Different general speed limits in Sweden. Effects on road accidents and on vehicle speeds.

(1)

STATENS VÄG- OCH TRAFIKINSTITUT National Swedish Road and Traffic Research Institute

DIFFERENT GENERAL SPEED LIMTTS IN SWEDEN Effects on Road Accidents and on Vehicle Speeds

By

Per-Olov Roosmark & Göran Nilsson

REPORT Nr 19

(2)

STATENS VÄG- OCH TRAFIKINSTITUT

National Swedish Road and Traffic Research Institute

DIFFERENT GENERAL SPEED LIMITS IN SWEDEN

Effects on Road Accidents and on Vehicle Speeds

BY

Per-Olov Roosmark 82: Göran Nilsson

REPORT Nr 19

(3)

Roosmark, P-O & Nilsson, G: Different General Speed ...

CONTENTS

Page

1. SCOPE OF INVESTIGATION 1

2. EFFECTS OF DIFFERENT GENERAL SPEED LIMITS ON ROAD 3 TRAFFIC ACCIDENTS

2.1 Comparison between Speed Limit of 90 km per h and 3

No Speed Limit

2.3 Comparison between Speed Limits of 110 and 90 km per h 7

No Speed Limit

3. EFFECTS OF DIFFERENT SPEED LIMITS ON ACTUAL VEHICLE 15 SPEEDS

4. ECONOMIC ASPECTS 16

(4)

Roosmark, P-O & Nilsson, G: Different General Speed ... l.

SCOPE OF INVESTIGATION

Field trials started in Sweden on May 8th, 1968, in order to study different

general speed limits. The speed limits under test were 90, 110, and 130 km

per h. After an interruption during July and August, these trials were

resumed on September 2nd of the same year.

The purpose of the investigation described in the present report was to study the total effect of the Speed limits on road traffic addicents, as well as to examine their effects on accidents in different road and traffic environments. Comparisons have been made between the respective frequencies of accidents occurring on the same roads under the following alternative conditions, viz.,

(1) 3 Speed limit of 90 km per h and no Speed limit, (2) a speed limit of 110 km per h and no Speed limit, (3) Speed limits of 90 and 110 km per h,

and (4) a Speed limit of 130 km per h and no speed limit. The road network

subjected to this investigation comprised all Europe Roads (E Roads) and some

of the other large National Main Roads in Sweden. This road network is

included in a road register which contains data on road parameters and

traffic Characteristics. By determining the locations of the accidents, it was possible to obtain the data on road environment and traffic charateristics

for each location of accident from the road register. The analysis of these

data was carried out by means of a digital computer. All those road sections

for which a Speed limit of 110 km per h had been adepted on September 2nd,

1968, were studied in this investigation. The total length of the road

network under investigation was about 4 500 km, including some 2 000 km of

roads where the speed limit was 110 km per h, and some 250 km of motorways,

where the speed limit was 130km per h, while the Speed limit on the remaining

roads was 90 km per h.

For comparisons with conditions in the absence of any Speed limits, use was

made of a reference year, from September, 1966, to August, 1967, which was chosen so as to coincide in months with the year of observation, from

September, 1968, to August, 1969. In order to reduce the disturbing effects of seasonal factors, the period of observation was divided into sub-periods, viz., autumn, winter, Spring, and summer, which coincided with the

(5)

Roosmark, P-O & Nilsson, G: Different General Speed ... 2.

Information was collected on the accidents which had occurred in the road system under investigation, on the geometric design features of the roads, expressed in terms of the width of carriageway and the width of shoulders, on the volume of traffic, expressed in terms of the average daily traffic in different road environments during the periods under investigation, on the condition of the road surface, described in terms of precipitation and

temperature, and on the actual vehicle speeds. This investigation covered

about 7 000 accidents in all. Speed observations have been made at 71

(6)

2.1

Roosmark, P-O & Nilsson, G: Different General Speed ... < 3.

EFFECTS OF DIFFERENT GENERAL SPEED LIMITS ON ROAD TRAFFIC ACCIDENTS Comparison between Speed Limit of 90 km per h and No Speed Limit

A speed limit of 90 km per h was found to have had a positive effect on the frequency of road traffic accidents as compared with that which had been

observed during the corresponding no-speed-limit periods. A regression analysis showed that there existed significant differences in the number

of accidents per day expressed as a function of the vehiclenkilometrage per

day during the autumn as well as the spring periods, with the exception of

low-traffic days; see Fig. 1. When the condition of the road surface was

good, this positive effect was greater than when the condition of the road

surface was bad. No significant difference between a speed limit of 90

km per h and no Speed limit was to be demonstrated for the winter period.

During the autumn and Spring periods, the effects of a Speed limit of 90

km per h on the frequency of accidents were positive on the roads 6 to 7 m

in width of carriageway, without shoulders, and on the roads 7 m in width

of carriageway, with shoulders. These effects relate to the road sections

where the average daily traffic during the period in question was greater

than 4 500 vehicles per day, see Fig. 2. When the speed limit was 90 km

per h, the frequency of accidents on all roads without shoulders was lower than in the absence of any Speed limits. This statement refers to those

road sections which were similar in respect of sight conditions.

If it is assumed that thernmüwn'of accidents has changed in proportion to the calculated increase in traffic, then the reduction in the number of accidents is found to have been 25 per cent for personal-injury accidents

and 19 per cent for propertyndamage accidents. These figures are based on comparisons which cover all periods of observation.

(7)

corresponding perl'ads in 1966 anå 1967, when no speed limit was b

;unposed in these roads.

'få

n the autumn of 1968 and in_the spring of.l969 compared with.thçtsameüroadshduring.theEy traffic: Roads where the Speed limit was 90 km/h

isection of the road and by average dai

Figure 2, _Breakdown of the number of 4560 u 70ü0

acciâents per million vehiclewkilometres by cross Average daily traffic i'

0 » Width of

0,3

0,2

0,1

3'-9,4 Width of shouldersO O (28.173: 1 ageway. O t 0 O .. .J -2 04 .1 .. i, L-ra nn it wq to e mawc on üh 1 .. .vi .... .. .Äm o 0 s.. .. .ut 'wa agna uo uo un un gn em o-awøo n. .. an oe .n ao uawa øuq nn nøo nc nt oa , n$ uonøa on un wo kuåa ou|wl w go tta. a»a nt a: nu" oup ua wo nun puo nc nc cn nn nun no ni um wt ni n 4 .a t 1 4 a a m . . 4. * .a u n i . .$... .. ...A. .mu... ... .,. u..".M .*. H.. .n...w. w.. .u* ...,1. .1. .. . h l i 0 . G i 0 n n. a »r n-run to v§| . i i i l h 0 I G U I ' Q O G gami ng ; i at t. .. m än .. n a uo .s .nnuøJ QA to lp as øn we ovm wau .. n 0 a a 0 a 0a u n a a n ia v .a a 0a a 0 a a 0 oo o 0 a a 0 a . i . h ' h C ' I i A ' ø' ÖQ o. . . Q U . . .-Jt aøo tum aa wn anøi *wul ,a A. A .. 4 a .. Ö 0 .M 0 C I 009 m r -r wb 1 .4 Q t i i l

.4 o . . .. .1 01 ., ... 7.. . .c

' 6,5

none 110118: . .u. u. m. n+ n. u. nu. u» .m an an af nc s A u .u n i. l i .. .r oa d .-.. .. .. .ur ououus nuc io uøn os n ;A q .4 5. 1 5 0 Å: 0 0 3 . 9 . 0

7,ø

1,

,ä 05

. . . ;. 4 lux. . a t t a o vo o J ul o i i l * ut * . 00 . a a ön a n o a t l 1 a. n. o 4 a i i D 9 W i A. D 0 .r 0 1 n . an awzun nd on no uo .. H. . H. n I .. |v. A. J. A 9

i

.a ;. u:v .. V. . . 2. .. A..1 ; .U .. ., .. , A :; 5. . . . . .4 v . : a A 1 . nys t a n . a n d a -ai a a i a i b n n : ab at m. .0 aa zooo .a .. on qn ae ae pa oøe oa na no ..o aa ac n .. .t oo th an an as .. na nn oa aa owøa aa aa oaon cg aa l. .1 .t h yu. ".u.a. u.u. .n ,u. nmu, n.u. u.u_uan .n. u.u. ".u. ". .. .0 53 .. . ; .3 . .l ast 4 ._ .i a.. ... ..xl .. ... .. ,0

5 22,0_

.k .._ 4. . . .1 .n är .: . A .. Q å 9 0 I 1 t .Ö | i ., O lb i' I .i 0 i t G .. .i .. . x . 4, 41 .. .. :3 ,1 .. 1. A. ,. .c .. uf .m .. .\ .. 4_ .. .. t o uuöo l o nA n c a . ..o m väg -3 ...c b t o o n øn n øøøa m i véa r c øa a n a r .. 9 1 . 0 0 . i . 0 Q 0. H. .. .i n. Må.. ns nuva a aåuuuwuum un nuuuuuum uu" ma n" "Hum mn uun uum unáån å. .. ?? Vunna t.: ; s k u-. a äo øa a n a m i oo n n z : S vug uøn áuuvvw . a aa h . b r i n b ln i q a n øg a ø . . 1 0 0 . a J . a a : I . 6 1 .. .. Qua nm no wn r-mb ci l .J u-Jua tn nt DA MM üm na w a : ... . A .s |. .7 .|..a .. i .. i ; I .. 1 3 : -v , . ...1. .J J .1 7. .. , n a a4 e. 0 01 00 » n o ;h 0a nu. n aa ai çu. . on an i... ? a n: i 0 0 r Q * o. o i *.. u". w,. .m. ... ._. .wm 3-.. .. .A ma nd a a áo i .. .n »wa tt 0 I xx: . . . 5 : L T

.v rst 0; . a a a w. u. t. t á, m. &Li nq rm m. .. m . "n un un uanc ua ua uuua na na nn uaup uo uc uo ug aøu U 0 . . . . . 0 öm 0 1 4 a. 4 5 , 9 1 a 1 ;. Mo no no MU no møa uun uc wobn :q ut cuo çua ao øn suc o.. i n a. 0. ! and. . i 0 ha n. .. a n a . .o m o 0 0. 0. .

. 0-0 .10 10 .r 0 .0 ,4 % .1 .0 av. .. * i avs at ta .. .-io _ . p t a øn o vo o b e a aq q o o åun n nn : 9 c . mt4a na uc na cwc at ba to -n at t-ao wn ka øn no io ø ik wnño an ni cb mo ni oc il : en e n . . a n c nui o o øn a n o tva øn un g -o! .h an na 'J bo nott co ca vo yøvi wc 0. 03.1 10 9. .. .o u 03 .3 3... . to ba k. . .1 .w0 1!40 0§n .t 01 u0 0 A Anmäl !) .0 a å o m4. .. .. . .Q .. 4. .c a 0ma n se .

.e ni in fi .. . : .. , a « avt a. < 1.. , .o x a . Ha noma g» I.

1 . I .. A. l n .vx ÅÄ. . J .. . ! 4 . . a t. : a tt a i n a .. on aseo nn xa nu :c bn ur nn am øi n m o t c a c un n b øo o n . t i c -o . a nv$ 4 n guQ ån om auc Mu-Ma at ua . n. .. 4 .. 19 o 'a 0. .: qu c om .. 0 D U A ... 1.0 J J.. .1 . 4 a a a s 0 0 I I wa tt tuwh a

nxu _{d 8 3} D.. TL T . mI ' 14 . ...n u G H» mm nu3» T... 6 nu... nxw 6 T... ,nu .aux rnu

NG Speed limit 1966, 1967

Accidents per million vehicleukilometres roads.

the correspanding periods in 1965 and 19679 whan na speed limit was impased on theäain the autumn Of 1968 and in the spring af 1969 compared with the same roads d

per day. Swedish Natiønal Main Road system, Roads where the Speed limit was 90 km/h

uring m

Figure 1.' Variatian in the numher af aøcidents per day with the vehicle-kilometrage

8 per day vehisléwkilametre 40 h. .ämm 3(2% L (*- ü

1:99

Speeå limi km

/h :gas

§1§69

1966?1967

NG speed limit Å 12 m LG 9 mm. i.. # áWKSpm: dav.I m

(8)

2.2

_gomparison between Speed Limit of 110 km per h and No Speed Limit

A speed limit of 110 km per h has had a positive effect on the frequency

of accidents but to a very small extent. The regression analysis did not

show that a significant difference between a Speed limit of 110 km per h and no speed limit had existed during any one of the periods under.

investigation, see Fig. 3. It was only on roads of broad cross section

(carriageway 7 m in width and shoulders 3 m.in width) that the frequency of accidents during the periods when the Speed limit was 110 km per h was

lower than in the absence of any Speed limit, see Fig. 4.

If the increase in the vehicle-kilometrage in the year of observation, as compared with the reference year, is taken into account, then it is found that the number of accidents on week-days, Monday to Thursday, was

unchanged, whereas the week-enda, Friday to Sunday, exhibited a certain

(9)

åüüidêütä per âay ₆ 10 NG apeeâ limit 5 : 5 "4

w%$üy, y 24%' . *mwâpeed limit 113 km/h 1968,L969

i i i ?2 .i B , i' 2 i , _ 35 3G 35 ae as 5% 55 se as ?Of'KllO

Vehiclewkiåametres par day

Figuägmäj Variatiün in the number mf accidents per day with the vehiclemkilometrage

per åay. Swadish National Main Røaá system- Knads where the Speeâ limit was 110 km/h

in the autumn af E968 and in the spring af 1969 campared with the same roaâs áuring the corrâsponåiäg perioâs in lâöê and 19675 when na Speeå limit was imposed an thüâê

raads.

Acciüenäg par miliimn vahicåawkilømetres

1,0 '4

=|| No Speed limiü 19663 1967

0,9

"

Q_-0p. . 'J

9:5 1. 35 ' , vi lt .. . \ . e 30 :9

-

' åSpeeá limit ziolkmfh 1968, 1959

' & . O ' d .1 D i t ä, _* ._ 3. .. , r; . a. a iân _*a d5 : f ' « ! * i 5 .ll 0. ; 1 : , J i: :it.9 7 _32 ' »L e , :4 :4 U U _f 4 k k . . 45 .1 20 * du* g; .. 5 , a 1 »G :ut :5 : i...o _{.N 4.*} -1: 0 9 . a . l \ "43

u' . ^ 'S-i Q n s. v 9 4-_* »* a t . 0 .gt I _4 .1 5_ . _u_ .. : I *4 .u. . a a .0 n u ' 1 ' * " 5 1 i #05 ciåi. . i .»om-; 5_u _{:- 2031:: 1}tar! _{l' a}' " m' . 'I' ; vanan a »_94

L_' _LU _Ailöb*sutnø T _...ut*4

' 7236"' #52". 1' "W" .

3 v 9 * sun .banan

' '210.0'_:wo dne-aa_;utåt _{21:13] '}"*W'J ;l&0|| xa. N, 0 '.3i 2 f _{9" 0}O .on . " 9 5 . 1 3 % " . .0. g 1t 0 I . . I 9 a :0: e: " #3 20 _7-' *0 'a.q 5 Q 0. 4. A1 "* ₊₇₅ 0 .l um " u\ 94 0 iI übbru. 4: ; .9 c I nu 9 r o( in - ta. 0004»

i_1:1994: uno.._{luta ,} _*i'a'U'

vin-v. 'hr-u* * :30,07

Simao 'kön . ,.Ö

3_' .nu_;Wutu. uns-eu_titta _:5.3.:3* l_ 4

är.. , tuta' 'K' v

- *0:00 * asa- _3:'iegå

-\ 7"1 4 ...få-.-E i I._;ya anwa _^'ett '

1 -.iI§R 0 L0_'a a v 's i 3 ; . 7 . . c 0 1 vad Oañii "" ,x '.:å'ZJ tr.. Å-V-G v ' . . .v .. .9; M*' Width af carriageway: Width of shoulders: 1,0m;

Average daily traffic:

Figure 4. Breakdown af the number of acciáents per million vehicleokilametres by cross

section of the røaå aná by average daily äraffic. Roads where the speed limit was 110 km/h in the autumn of 1968 anå in the apring of 1969 campared with the same roads during the correSpanding pericds in 1966 and 1967, when no Speed limit waS imposed an these roads.

(10)

2.3

Comparison between Speed Limits of 110 and 90 km per h

A comparison was made between the roads where the speed limit in the summer of 1969 was 110 km per h and the same roads where the speed limit in the

summer of 1968 had been 90 km per h. This comparison showed that there

existed a significant difference in the number of accidents between speed

limits of 110 and 90 km per h, see Fig. 5.

The frequency of accidents in the presence of a speed limit of 90 km per h was substantially lower than that which correSponded to a Speed limit of

110 km per h on the roads with carriageways 7 m in width and with shoulders.

The reduction in the frequency of accidents on the roads which differed in

the width of shoulders were roughly equal, see Fig. 6. Thus, it was found

that a speed limit of 90 km per h also had a positive effect on the

fre-quency of accidents on roads which conformed to a high standard.

If it is assumed that the number of accidents has changed in proportion to the calculated increase in traffic, then it is found that the frequency of

accidents which corresponded to a speed limit of 110 km per h was about 40

per cent higher than that which was observed during the period when the Speed

limit Was 90 km per h. The greatest increase occurred during weekends. The

values of the increase in the frequency of personal-injury accidents and property-damage accidents were approximately equal.

In order to provide a check, the roads where the speed limit in the summer of 1969 was 90 km per h have been compared with the same roads in the summer of

1968, when the speed limit had likewise been 90 km per h, see Fig. 7. The

analysis showed that the frequencies of accidents during the summer periods

subjected to comparison were approximately equal.

A later comparison between speed limits of 110 km per h and 90 km per h has also been made for the summer period (May « June) of 1969 110 km per h -and the corresponding summer period of 1970 - 90 km per h-. The results are almost identical with those received from the comparison between 90 km

per h in the summer of 1968 and 110 km per h in the summer of 1969. These results are shown in figures 8 - 12.

(11)

Acciåentü par áay m 13. 10_ C h i I 22 i 1 an aa sa 55 Ma) a5_Y70 K

Vahicleukiiometres per day

L_ p...- L...

Figura 5. Variatiøn in the number of acciâents per day with åke vehicleukilometrage

per day. Swediäh Natianal Main Rcaá system. Roaás where the Speed limit was llüêüüñ in the summer of 196? compared with the same røadg áuring the summer of 1968, wbam

the Speed limit in thesa roaás was 90 km/hç

to O Nm 4,...)

v.

0 *avV1 ₁₁₀

Accidents per millian vehiclewkilametres

150 ;IITSpeed,Eimit 110 km/h ajg â" 1969 www Speed limit 90 km/h §38 1968 ü,? ü,â ü,5 "

m3

G,á i mm ü,2, 0,2 ü,1 Width of carriagüway: ?,0 ?,G

Wiáth of shüuldera: mama läüm1,5 2,0w2,5 23,3

gåågra 6, Erüakdøwn Of the numbâr of accidentg ?är million vehialeukilametres

by crcss sactian af the rüaá. Roaâs whara the Spead,limiä was läü'km/h in ähe

summer Of i§69 cümpareá with tha sama rüaäs in the gummer mf 19685 when the speeá limit maa §0 kmfh.

(12)

U; 10 53 .fr u .1; ut 1

. ;anta per åay

Speed limåc 90 kmfh iâöâ

! i 5 613 65 78 * K Vebiclaakålømetres_per áay. V 1 i 25% 30 35 540 :'45 '36 mv nn- ;m n... U3 .9.. . 90

Figure_z, Variatiøn ia the number af acciåents per áay

With äüé vehiclawkiåømetragä per day; Sweáish National

Main Road gyâtem. Rcaâs wherä thê Speäd Åimiä was 9G kmfh in the gummer af lgêg campared Wigh thê gama roadg during :ha sammer of lêöâs when the spaaâ.åimit on these reads was ?O km/h.

(13)

A üc i âg nts pe r áa y 10 . m ät s J' ' r ,å . . 2.14 4 af: 4 90 km /h 19 79 m m Qin 325, M :I: N Ve bi cl ewk il øm et re s pe r da y Fi ur euê, Re gre$ si on Ei ne a fo rth e num be r ef ac ci dent s pe r âa y wi th th e ve hi cl em ki ia me trag e ?e råa y. Swe åi sh Na tâa nal Ma in Ro ad sys te m. Read a wh er e th e Sg ee d limi t wa s 11 0_ km /h in the Sum me r af 1%69 (M aq uün a) ce mp areå wi th th e sa me ro ad å âur in g th e sum ma ? of 19 70 (M aq ugna ), wh en th a Sp ga d li mi t in :h ag e ?Dáåâ wa â 90 km /h . *i m ;3 a 1; ;.vv; FÅ' FL ; ;4. 4; 53 4. .. Eäx* V. . .. _ . .. 4 . 4 , . . . A . « < -. . \ V4 *' \' 4 ' \' .y' <" H-': "\ " A ._ A: -v

50 "5

5"

s

e

'

w

ss

10*

(14)

Ac ci de nt s pe r da y 8 4 4-1: -* 33 1 -*i a' 3 -wi n. '' ' : r a -' 2 : -:v " r :2 1. :: ::? '§»" '. ç.\ .-' * g Jua n 1+ :r u-'7:711' , ' _. §11:;V)*-,'.fh ; . I, (73 :2 g ig üa wg g :år iga -'5 1:1 1: ' :1 :w: . h r -n :wa â' çn ' '-25 30 35 40 45 SO _5 5 . 65 70 Ve hic1 e«k il om etre s pe r áa y Fi ur e 9. Di ff ef ence be twe en th e regr es si on li ne sfo r 11 0 km /h in th e sum me r of 1969 (M ay « Jun e) an d 90 km /h in th e sum me r of 1970 (M ay 0 Jun e) wit h co nf id en ce re gi on c o r r e s p o n d i n g to th e 5 Z le ve l of si gn if ic an de. V11.

(15)

0,6 055

0,4

Accidents

Width of carriageway [m]

Width of shoulder Eng:

Figprelü.

0 t

per million vehicleukilometres g//ilo km/h 1969

» »we 90 km/h 1970

Düüê

Breakdown af the number of accidents per million.vehicleukilometres

by brasa section Of the røadü Roads where the speed limit was 110 km/h in the summer af 1969 (May » June) campared with the same reads in the summer of 1970 (May « June), when the speed limit was 90 hm/h.

Acciáents permillion vehiciewkilometres

0,6

0,5

0,4

053

0_3,4.,'3

0,1

nu...

Wiáth of carriagsway img : _6,0m6,5 name Width of shøuldar [m3 : W

_{9G km/h 1969 90 han: 1970} flat

_.xV _.ån -'- i I 4 . -._-:b;r <> _ : J» A .i ?,0 acne '

Fi'urellL Breakåüwn af the numbar of acaidents ger million vehiclenkilometres by eröSé sectiøn af the road. Roads where the Speed limit waä 90 km/h in the

summer af 1969 (May « June) campared with the same rcaâs inLthe summer of 1970

(16)

Ac ci de nt spe r áa y 1G Ö . 4

;25

4

30

35

40

45

50

55 .

60

V áh i c l e -k i l a met r e s pe r da y Fi ur e 12 . R egr e s s i øn li ne s fa r th e num ber of ac ci de nt s pe r åa y wi t h th e ve h i c l e a k i l üm a âr a g e pe ráa y. : Swe ái sh Na ti on al Ma in Rça d sys te mg Ro ad s wh er e the sp ee á li mi t wa s 90 km /h in th e sum me r af 19 69 (M aquun e) ce mp ar eá wi th th e sa me ro ad s éur in g th e ga mm er of 19 70 (M ayuj un e) , wh en th e Sp ee dli mi t in th es a. ro aâ$ wa s 90 km /h . ' 13,

(17)

2.4

Comparison between Speed Limit of 130 km per h and No Speed Limit

The comparisons between a speed limit of 130 km per h and no speed limit on motorways indicated that the corre8ponding numbers of accidents had been much too small to make it possible to draw any conclusions as to the effects of Speed limits.

(18)

EFFECTS OF DIFFERENT SPEED LIMITS ON ACTUAL VEHICLE SPEEDS

The speed measurements which had been made during the year of observation showed that the median Speed of all vehicles in the presence of a speed limit of 90 km per h was 6 km per h lower than that which corre8ponded to a speed limit of 110 km per h, when the other factors which influenced the

speed were taken into consideration. The corresponding difference in the

median Speed of private cars alone was somewhat greater, and amounted to

about 8 km per h. The difference in the 90th-percentile Speed, i.e. in the

speed which was exceeded by 10 per cent of the total number of vehicles, was

about 9 km per h for all vehicles as well as for private cars only. In

other words, for the whole traffic, the reduction in the higher speeds due

to a speed limit was greater than the correSponding decrease in the median

speeds. The dispersion in speeds in the presence of a speed limit of 90 km

per h was smaller than that which correSponded to a speed limit of 110 km per h, and this implies that the difference in speed between vehicles has

decreased. If the calculated median speeds and 90th-percentile Speeds are

brought into relation with the frequencies of accidents which have been

observed during different periods, then it is found that low median speeds

and 90th-percenti1e speeds corresponded to low frequencies of accidents, whereas higher median speeds and 90th-percentile speeds corresponded to

(19)

ECONOMIC ASPECTS

Speed limits should of course also be viewed in an economic perspective. A simple numerical example has been calculated in order to show in outline

how the economic consequences of speed limits can be illustrated. The

values of the cost of accidents and the cost of time were taken from the

Road Plan 70 which has been published in Sweden (see State Official

Reports Nos. 1969:56 and 1969:57).

110 and 90 km per h has been made for the roads where the speed limit had A comparison between speed limit had been 110 km per h in the summer of 1969 and 90 km per h in the summer of 1968.

limit of 90 km per h on the roads in question had caused a reduction in the

The results of this comparison showed that the adoption of a speed cost of accidents which was greater than the increase in the cost of transportation, see Fig. 13.

(20)

17.

åccident cost {1000 swedish kronor per million vehicle-kilometrage]

30 '

No Speed i i , limit

25 g ' .

(X

110 km/h \ 90 km/h & N, 70 km/h ut i* .-7 .6

V 315

i ' 320 '

325' I' '3305

'335

340 345,

.Transport cost [1000 swedish kronor per million vehicle-kilometrage] Figure 13. The connection betWeen the transport cost and the accident cost at different

speed limits.

Accident cost = 50.000 swedish kronor per

(21)

CONCLUSIONS

The data which served as a basis for the investigation described in the present

paper were of course deficient in several reSpects. In a report publisched by the National Swedish Road Research Institute (see Special Report No. 60), it has been shown that the number of accidents which are reported to the police

amounts to only a part of the total number of accidents which occur in Sweden. It has not been possible to carry out special traffic counts for this

investi-gation.' For various reasons, the measurement of certain factors, e.g. the degree of supervision in road traffic, was not practicable.

It is obvious that these deficiencies of the data can have influenced the results. However, it has been possible to check the reliability of the results in some reSpects.

Repeated comparisons with conditions in the absence of any Speed limits showed

that a Speed limit of 90 km per h had a positive effect on the frequency of

accidents. This results was obtained both during the autumn period and in the

course of the spring period, and was found to hold true when the whole road

network under investigation was considered, as well as when this network was

divided into several regions. The comparisons related to the same roads, but referred to different periods. Therefore, the differences in the frequencies

of accidents cannot be attributed to dissimilarities in the road standard. When the results were checked, it was found that the differences remained

significant in most cases, even though the actual increase in traffic differed

substantially from that which had been assumed.

A comparison between a speed limit of 110 km per h and no Speed limit, which

was carried out in the same way as that between a Speed limit of 90 km per h and no speed limit, showed that the respective frequencies of accidents exhibited no statistically justifiable differences during any one of the periods of observation.

A comparison of a speed limit of 90 km per h with that of 110 km per h was

made between the summer periods of 1968 and 1969, and related to the same

roads. This implies that the differences obtained from the comparison in questionlunnanot been influenced by any differences in the road Standard. In this case too, it was found by checking the results that the difference in the

(22)

frequency of accidents remained significant unless the actual increase in traffic differed very considerably from that which had been assumed.< Possible effects of other factors on the results can be checked by comparing the roads where the speed limit has been 90 km per h in the summer of 1968 as well as in

the summer of 1969. Therefore, the differences between speed limits of 110

and 90 km per h cannot be attributed in any notable degree to differences in

other factors, e.g. supervision, weather, and composition of traffic. Various

comparisons betWeen speed limits of 90 and 110 km per h showed that the lower

speed limit had a positive effect on the frequency of accidents even when the cross section of the road\was wide.

(23)