This report was prepared by E. H. Pechan and Associates for the U.S. Environmental Protection Agency, Office of Policy, Planning, and Evaluation. For further information, please contact Bob Noland at 202-260-2418. Analysis of the Effects of Eliminating the National Speed Limit on NOx Emissions Introduction Highway vehicles contribute approximately one third of the oxides of nitrogen (NOx) emissions released to the atmosphere in the United States annually (EPA, 1994). Since highway vehicles contribute such a large percentage of NOx emissions, proposed modifications in the National Highway System bill are examined in this analysis to determine their effect on this important source category. Specifically, the U.S. Senate has recently proposed to abolish the national maximum speed limit. Such a policy modification may have a significant impact on the magnitude of NOx emissions from motor vehicles. Motor vehicle NOx emissions result from combustion processes and tend to increase with increasing speeds above 48 miles per hour (mph) (Pechan, 1992). This analysis examines the potential consequences of the proposed Senate changes to the National Highway System bill on highway vehicle NOx emission levels. The Senate recently voted to repeal the national maximum speed limit on federally financed highways. The national maximum speed limit rule currently restricts vehicle speed limits to 65 mph on rural freeways, and 55 mph for all other corridors. The Senate proposal to repeal the national speed limit passed on June 19, 1995 with the stipulation that federal speed limits still apply to commercial vehicles such as trucks and buses. If the bill passes through the House of Representatives, States would have the authority to determine the maximum allowable speed limits for automobiles within their State boundaries. Trucks and buses would still be subject to the 55 mph urban and 65 mph rural limits. The national maximum speed limit was established in 1974; it restricted speed limits to 55 mph. The impetus for this regulation was to conserve fuel during the 1973 oil embargo and subsequent energy crisis. Prior to the national limit, States determined speed limits for all corridors within their jurisdictions. Table 1 presents the speed limits maintained by States in January 1973, prior to implementation of the national maximum speed limit. Also presented in Table 1 are current speed limits for automobiles and trucks, by State. In 1987, the maximum speed limit on rural freeways was increased to 65 mph. Rural freeways are defined as freeways located in designated "rural" areas, with a population less than 50,000. Freeways are "controlled access facilities" which means that access is by ramp only and freeways are divided highways which usually consist of four lanes. The likely effect of the recent Senate proposal abolishing the national speed limit will be increases in rural freeway speed limits for automobiles. Since most States adhered to the 55 mph speed limit in urban areas before the national maximum speed limit was established, changes in speed limits around metropolitan areas are expected to be minimal (FHWA, 1995). As shown in Table 1, speed limits on rural freeways before the national maximum limit of 65 mph did not exceed 75 mph, with the exception of Montana and Nevada -- which did not establish maximum limits. Most maximum speed limits for rural corridors ranged between 65 and 75 mph with the majority of States setting rural speed limits of 70 mph. This analysis assumes that States are most likely to raise the maximum speed limits on their rural freeways to limits established prior to the national maximum speed limit and retain the 55 mph limit in urban areas. Current Effects of Speeds Limits Before estimating the likely effects that abolishing the national speed limit will have on interstate speeds, it is helpful to examine the effectiveness of the 55 mph speed limit. Based on data from the Federal Highway Administration (FHWA) published in the 1993 Highway Statistics report that analyzes trends in speeds on highways signed for 55 mph, a majority of vehicles exceed this posted limit (FHWA, 1994). In 1993, 70 percent of drivers exceeded 55 mph on urban interstate highways and 78 percent exceeded the speed limit on rural interstate highways. Only highways with a posted 55 mph speed limit are included in this average. The average speed recorded on the urban interstate highways was 59 mph and approximately 61 mph on rural interstate highways. In addition, 18 percent of drivers on urban interstates and 24 percent of drivers on rural interstates were already exceeding 65 mph on 55 mph roads. Prior to the setting of the national speed limit, FHWA reports that the average speed of free-moving traffic on level, straight, uncongested sections of the rural interstate system reached a peak of approximately 65 mph in 1973. This compares to a current average speed on rural interstates posted with 55 mph limits of 61 mph. It is important to consider that the average speed in 1973 was calculated at ideal conditions whereas the current average speed includes all vehicle travel on the rural interstate system, not just the straight, uncongested portions analyzed for the 1973 data. Analysis Methods This analysis was performed within the constraints of the MOBILE5a model. Therefore, the maximum speed that could be examined was 65 mph. Two cases were compared with the 1994 highway vehicle NOx emission inventory prepared for EPA's Emission Trends report--a maximum effects case and a likely effects case. In the base case analysis, the speeds modeled for each road type and vehicle type were determined using data from FHWA's 1987 through 1990 HPMS impact analyses. Speeds varied less than 1 mph over this time period for any given vehicle type/road type combination. Therefore, the 1990 data were used and aggregated to determine average speeds for three vehicle classifications (light duty vehicles, light duty trucks, and heavy duty vehicles). Table 2 presents the national distribution of VMT by vehicle class and road type. Table 3 summarizes the speeds modeled in the four road classes of interest (rural interstates, rural principal arterials, urban interstates, and urban other freeways and expressways) in this analysis. Emissions were estimated at the county, monthly, road type level. As seen in Table 2, these four road types account for 39 percent of the total VMT. To get an estimate of the maximum effects of abolishing the national speed limit, speeds for all vehicle types on all of the road types listed in Table 3 were modeled at a speed of 65 mph. This assumes that urban speed limits would be increased as well as the rural speeds and that heavy duty vehicles would also be able to increase to 65 mph. Due to factors such as congestion from heavy peak hour volumes, it is unlikely that urban speeds would ever reach an average of 65 mph even if the speed limits on these roads were increased. However, modeling this case gives an estimate of the upper bounds of possible emission increases. Other than the change in speeds for these four road types, all other modeling was done identically to the base case. A more realistic case was also examined. In this case, the base case speeds were modeled for the heavy duty vehicles. This corresponds with the version of the bill that has passed the Senate, in which these vehicle classes would be subject to the current limits of 55 mph or 65 mph. In addition, it was assumed that urban speeds for all vehicle types would remain unchanged from the base case, reflecting the reality of the lower average speeds observed on urban interstates. Finally, the nine States shown in Table 1 to have maximum limits of 65 or lower prior to the setting of the national speed limit were modeled at the same speeds used in the base case. As seen in Table 2, light-duty VMT on rural roads accounts for only 18 percent of the total 1994 VMT. With nine States modeled at base case speeds, the percentage of VMT modeled in this case at 65 mph would be even less than 18 percent. Again, all inputs other than speed used in the base case were modeled in this case as well. Results Tables 4, 5, and 6 present the NOx emissions calculated for the base case, the maximum effects case, and the likely effects case, respectively. These results are presented at the State level, and with emissions from heavy duty vehicles separated from emissions from light duty vehicles. In this manner, results from the three cases can easily be combined in other configurations if changes occur in the present status of disallowing heavy duty vehicles from the higher speeds, or if urban speeds in certain States are likely to approach the higher rural speeds. Table 7 compares the results from the likely case with the base case results. As seen in this table, NOx emissions are projected to increase by 5.2 percent with the elimination of the national speed limit. States such as Montana, North Dakota, and South Dakota that have high rural interstate VMT relative to urban VMT show the greatest increases in NOx. In contrast, States like New Jersey, California, and Illinois that have high urban VMT relative to rural VMT show relatively small emission increases resulting from the change in speeds. Related Issues This analysis was completed using MOBILE5a. The MOBILE model is designed to model vehicle speeds up to 65 mph. Therefore, it was not possible to use to the MOBILE model to estimate emissions resulting from vehicle speeds in the 70 to 75 mph range. Forty-two States posted speed limits above 70 mph prior to the maximum national speed limit set in 1974. It is, therefore, realistic that the majority of States will post speed limits in this range again. A rough estimate using the State of Texas as a sample indicates an increase in NOx emissions of an additional 11 percent for light duty vehicles on rural corridors if emissions are modelled using an emission factor reflecting emission rates applicable to vehicles travelling at 70 mph. This example was calculated assuming a linear increase in the emission factor with speed after 48 mph. This example illustrates that the effects of repealing the maximum national speed limit on NOx emissions may be more dramatic than the results in this analysis -- using MOBILE5a -- imply. Although this analysis examines the potential increase in highway vehicle NOx emissions as a result of increased automobile speeds on rural highways, carbon monoxide (CO) emissions are likely to increase as well. Both CO and NOx emissions result from combustion processes. At lower speeds, around 15 mph, motor vehicle emissions of CO and NOx decrease with increases in vehicle speed as a result of more efficient combustion. However, after 48 mph, increases in vehicular speeds are accompanied by increases in emissions of both CO and NOx (Pechan, 1992). Thus, increases in highway vehicle CO emissions are also likely as a result of the proposed changes to the National Highway System bill. Using the same reasoning, particulate matter (PM) emissions may also increase. Conclusions The results of this analysis indicate a 5 percent increase in motor vehicle NOx emissions annually on a national level. Current controls on NOx emissions are focused in ozone nonattainment areas where NOx and HC emissions react with sunlight to form tropospheric ozone. Although NOx emissions will increase on a national level as a result of repealing the maximum national speed limit on rural corridors, the net effect on ozone levels will likely be much less significant. Ozone nonattainment areas are predominantly urban areas where automobile speed limit changes are not as likely to occur. Despite the effect of transport, the increases in NOx shown in this analysis on a national level may not necessarily contribute to parallel increases in ozone formation. In order to more accurately assess the implications on air quality of the proposed Senate rule, a more thorough analysis should be conducted which analyzes increases in highway vehicle CO and PM emissions as well. Moreover, emission estimates should be determined using an appropriate correction factor to estimate emissions generated at 70 mph speeds. The effect of increased motor vehicle NOx emissions resulting from light duty vehicle speed increases on rural corridors on ozone nonattainment areas also deserves more in depth attention. References EPA, 1994; U.S. Environmental Protection Agency, "National Air Pollutant Emission Trends, 1900-1993" OAQPS, Research Triangle Park, North Carolina, October, 1994. FHWA, 1994; Federal Highway Association, "Highway Statistics 1993" Washington, DC, 1994. FHWA, 1995; Federal Highway Association. Personal Communication with Ms. Julie Cirillo, June, 1995. Pechan, 1992; E.H. Pechan & Associates, "Sensitivity Analysis of MOBILE4.1 Emission Factors" Prepared for EPA Ozone/Carbon Monoxide Programs Branch Springfield, Virginia, July 1992. Table 1 Maximum State Speed Limits State State Speed Limit Current Comments Prior to 1974 Speed Limits (mph) (mph) Automobiles Trucks Alabama 70 65 65 Alaska 70 65 65 Arizona 75 65 65 Arkansas 75 65 65 California 70 65 55 Colorado 70 65 65 Trucks lower in mountains Connecticut 60 55 55 Delaware 60 55 55 DC 60 55 55 Florida 70 65 65 Georgia 70 65 65 Hawaii 70 55 55 Idaho 70 65 65 Illinois 70 65 55 Indiana 70 65 60 Iowa 75 65 65 Kansas 75 65 65 Kentucky 70 65 65 Louisiana 70 65 65 Maine 70 65 65 Maryland 70 55 55 65 mph on July 1, 1995 Massachusetts 65 65 65 65 mph on Mass. Tnpk only. All other locations 55 mph Michigan 70 65 55 Minnesota 65 65 65 Mississippi 70 65 65 Missouri 70 65 60 Montana unlimited 65 65 Nebraska 75 65 65 Nevada unlimited 65 65 New Hampshire 70 65 65 New Jersey 70 55 55 New Mexico 70 65 65 New York 55 55 55 65 mph on August 1, 1995 - all vehicles North Carolina 70 65 65 North Dakota 75 65 65 Ohio 70 65 55 Oklahoma 70 65 65 Oregon 75 65 55 Pennsylvania 65 55 55 65 mph July 13, 1995 Rhode Island 60 55 55 South Carolina 70 65 65 South Dakota 75 65 65 Tennessee 75 65 65 Texas 70 65 60 Trucks limited to 60 mph daytime and 55 mph nighttime Utah 70 65 65 Vermont 65 65 65 Virginia 70 65 65 Washington 70 65 60 West Virginia 70 65 65 Wisconsin 70 65 65 Wyoming 75 65 65 Source: Department of Transportation/Federal Highway Association Table 2 1994 Vehicle Miles Travelled (VMT) by Road Type and Vehicle Class Road Total VMT Percent Percent Percent of Type of Total of Total Total VMT VMT VMT (Heavy (Light Duty) Duty) RURAL Interstate 214,757 1.0 8.1 9.1 Other Principal Arterial 209,017 1.0 7.9 8.9 Minor Arterial 153,503 0.7 5.8 6.5 Major Collector 183,281 0.9 6.9 7.8 Minor Collector 49,932 0.2 1.9 2.1 Local 105,430 0.5 4.0 4.5 Total Rural 915,919 4.5 34.6 39.0 URBAN Interstate 322,023 0.6 13.1 13.7 Other Freeways 144,284 0.3 5.9 6.1 Expressways Other Principal Arterial 360,146 0.7 14.6 15.3 Minor Arterial 279,419 0.5 11.4 11.9 Collector 122,536 0.2 5.0 5.2 Local 202,918 0.4 8.3 8.6 Total Urban 1,431,325 2.8 58.2 61.0 TOTAL 2,347,244 7.2 92.8 100.0 Table 3 Average Speeds by Road Type and Vehicle Type Modeled for the Base Case Rural Urban Other --------------------- ----------------------- Vehicle Type Interstate Principal Interstate Freeways Arterial Expressways Light Duty 60 mph 45 mph 45 mph 45 mph Vehicles Light Duty Trucks 55 mph 45 mph 45 mph 45 mph Heavy Duty 40 mph 35 mph 35 mph 35 mph Vehicles Table 4 1994 Highway Vehicle NOx Emissions with Current Speed Limits (Base Case) 1994 Base Case NOx Emissions (tons) Light-Duty Heavy-Duty All State Vehicles Vehicles Vehicles Alabama 108,578 53,320 161,898 Alaska 10,843 4,500 15,343 Arizona 78,660 35,363 114,023 Arkansas 57,374 30,694 88,067 California 500,401 213,317 713,718 Colorado 75,598 32,384 107,982 Connecticut 64,465 23,245 87,710 Delaware 15,142 6,902 22,044 District of Columbia 7,737 2,286 10,023 Florida 252,135 108,530 360,665 Georgia 179,504 81,615 261,119 Hawaii 16,470 6,983 23,453 Idaho 28,555 14,902 43,457 Illinois 213,507 83,066 296,573 Indiana 147,618 68,419 216,037 Iowa 63,270 31,458 94,727 Kansas 58,571 27,933 86,504 Kentucky 93,233 45,802 139,035 Louisiana 76,049 38,144 114,193 Maine 30,784 16,103 46,887 Maryland 98,724 38,949 137,673 Massachusetts 108,300 36,795 145,095 Michigan 206,614 83,471 290,085 Minnesota 105,734 45,987 151,721 Mississippi 62,248 35,023 97,270 Missouri 131,072 59,523 190,594 Montana 22,571 11,854 34,425 Nebraska 36,876 18,277 55,152 Nevada 25,996 11,843 37,839 New Hampshire 26,392 12,325 38,717 New Jersey 129,949 47,339 177,288 New Mexico 44,555 22,952 67,507 New York 255,046 98,126 353,172 North Carolina 162,538 78,311 240,850 North Dakota 16,118 8,190 24,308 Ohio 233,350 97,841 331,190 Oklahoma 83,252 39,856 123,108 Oregon 68,076 32,079 100,156 Pennsylvania 211,943 92,727 304,670 Rhode Island 16,431 5,371 21,802 South Carolina 83,695 43,879 127,574 South Dakota 19,791 10,472 30,263 Tennessee 123,994 56,331 180,325 Texas 379,434 160,823 540,257 Utah 41,066 16,572 57,637 Vermont 15,528 7,698 23,227 Virginia 151,518 70,200 221,718 Washington 111,867 43,970 155,838 West Virginia 40,811 21,793 62,603 Wisconsin 120,804 55,905 176,709 Wyoming 17,972 9,503 27,475 Total 5,230,757 2,298,948 7,529,705 Table 5 1994 Highway Vehicle NOx Emissions with Maximum Effects of New Speed Limit (Max Case) 1994 Max Case NOx Emissions (tons) Light-Duty Heavy-Duty All State Vehicles Vehicles Vehicles Alabama 127,531 68,944 196,476 Alaska 12,626 5,977 18,603 Arizona 91,844 46,780 138,623 Arkansas 70,699 42,242 112,941 California 659,405 296,651 956,056 Colorado 93,065 45,069 138,134 Connecticut 84,009 31,378 115,387 Delaware 18,844 9,096 27,939 District of Columbia 9,211 2,709 11,920 Florida 299,930 144,682 444,612 Georgia 217,807 107,692 325,499 Hawaii 20,091 8,159 28,250 Idaho 33,742 19,813 53,555 Illinois 253,580 107,783 361,363 Indiana 174,579 89,146 263,725 Iowa 76,480 43,497 119,978 Kansas 71,893 37,959 109,851 Kentucky 113,245 61,522 174,767 Louisiana 89,839 49,970 139,809 Maine 35,515 20,660 56,175 Maryland 126,817 53,138 179,955 Massachusetts 137,296 48,748 186,044 Michigan 252,893 109,273 362,166 Minnesota 132,464 62,094 194,558 Mississippi 72,498 45,020 117,518 Missouri 167,001 83,125 250,126 Montana 27,322 17,026 44,348 Nebraska 43,766 24,731 68,497 Nevada 30,988 16,321 47,309 New Hampshire 32,351 16,897 49,248 New Jersey 159,460 61,426 220,886 New Mexico 52,269 31,958 84,227 New York 317,253 126,359 443,612 North Carolina 192,285 100,044 292,330 North Dakota 19,531 11,508 31,039 Ohio 282,603 126,366 408,969 Oklahoma 100,295 52,658 152,953 Oregon 84,930 45,058 129,988 Pennsylvania 254,841 121,159 376,000 Rhode Island 20,489 6,998 27,487 South Carolina 98,405 58,821 157,226 South Dakota 23,804 14,612 38,416 Tennessee 148,259 75,407 223,667 Texas 476,931 216,639 693,570 Utah 50,521 23,425 73,946 Vermont 18,131 10,162 28,293 Virginia 182,925 93,046 275,971 Washington 142,421 60,383 202,804 West Virginia 48,686 29,510 78,196 Wisconsin 144,875 74,397 219,271 Wyoming 21,408 13,445 34,853 Total 6,417,650 3,069,488 9,487,138 Modeled with 65 mph speeds on rural interstates and principal arterial roads and on urban interstate and other freeways and expressways for all vehicle types. Table 6 1994 Highway Vehicle NOx Emissions with Likely Effects of New Speed Limit (Likely Case) 1994 Likely Case NOx Emissions (tons) Light-Duty Heavy-Duty All State Vehicles Vehicles Vehicles Alabama 119,344 53,320 172,664 Alaska 11,747 4,500 16,247 Arizona 84,239 35,363 119,602 Arkansas 66,001 30,694 96,694 California 527,430 213,317 740,748 Colorado 82,742 32,384 115,125 Connecticut 64,465 23,245 87,710 Delaware 15,142 6,902 22,044 District of Columbia 7,737 2,286 10,023 Florida 274,081 108,530 382,611 Georgia 193,385 81,615 275,000 Hawaii 16,561 6,983 23,544 Idaho 32,405 14,902 47,307 Illinois 226,292 83,066 309,359 Indiana 161,397 68,419 229,816 Iowa 73,464 31,458 104,922 Kansas 66,126 27,933 94,059 Kentucky 104,264 45,802 150,065 Louisiana 83,044 38,144 121,188 Maine 34,433 16,103 50,536 Maryland 105,618 38,949 144,567 Massachusetts 108,300 36,795 145,095 Michigan 222,395 83,471 305,866 Minnesota 105,734 45,987 151,721 Mississippi 69,836 35,023 104,859 Missouri 146,705 59,523 206,227 Montana 26,957 11,854 38,811 Nebraska 42,201 18,277 60,478 Nevada 28,532 11,843 40,375 New Hampshire 29,945 12,325 42,270 New Jersey 136,708 47,339 184,047 New Mexico 50,320 22,952 73,271 New York 255,046 98,126 353,172 North Carolina 177,367 78,311 255,678 North Dakota 19,168 8,190 27,358 Ohio 248,014 97,841 345,855 Oklahoma 91,459 39,856 131,315 Oregon 77,448 32,079 109,528 Pennsylvania 211,943 92,727 304,670 Rhode Island 16,431 5,371 21,802 South Carolina 93,254 43,879 137,132 South Dakota 23,296 10,472 33,768 Tennessee 135,117 56,331 191,448 Texas 407,912 160,823 568,734 Utah 44,536 16,572 61,108 Vermont 15,528 7,698 23,227 Virginia 165,109 70,200 235,309 Washington 120,682 43,970 164,653 West Virginia 46,365 21,793 68,158 Wisconsin 136,212 55,905 192,118 Wyoming 20,931 9,503 30,434 Total 5,623,370 2,298,948 7,922,318 Modeled with 65 mph speeds on rural interstates and principal arterial roads for light duty vehicles and trucks only and only in states with former limits above 65. Table 7 Comparison of Base Case and Likely Case 1994 Highway Vehicle NOx Emissions 1994 Total NOx Emissions Increase in Percentage 1994 from All Vehicle Types NOx Increase (tons) Emissions State Base Case Likely Case (tons) in Emissions Alabama 161,898 172,664 10,766 6.65 Alaska 15,343 16,247 904 5.89 Arizona 114,023 119,602 5,579 4.89 Arkansas 88,067 96,694 8,627 9.80 California 713,718 740,748 27,030 3.79 Colorado 107,982 115,125 7,143 6.62 Connecticut 87,710 87,710 0 0.00 Delaware 22,044 22,044 0 0.00 District of Columbia 10,023 10,023 0 0.00 Florida 360,665 382,611 21,946 6.08 Georgia 261,119 275,000 13,881 5.32 Hawaii 23,453 23,544 91 0.39 Idaho 43,457 47,307 3,850 8.86 Illinois 296,573 309,359 12,786 4.31 Indiana 216,037 229,816 13,779 6.38 Iowa 94,727 104,922 10,195 10.76 Kansas 86,504 94,059 7,555 8.73 Kentucky 139,035 150,065 11,030 7.93 Louisiana 114,193 121,188 6,996 6.13 Maine 46,887 50,536 3,649 7.78 Maryland 137,673 144,567 6,895 5.01 Massachusetts 145,095 145,095 0 0.00 Michigan 290,085 305,866 15,781 5.44 Minnesota 151,721 151,721 0 0.00 Mississippi 97,270 104,859 7,588 7.80 Missouri 190,594 206,227 15,633 8.20 Montana 34,425 38,811 4,386 12.74 Nebraska 55,152 60,478 5,325 9.66 Nevada 37,839 40,375 2,536 6.70 New Hampshire 38,717 42,270 3,553 9.18 New Jersey 177,288 184,047 6,758 3.81 New Mexico 67,507 73,271 5,764 8.54 New York 353,172 353,172 0 0.00 North Carolina 240,850 255,678 14,828 6.16 North Dakota 24,308 27,358 3,050 12.55 Ohio 331,190 345,855 14,664 4.43 Oklahoma 123,108 131,315 8,207 6.67 Oregon 100,156 109,528 9,372 9.36 Pennsylvania 304,670 304,670 0 0.00 Rhode Island 21,802 21,802 0 0.00 South Carolina 127,574 137,132 9,559 7.49 South Dakota 30,263 33,768 3,505 11.58 Tennessee 180,325 191,448 11,123 6.17 Texas 540,257 568,734 28,477 5.27 Utah 57,637 61,108 3,471 6.02 Vermont 23,227 23,227 0 0.00 Virginia 221,718 235,309 13,591 6.13 Washington 155,838 164,653 8,815 5.66 West Virginia 62,603 68,158 5,554 8.87 Wisconsin 176,709 192,118 15,409 8.72 Wyoming 27,475 30,434 2,959 10.77 Total 7,529,705 7,922,318 392,613 5.21