Considering Safe Distance between Moving Vehicles

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Considering Safe Distance between Moving Vehicles

¹

Kiyofumi

^lcHIKAWA

(Key words : headway time, stopping distance, safe distance) Abstract

The purpose of this study was to measure time headway between moving vehicles. We observed 2,004 vehicles running on suburban and city roads at night and daytime. We analyzed time headway from the viewpoints of location, time, lane, weather, 'fleet' , and type of vehicles. The safe distance at 60 km/h, recommended by driving schools in Japan today, was tentatively equivalent to a time interval of 2. 7 seconds. From the overall standpoint, nearly 80% of 2,004 vehicles maintained an unsafe and risky time headway. When adopting the safe criterion in the UK, 92 % of vehicles in Japan kept unsafe time gaps under rainy condition. We proposed "how to maintain safe distance between moving vehicles" using time headway in concrete terms.

Spacing behavior between moving vehicles while driving is connected with personal space - the concept of space as a protector of the personal territory that originates in Hall ( 1966) . Whitlock (1971) explained male driver's aggressive conduct as being that of a protector of personal territory in terms of the traditional role of the male. This study was concerned with the "safe distance" between oneself and the vehicle ahead as proxemics in driving. Japanese driving schools recommend the safe distance while driving referring to 'stopping distance' which changes with speed. In the UK, New Zealand, Australia, and the USA, a two seconds' time gap has been adopted as the recommended safe distai:ice between a vehicle and the vehicle in front (Tominaga, 1993). In the UK, double time gap, that is, four seconds at least is recommended while raining ( wet road). On the other hand, the road code in

1 I wish to express my gratitude to Ms A. Doy and Ms Y. Yoshihara for their help in carrying out the study and analyzing the data. This study was presented at the XXV International Congress of Applied Psychology (held at Singapore in 2002) .

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Kiyofumi lcHIKAWA

Australia states that, "drivers need to develop their judgement skills about what distance at different speeds represents two seconds in the following conditions: When visibility is poor; If conditions are dark; If conditions are wet or slippery; When you are tired; When you have a heavy load; When the road is unmade. This skill will only come with lots of supervised driving experience" (Vic roads, 2000) .

As for measuring time headway in Japan, Nakajima (1987) has reported on the results of time headway on Tomei Expressway at daytime and night. According to this report, the mode of the time gap was . 9 and 1. 0 seconds at daytime and night, respectively. The median was 2. 6 seconds at daytime and 2. 2 seconds at night. Even tho_ugh these reports were obtained only for the overtaking lane on the expressway because of noise affecting the measurement instrument, the observed time gap between moving vehicles on the expressway was in reality highly dangerous. However, the study of time headway on open roads in Japan has not tried until Ichikawa (2003a) .

The purpose of this study was to consider the safe distance between two moving vehicles.

Ichikawa (2003a) observed 1, 415 vehicles running on open roads to measure time headway to obtain recent realistic actual information in order to consider the safe distance while driving.

Table 1. List of distance criteria while driving at different speeds (**)

Speed/h (A) (B) (B/A) ^(D) (D/A) ^(H) (H/A)

30km/h 8.33m/s 9m 1.08 s 30m 3.60 s 15 m 1.80 s 40 km/h 11.1 lm/s 22m 1.98 s 40m 3.60 s 25m 2.25 s 50km/h 13.88m/s 32m 2.30 s 50m 3.60 s 35 m 2.52 s 60km/h 16.66m/s 44m 2.64s 60m 3.60 s 45m 2.70 s 70km/h 19.44m/s 58m 2.98 s 70m 3.60 s

80km/h 22.22m/s 76m 3.42 s 80m 3.60 s 80m* 3.60s 90km/h 25.00m/s 93m 3.72 s 90m 3.60 s 90m* 3.60s 100 km/h 27.77m/s 112 m 4.03 s 100m 3.60 s lOOm* 3.60s

Note: (A): Traveling distance per second for each speed.

(B): Traveling distance until stopping taught at Japanese driving schools these days.

(D): Ideal gap between one and the vehicle ahead, taught at driving schools in the 1970's.

(H): Rough standard of the safe distance between one and the vehicle ahead which is calculated by [speed minus 15 (m)], taught at driving schools these days. Tp.is standard applies only to the speed of between 30 and 60 km/h.

(*): As for how to maintain safe distance on expressways at speeds more than 80 km/h taught at Japanese driving schools todays is the same as column (D). The fact that column (D) and (H) for speeds above 80 km/h (highway speeds) is the same; While (D) and (H) for speeds below 80 km/h differs confuses drivers easily.

(**): This table was reprinted from Ichikawa (2003a).

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In addition to the viewpoint in Ichikawa (2003a) , the weather conditions and types of vehicles were introduced into the analysis. Consequently, we examined the reality of time headway as to weather (not raining, raining), lane (cruising, overtaking), location (city, suburbs), time (daytime, night), 'fleet' (tail, in group) and vehicle type.

Table 1 showed a list of distance criteria while driving at different speeds. In Japanese driving schools, 'stopping distance' (see column [BJ ) was defined (moving distance after depressing the brake pedal until the brake start working) plus ( traveling distance until stopping after coming on effective braking) . The numerical value in column [BJ (i. e. , stopping distance) is calculated as 'moving distance per second at each speed' plus 'braking distance'. The former means "after perceiving danger and putting on the brake, until the brake start working". This is conceived approximately as taking one second. This concept of 'stopping distance' was explained to apply only in good conditions. When a driver suffered from fatigue, it might cause a longer moving distance because of the delayed perception of danger. The list also explained that stopping distance (see column [BJ ) might be sometimes double when it is raining or tires are worn down and so forth. Column (H) was used to explain the rough standard of 'safe distance' at different speeds recommended by Japanese driving schools today. This standard meant 'reading the numerical value on the indicator of the speed-meter' minus 15 (m). For example, when driving at 60 km/h, the safe distance would be (60-15=) 45m. However this concept applies only at the speeds between 30 and 60 km/h and also only being applicable on dried paved roads; that is, in good condition. We should recognize that the safe interval between cars is changeable according to the speeds of the car as indicated:in Table 1 (see column [D] and [HJ ) .

Can people subtract or calculate something figures or perform arithmetic calculations without problems. while driving? This is one of reasons why maintaining the safe distance (as the criterion in meters) is difficult for drivers. Can drivers perceive the distance between themselves and the vehicle in front of them immediately while driving? I dare say that the policy of teaching 'safe distance' in Japanese driving s.chools is too unrealistic because of the difficulties in perceiving accurate distance in meters while driving at any changeable speed.

Why is the rough standard of safe distance while driving applied only to the restricted speeds between 30 and 60 km/h? We found a similar unrealistic concept in the 'Basic Theory of Driving' in Singapore for the safe distance while driving. As for how to maintain safe distance while driving, the following was stated in the code of driving (Wan Hao Department Store, 2002): "To be able to adjust your speed so that you can stop within the space between you and the vehicle in front, you must allow at least one car length for every 16 km/h (10 mph) of your speed." Can drivers perceive and recognize several-fold of their car length while driving? Counting car length would be too difficult perception and also should be impractical while driving.

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Kiyofumi lcHIKA w A

Taking an example of most prevalent speed on open roads in Japan, the recommended rough standard of safe distance taught by Japanese driving schools today is 45m at the speed of 60 km/h as shown in Table 1. Thus the tentative equivalent moving distance of time headway for vehicles at 60 km/h while keeping the 'safe distance' is 2. 70 seconds. We tried to investigate the time headway for vehicles running on open roads in Japan to consider

"how to maintain safe distance while driving", making reference to some criterion.

Regarding points of analysis, in addition to variables anticipated to affect the distance while driving, dealt with in Ichikawa (2003a), we also focused on the differences of weather and vehicle types.

In Japan, vehicles are generally classified broadly into two categories, namely heavy automobiles and 'ordinary vehicles' , as well as two-wheeled vehicle, large sized and small sized special kind cars. If vehicles are corresponded one of the following requirements; this vehicle comes under the category of heavy automobile: Gross vehicle mass is 8, 000kg and over, or maximum capacity loading is 5, 000kg and over, or riding capacity is 11 persons and over. If one automobile satisfies all of following requirements, that is, it is below 8, 000 kg in gross vehicle mass, under 5,000 kg for maximum loading weight, and with a riding capacities of under 11 persons, then this vehicle is categorized as an 'ordinary vehicle'.

However, usually we use some kinds of another classifications in daily life. For examples, we use categories of 'light cars' with an engine size between 50 and 660cc which might be peculiar to Japan, standard-sized cars, passenger cars, heavy trucks, medium-sized trucks with maximum load of 4, 000kg, small-sized trucks with one or two tons of maximum load and so forth.

Personally I am also interested in the reality of professional drivers, for examples, taxi drivers, bus drivers, and so on. We added further separate categories of vehicles for these professional drivers. Finally, we used the following categories in this study to obtain information as to whether the type of motor vehicle made any differences in time headway: taxi, small-sized truck (maximum loading weight 2 tons), heavy truck, middle-sized truck (4 tons of maximum load), light car, light car truck, and passenger car. In relation to actual procedure to confirm heavy trucks, we operationally defined them such as vehicles equipped with more than 10 wheels, and the vehicles were then categorized as heavy trucks, except for trailer types.

Method

Procedure:

We observed vehicles for 20 minutes running on open roads on the 7 th (Sunday) , 9th (Tuesday) and 10th (Wednesday) in Jan'uary 200 l.

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An 8mm-video system was placed on the side of the road lanes to videotape vehicles running. on open roads. We selected two roads located in a city and a suburb. The city road was located in a center of Takamatsu city in Japan and the road observed in suburbs was on Route 32 (on the way from Takamatsu to Kotohira in Kagawa prefecture) which was approximately 6 km distant from Takamatsu. Observation time was at daytime (between 2:00 and 3:00 PM) and at night (18:00 to 20:00 PM). The weather condition was 'not raining' and 'raining' for each time and location conditions. The lanes had two lanes on the road in suburbs. However, the city lane had three lanes. The third lane was a specific lane for right turning on which sometimes vehicles ran very slowly depending on turning conditions.

Therefore, we called the center lane in the city lane as an overtaking lane in this study. The speed limit on the city road was 50 km/h and 60 km/h on the route 32 in the suburbs. It was supposed that the practical speeds on these roads were around 60 km/h.

We considered weather conditions as an analysis variable. The weather condition of 'not raining' recorded on an 8-mm videotape, was not fine, but cloudy. The 'not raining weather condition' was coincidentally cloudy for all observational conditions. When we fixed a recording position through the 8-mm camera, a landmark was selected in order to measure time headway with ease. We could measure time headway using this landmark as vehicles' tails or heads passed it. The precise definition of time headway between vehicles was judged to be from when the tail of the vehicle in front passed the landmark, until just before the front side of a vehicle backward passed the same landmark. We used a digital stopwatch with a measuring unit of precision of 1/100 seconds to gauge this time in seconds.

The flow of vehicles on the road in the city center was influenced by a traffic signal.

When the signal changed to red, tape-recording was suspended until the flow of vehicles returned to normal flows of speed after changing back to green.

Two judges participated in measuring the time gap between moving vehicles. We trained the two judges until agreements of inter-rater' s reliability for time headway was more than 70

% . After attaining this judgement criterion, each judge measured time headway between moving vehicles independently.

Another variable for analysis in addition to the location, time, weather, and lane, was 'fleet', to analyze the time headway. In relation to the definition of 'fleet', Nakajima (1987) introduced the following two criteria: Edie et al., (1963)² defined the "platoon" holding 5.1 seconds when the speeds was less than 43 km/h, and Underwood (1963) ²thoughtthat vehicles running under 5. 0 seconds time headway were "in-group". Consequently, considering all the various bases together, we operationally defined the criterion of 'fleet' to be when a fleet group ran maintaining more than 5 seconds time headway in the front andbackward interval.

2 Both were quoted in Nakajima ( 1987), p. 62.

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Kiyofumi lcmKA w A

We analyzed all vehicles for observational targets except for two-wheeled vehicles and also special kinds of cars. All the vehicles observed including special kind cars have been analyzed in another paper (Ichikawa, 2003b) .

Results

Time headway observed:

We analyzed a total of 2, 004 vehicles running on open roads in Kagawa prefecture. The number of vehicles observed for twenty minutes in each condition is shown in Table 2. The observational ratio between location (city : 51. 6% ; suburbs : 48. 4 % ) , time (daytime : 54. 2

% ; night :

45. 8%)

and weather (not raining :

51. 0% ;

raining :

49. 0%)

attained approximately the same magnitude of number.

Table 2. Number of vehicles observed

~

^Cloudy^Daytime^Raining ^Cloudy^Night^Raining ^Cloudy^Total^Raining

Suburb (N =970) 259 285 275 151 534 436

City (N = 1,034) 259 283 230 262 489- 545

518 568 505 413 1,023 981

Total (N = 2,004)

1,086 918 2,004

Figure 1 shows the distribution of time headway for all vehicles. The mean time headway obtained was 2. 11 (SD

=

1. 00) . However, taking notice of mode in this distribution, 1. 54 seconds of time headway meant that most vehicles ran on open roads with a very narrow and unsafe distance between themselves and the vehicle ahead. We could observe that most vehicles usually ran with nearly a 20m (time headway being converted into meters) between their vehicle and the vehicle in front at the speed of 60km/h. This figure is half the safe distance in meters recommended by Japanese driving schools today.

We carried out a four-way ANOV A - time (daytime, night) X weather (not raining, raining) X location (suburb, city) X lane (cruising, overtaking) for time headway. As to main effects, weather, F (l, 1988)

=

22. 13, P

<.

001, location, F (l, 1988)

=

11. 63, P

< .

⁰⁰l, and lane, F (l, 1988) = 21. 55, P

< .

⁰⁰l attained significance for time headway.

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(Frequency)

500 - - - . - - - - 450

400 350 300 250 200 150 100

so

0 -+-,...,.. ... ,_..,....,...,....,...~..,..._.,.~..;.-. ... ...,_.~

0 .5 1 1.5 2 2.5 3 3.5 4 4.5 5. 5.5 Time headway

M = 2.11 SD= 1.00 Min.= .20 Max.= 4.99 Med. = 1.88 Mo. =1.54 Skew.= .85 N = 2,004

Figure 1. Distribution of time headway for all vehicles (Frequency)

300

71"""'=-==-=---

250 200 150 100

so

0 .5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 Time headway

M = 2.00 SD= .97 Min.= .44 Max.= 4.99 Med. = 1.76 Mo. =1.54 Skew.= 1.00 N = 1,023

Figure 2. Distribution of time headway in cloudy (Frequency)

225 .,..,...,..., _ _ _ _ ...,,_ _ _ """""" _ _ =====- 200

175 150 125 100 75

so

25

0

-+-,i....,...._~..,...,. ... ~...,..,..,.,,..;,-..;...::-.~~

0 .5 1 1 .5 2 2.5 3 3.5 4 4.5 5 5.5 Time headway

M = 2.21 SD = 1.01 Min.= .20 Max.= 4.95 Med. = 2.01 Mo.= 1.54 Skew.= .72 N= 981

Figure 3. Distribution of time headway in raining

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Kiyofumi ^lcHIKAw A

The significant difference in weather condition meant that vehicles running in the rain maintained larger time headway compared with ones not in the rain (in reality, being only cloudy) condition (see Figure 4). However, these results meant the relative differences.

We should remind the absolute value of seconds (M = 2. 21, SD = 1. 01) in the raining condition.

(Sec.)

2.22 - . - - - - 2.2

2.17 2.15 2.12 2.1 2.08 2.05

2.03 2 _ _ _ _ _ _ _ _ _ _ _ ... _ _ _ ....

Raining ^Cloudy

Figure 4. Main effect of weather (P< .001)

As for main effect of location, vehicles running on the city road were observed to be maintaining significantly longer time headway than vehicles in the suburbs (see Figure 5) . The main effect of lane was that vehicles running in the overtaking lane kept significantly shorter time headway than ones running in the cruising lane (see Figure 6) . The interaction of location with lane attained significance, F ( 1, 1988)

=

^{8. 27,} P

<.

004. This meant that vehicles running in the cruising lane were observed maintaining longer time headway only

(Sec.)

2.18 - - . - - - , . - - - - 2.16

2.14 2.12 2.1 2.08 2.06 2.04

2.02 _ _ _ _ _ _ _ _ _ _ _ _ _ _ __.

Suburbs City

Figure 5. Main effect of location (P< .001)

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(Sec.)

2.25 - - - -..

2.23 2.2 2.18 2.15 2.12 2.1 2.08 2.05

2.03 ..._ _ _ _ _ _ _ _ _ _ _ _ '--_ _ _.

Cruising Overtaking Figure 6. Main effect of lane (P< .001)

(Sec.) 2.45

2.4 2.35 2.3 · 2.25

2.2 · 2.15

2.1 2.05

2

.

^..^-·.^_^{. . .}^-

1.95 ..._ _ _ _ _ _ _ _ _ _ _ _ _ ___.

Suburbs City

•

Cruising lane

• - Overtaking lane

Figure 7. Interaction of location with lane (P<.004) (Sec.)

2.35

2.3

•

2.25 ^\

\

2.2

\

2.15 2.1

'~\ \. ^•

- • City ^Suburbs 2.05

2 1.95

Raining Cloudy Raining Cloudy

Daytime Night

Figure 8. Interaction of time with weather and location (P< .087)

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Kiyofumi ^lcHIKAw A

in the city road. Vehicles running on the overtaking lane kept shorter time headway in both the city and suburbs, as indicated in Figure

7.

The interaction of time with weather and location indicated a strong trend but came up short of significance, F (1, 1988)

=

2. 92, P

<.

087 (see Figure 8).

In addition to the four variables mentioned above, we analyzed the effect of 'fleet' on the time headway adopting the criterion of 5. 0 seconds. The number of 'fleets' i. e. , bands of running groups observed were as follows: We distinguished 187 fleet (groups) in the suburbs (98 fleet under non-rainy weather (actually cloudy) and 89 running groups under rainy conditions). In relation to the city, 201 groups were observed (in detail, 103 groups in 'cloudy' and 98 groups in 'raining'· ) .

(Sec.) 2.45

2.4 2.35 2.3 2.25 2.2 2.1 S 2. 1 2.05 2

Tail In group

Figure 9. Main Effect of Fleet (P< .001)

Figure 9 shows the effect of 'fleet' on the time headway. Vehicles running at the tail in the fleet maintained significantly longer time headway than vehicles running inside the 'fleet' . Vehicles more than 5 seconds behind maintained a larger distance between themselves and the vehicle in front.

The results of time headway in each vehicle category are shown in Figure 10. The operational definition of vehicle type was discussed in method previously. An one-way analysis of variance for the type of motor vehicle attained significance, F (1, 1997)

= 3.

67, P

<.

001. Fisher's multiple comparisons acquired following significant differences: taxis maintained longer time headway than small trucks (P

<.

008), heavy trucks (P

<.

013), and passenger cars (P

< .

002) ; light cars kept longer time headway compared with small trucks (P

< .

046) and passenger cars (P

< .

004) .

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(Sec.) 2.5

N=87 2.4

N=47 2.3

~ ^2.2

"tj ~ ~ N = 1,432

2.1 ..c:: (I)

e

(I) ₂

~ 1.9

N=43 1.8

1.7 Taxis Small Heavy Middle Light Light Passenger trucks trucks sized cars car cars

trucks trucks

Figure 10. Time headway in each vehicle category

54, N

=

6) was obse~ved for taxis in suburbs. The latter was much narrower time headway for taxis running on suburb roads.

However, taxis observed in suburbs were very small number. In conclusion, the over all

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Kiyofumi lcttIKA w A

result of taxis was influenced mainly by the time headway of taxis running on the city roads.

This meant that most taxis observed were running on city roads. Given the location and direction of the city road observed, it is reasonable to supporse that most of the taxis were either looking for passengers or en rout back to the city area.

Considering how to maintain safe distance:

We have several criteria in Japan to consider the safe distance between oneself and the vehicle ahead, as indicated in Table 1. The fore-mentioned suitable and reasonable concept of safe distance was to adopt the notion of time headway instead of spacing or distance in meters taught at driving schools in Japan. Table 3 showed the time headway equivalent for safe distance taught at driving schools in Japan these days. Japanese driving schools today recommend rough standards of safe distance corresponding to different speeds while driving.

For example, if we drive at a speed of 60 km/h, then the recommended safe distance would be 45m. So we calculated a tentative equivalent time headway for moving vehicles at the speed of 60 km/h while keeping the 'safe distance' taught by driving schools today. In concrete terms, we figured this out as 2. 7 seconds instead of spacing in meters as a safe distance recommended by Japanese driving schools today, (dividing 45m by 16. 66m for moving distance per second while driving at 60 km/h) .

Observation of vehicles according to this criterion of time gap was indicated in Table 3.

Taking the whole sum of vehicles and expressing in them specific figures, most drivers, that is, 76. 1 percent of vehicles maintained an unsafe distance on open roads in Kagawa prefecture. Above all, in the not raining condition (cloudy), we are obliged to face up to the reality that nearly 80 ( 79. 4) percent of all vehicles kept unsafe distances between themselves and the vehicles in front of them while driving on open roads. Considering the raining situation, even though slightly more ( 27. 3 % ) , compared with cloudy condition ( 20. 6

Table 3. Time headway equivalent for sale distance taught at driving schools in Japan

~

No. of cars ^Cloudy(%) No. of cars ^Raining(%) No. of cars ^Total (%) Time headway ~2.70s 211 (20.6) 268 (27.3) 479 (23.9) Time headway < 2. 70s 812 (79.4) 713 (72.7) 1,525 (76.1)

Total 1,023 (100.0) 981 (100.0) 2,004 (100.0)

Note: The criterion is a tentative equivalent time headway for vehicles moving at 60 km/h while maintaining the 'safe distance' recommended by Japanese driving schools these days.

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% ) , maintained safe time gaps while driving, the absolute percentage driving safely was under 30% of all vehicles (see Table 3).

We here considered the safe criterion recommended in the UK. Table 4 and 5 show the criteria in the UK for safe distance in the measure of time gap. According to the regulations in the UK, a two-second gap between oneself and the car in front is sufficient if conditions are good. On the other hand, leaving a time gap of at least 4 seconds is recommended on a wet road (The British School of Motoring Ltd., 1996). When we adopted the UK criterion, 60. 9 percent of vehicles in 'not raining' maintained less than recommended time headway in the UK. Nearly 40% (39. 1) of vehicles kept 2. 0 seconds and over in not raining, that is, practically cloudy condition. Turning attention to Figure 2, mode of time headway was 1. 54 seconds and median was 1.

76

seconds in 'not raining' condition.

Table 5 shows the results of time headway in raining condition, making reference to the criterion in the UK. Surprisingly enough, more than 90% of vehicles running on open roads in raining maintained unsafe time gap following the UK criterion. Considering the mean time headway in raining was 2. 21 seconds (SD = 1. 01) , this represented nearly half the recommended gap in the UK.

Table 4. Safe criterion in UK while not raining

~

C

No. of cars (%) Mean SD

Time gap

~

2.0 sec. 400 (39.1) 2.99 .78 Time gap < 2.0 sec. 623 (60.9) 1.37 .36

Total 1,023 (100.0) 2.00 .97

Table 5. Safe criterion in UK while raining

~

C

No. of cars (%) Mean SD

Time gap~ 4.0 sec. 75 ( 7.6) 4.44 .29

Time gap< 4.0 sec. 906 (92.4) 2.03 .81

Total 981 (100.0) 2.21 1.01

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Kiyofumi ^lcHIKAWA

Discussion

We explored the reality of measuring distance between moving vehicles, from the viewpoint of time headway. We analyzed

2,004

vehicles in total running on the open roads in light of .location, time, weather, lane, fleet, and the type of vehicles. Making a summarized report of the results as follows: Mean time headway for all vehicles analyzed was

2. 11

seconds (SD

= 1. 00) ,

the median was

1. 88

seconds and the mode indicated 1. 54 seconds, respectively. These meant, considering all the various factors together, that most vehicles running on open roads maintained unsafe spacings between themselves and the vehicle ahead of them, making judgement from the aspect of time headway instead of distance in meters. Both criteria we used, as examples, i. e. , tentative equivalent time headway for vehicles moving at

60

km/h while maintaining the 'safe distance' recommended by Japanese driving schools or time gap criteria in the UK, attained undesirable consequences.

Explaining it in specific figures, when we applied the converted criterion for 2. 7 seconds of safe distance taught at driving schools in Japan,

79.

4 % of vehicles kept shorter time headway and only

20.

6 % maintained ideal spacing between themselves and the vehicles ahead of them. On the other hand, adopting the criteria for not raining condition in the UK, nearly

40%

of drivers maintained recommended time gap by the UK. However, focussing on the raining condition, the results were disappointing and undesirable as

92.

4 % of vehicles maintained unsafe time gaps, in other words, only 7. 6 % of drivers maintained the safe distance in Japan. We can thus state that most Japanese drivers maintain risky time headway, especially when raining at any time.

Promoting our awareness was the facts that heavy trucks kept the narrowest time headway among all vehicles. It is not surprising, therefor, to hear of chain collisions caused by heavy trucks, especially on highways in Japan.

According to Nakajima

(1987),

Buseck., et al.,

(1980)

asserted that if the time gap between moving cars would be between

0. 0

to

0. 1

seconds, then even prompt drivers cannot avoid accidents or car crushes, when vehicle ahead of them make sudden stops. If the time headway is between

1.

0 to 2. 25 seconds, then alert drivers can stop when vehicles in front put on the emergency brake. However, when alert drivers relax their guard, car accidents may arise.

We should remind ourselves here that we cannot perceive accurate distances between ourselves and vehicles in front of us while driving immediately. Also the stopping distance or safe distance between moving vehicles taught at driving schools in Japan, were changeable while driving. There might be a gap between theory and practice as for how to maintain safe distance between two moving vehicles. Considering these stand points, the concept of 'time headway' is very practical compared to the operational 'concept of distance' between

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moving vehicles.

We obtained valuable results observed in a non-metropolitan district. However, we need to refer to results in big cities. Ogawa and Shiina (1984) observed walking distance per second passing through on an intersection which located at the capitals of 19 prefectural capitals or a sidewalk in front of municipal governments in Japan. For example, people in Osaka walked 1. 60m per second, 1. 56m/s in Tokyo. These meant that people in big cities (Osaka and Tokyo) were the most restless; Ogawa and Shiina (1984) defined restless as 'social speed'. On the other hand, people in Kagawa prefecture showed a 1. 35m gait per second which was the second shortest after people in Kagoshima ( 1. 33m/s) . Considering these facts, it might be useful to obtain the information as to distance or time headway between moving vehicles in other countries or locations in Japan. Our results were the first observation on open roads in Japan, · after Nakajima (1987) had indicated the results on Tomei Expressway. And also we should obtain the results including both open roads and expressways from the aspects covered this study.

Another point to consider is the 'progressive aging of society'. Generally speaking, aged drivers would response slower for selective reactions and visual power, for example when braking. We should remind ourselves that 'stopping distance' taught at Japanese driving schools today was conceptualized that 'moving distance after depressing the brake pedal until the brake start working' plus 'braking distance that meant travelling distance until stopping after commencement of effective braking'. By and large, the former 'free running distance' is thought to take one second. This should be connected with individual differences, especially in the aged people. We think that more than one second would be the natural response time for elderly people. This reasoning leads us to maintain the ideal safe distance.

Taking an example at ·the speed of 60 km/h and 'free running distance' for an elderly person to be two seconds, then (16. 66m X 2 sec.) plus 27m (braking distance) was approximately 60m. This leads my assertion of the safe distance being 3. 6 seconds (see column

[D/ A] ) in Table 1.

According to the announcement to the public for Automobile Assessment for several vehicles as to brake performance by Transport Ministry (now, Land, Infrastructure and Transportation Ministry) and National Organization for Automotive Safety & Victims' Aid (2000), braking performance has undoubtedly improved today. It might be caused by n9t uncommon equipment of ABS (Anti Lock Brake System) these days.

Generally speaking, braking tests are usually carried out by professional drivers who have a mental set to braking hard suddenly. Consequently, the results of these braking tests should be considered as those of ideal examples of braking function under optimum conditions.

However, we should take into consideration a gap between theocy and practice in Japan regarding the concept of safe distance as to "how to maintain spacing between moving

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Kiyofumi lcHIKA w A

distances".

In conclusion, we would like to suggest the adoption of the concept of time headway in stead of '<listanc~' for safe distance. We recommend in Ichikawa (2003a) maintaining time headway of 3. 5 seconds for any speed (see Table 1 column [D/A] ) . This is especially true for newly licensed drivers being trained at Japanese driving schools. Wherever possible we suggest that these new drivers be taught in concrete terms how to count or caliculate the time headway accurately.

References

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( *) Translated into English by the author.

Considering Safe Distance between Moving Vehicles