing vulnerability to injury, older people are more likely to be seriously or fatally injured in a crash of specified dimension (Evans, 1991).  

What are the implications of these changing demographics for speed and speed limits? First, we are seeing an increasing number of older drivers on our roads. Second, there is no question that, as a group, their driving proficiency decreases with increasing age. Third, their transportation needs do not decrease simply because they can no longer drive. Data from the Nationwide Personal Transportation Survey show that, for all ages, work‑related trips account for fewer than one‑third of trips taken. Up until the mid‑80s, older people take about as many, if not more, non‑work‑related trips as those aged 16 to 64 (Rosenbloorn, in press). Fourth, there will be fewer younger drivers available to meet the transportation needs of these older people. There is clearly a need to enable older drivers to continue to meet their own transportation needs for as long as possible.  

There is currently considerable effort to do just that. Federal Highway Administration is seriously addressing modifications in the highway system to assist older drivers, and hence everyone else who is not functioning at optimum level (Staplin, Lococo, and Byington, 1998). Other work continues on facilitating the driving task, with a focus on older drivers (Schieber, in press., Staplin, in press), including how intelligent transportation systems may be of assistance (Caird, in press). In the private sector, vehicle manufacturers are at last addressing the needs of older drivers (Pike, in press).  

There is also considerable attention being paid to licensing, original and renewal, of older drivers, but here the problems are much more challenging. In some states, the laws specifically forbid any special requirements for licensure based on increasing age. Thirty states have no safety-related provisions whatsoever for renewal licensure of older drivers. Some "special provisions" for older persons are designed to make it easier for the older applicant, e.g., in Tennessee, licences issued to people aged 65 and older do not expire, and in North Carolina, applicants aged 60 and older are exempt from parallel parking on the road test. The most common special provision is a shortened renewal cycle or a requirement for in‑person renewal (Insurance Institute for Highway Safety, 2001). There is some evidence that vision testing of older drivers is effective in reducing crashes (Shipp, 1998), and that more sophisticated vision testing may possibly be more effective (Owsley, in press). However, on the whole, measures currently used in driver licensing renewal have shown little or no relationship to prior or subsequent driving performance as indicated in driver records. Although there is considerable research being conducted in this area, and from time to time, we hear that someone has discovered the “magic bullet," we still do not know how to identify those older drivers who are more likely to have future crashes.  

We do know, however, that as drivers age and become less proficient, they respond as other drivers do when the task becomes more complex - they reduce their speed. Older drivers are increasing their presence on our roadways, and many of them cannot drive safely at the higher speeds. To ban them from the interstate or other high speed roadways means excluding them from the safest roads in existence. In 1995, our Congress saw fit to remove the federal requirement that states retain the 55 mph speed limit on rural interstates. In those states that raised the speed limit, highway fatalities increased on interstates, an average of 15 percent, and fatality rates increased 17 percent (Farmer, Retting and Lurid, 1999).




Higher speeds reduce time to respond to emergency situations, increase stopping distance, and increase both crash risk and crash severity. On virtually every dimension considered, including fuel economy, safety, consumer and societal costs, environmental pollution, national security, and time costs, the lower speed limits make more sense. Most importantly, they make more sense in light of the unprecedented changes occurring in our population and the increasing presence of older drivers on our highways. It is to everyone's advantage to enable these drivers to continue to meet their own transportation needs without posing undue hazard to themselves or other road users. If we are fortunate, eventually we will all be the beneficiaries of such a policy.  



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