The effects of interstate speed limits on driving speeds: Some new

evidence

Fred Mannering

Background

Emergency Highway Energy Conservation Act in 1974, mandates the 55 mph national maximum speed limit on interstate highways

National Highway System Designation Act of

1995 that gave states complete freedom to set interstate speed limits

Interstate speed limits now are set 55 mph to 65 mph, 70 mph, or more

Consequences of higher speed limits:

Many studies concluded the lower 55 mph speed limits saved lives

Kockelman and Bottom (2006): From 55 to 65 mph results in a 3% increase in the

crash rate and a 24% increase in the probability of a fatality once an accident occurred

From 65 to 75 mph resulted in a lower 0.64% increase in the crash rate and in a lower 12% increase in the probability of fatal injury once an accident occurred

Consequences of higher speed limits:

Possible reasons for lower increases with speed limits 65 to 75mph:

Drivers' heightened awareness of risk at higher speeds

Roads assigned the higher 75 mph in their study’s sample may have been inherently safer

Are higher speed limits safer?

Lave and Elias (1994) argue increase from 55 mph to 65 mph saved lives because of:

1. shifts in law enforcement resources

2. higher-speed-limit interstates to attract riskier drivers away from inherently more dangerous non-interstate highways

3. possible reductions in speed variances

Why conflicting safety results?Time varying changes in factors such as:

1. highway enforcement, 2. vehicle miles traveled, 3. vehicle occupancy, 4. seat belt usage, 5. alcohol use and driving, 6. vehicle fleet mix (proportions of passenger cars,

minivans, pickup trucks, and sport utility vehicles), 7. vehicle safety features (increasing adoption of air

bags, antilock brakes, other active safety systems),

8. speed limits on other road classes and in other states,

9. driver expectations, and driver adjustment and adaptation to risk

Current study:

Focus not on safety but normal speed selection under low-volume conditions

Consider a range of concurrently existing interstate speeds

Effect of Speed Limits on Drivers' Choice of Speed

Individual drivers have an “optimal” speed (In the absence of speed limits) …. trading of safety and speed

In the presence of speed limits, drivers adjust their speeds to account for the risk of detection by law enforcement, respect for the law, etc.

Choice of Speed

Following Peltzman (1975), Viscusi (1984), Winston, Maheshri, Mannering (2006)

Individuals maximize driving utility by trading off safety, S, and driving intensity (speed, risk-taking at intersections), s

There will be a marginal rate of transformation (MRT) between S and s.

A

Speed limit below point A speed

●

●

●

C

B

S

S*

ss*

Ss

U0

Speed limit above point A speed

D●

●

E

Saf

ety

(pro

babi

lity

of

avoi

ding

an

acci

dent

)

Speed

In this figure:

With no speed limit, Point A is the driver’s optimal speed

With speed limit set below Point A, driver selects a new equilibrium Point C, which is likely between Points A and B

With speed limit set above Point A, driver selects a new equilibrium Point E, which is likely between Points A and D

A

Speed limit below point A speed

●

●

●

C

B

S

S*

ss*

Ss

U0

Speed limit above point A speed

D●

●

E

Saf

ety

(pro

babi

lity

of

avoi

ding

an

acci

dent

)

Speed

Objective:

Empirically study factors that determine selection of Points C and E (speed selection in the presence of speed limits)

Data Survey of Purdue students and faculty

Key questions:

1. On an interstate with a 55 mph speed limit and little traffic, about how fast do you normally drive?

2. On an interstate with a 65 mph speed limit and little traffic, about how fast do you normally drive?

3. On an interstate with a 70 mph speed limit and little traffic, about how fast do you normally drive?

OF 204 respondents:

1 person normally drove below the 55 mph speed limit (9 reported they drive at the 55mph speed limit)

3 people normally drove below the 65 mph speed limit (11 reported they drive at the 65 mph speed limit)

6 people normally drove below the 70 mph speed limit (20 reported they drive at the 70 mph speed limit)

Methodology

Speed choices under three speed limits

Estimating separate equations would result in inefficient coefficient estimates

Approach

70 70 70 70Z X Speed

65 65 65 65Z X Speed

55 55 55 55Speed Z X

Where: Speed70, Speed65 and Speed55 are the number

of miles per hour respondents normally drive above the speed limit (with little traffic) for 70, 65, and 55 mph speed limits

Z is a vector of driver and driver-household characteristics,

X is a vector of vector of driver preferences and opinions,

's, 's, are vectors of estimable parameters, and

's are disturbance terms

Use seemingly unrelated regression estimation (SURE)

To account for cross-equation correlation

Instead of

Use generalized least squares

1T Tˆ X X X Y

11 1T Tˆ X X X Y

Variable Mean Standard deviation Minimum/Maximum

Dependent variables

Miles per hour normally driven above a 55 mph speed limit 10.96 6.21 -5/35 Miles per hour normally driven above a 65 mph speed limit 9.06 5.10 -5/25 Miles per hour normally driven above a 70 mph speed limit 7.88 5.35 -10/20

Driver/household attributes

Male indicator (1 if driver is male, 0 otherwise) 0.742 0.439 0/1 Driver age (years) 25.21 6.58 17/51 High income indicator (1 if household’s total annual income

is $75,000 or greater, 0 otherwise) 0.268

0.444

0/1

Low income indicator (1 if household’s total annual income is

less than $30,000, 0 otherwise) 0.289

0.454

0/1

Number of children under the age of 6 years old in the

household 0.196

0.522

0/3

Late license indicator (1 if driver was first licensed at age 17

or greater, 0 otherwise) 0.361

0.481

0/1

Driver opinions

Good pavement indicator (1 if driver believes pavement quality on Indiana interstates is good or very good, 0 otherwise)

0.531

0.500

0/1

German prestige indicator (1 if driver believes if German-brand vehicles are the most prestigious, 0 otherwise)

0.634

0.483

0/1

Japanese prestige indicator (1 if driver believes if Japanese-brand vehicles are the most prestigious, 0 otherwise)

0.119

0.324

0/1

Variable

Estimated Coefficient

(55 mph speed limit)

Estimated Coefficient

(65 mph speed limit)

Estimated Coefficient

(70 mph speed limit)

Constant 11.11 (12.37)** 10.88 (10.27)** 10.86 (8.44)**

Driver/household attributes

Male indicator (1 if driver is male, 0 otherwise) – 0.470 (1.37)* – Driver age (years) – -0.088 (-2.47)** -0.129 (-2.83)** High income indicator (1 if household’s total annual income

is $75,000 or greater, 0 otherwise) 1.958 (2.06)**

2.099 (2.71)**

1.255 (1.52)*

Low income indicator (1 if household’s total annual income is

less than $30,000, 0 otherwise) -0.763 (-1.17)

–

–

Number of children under the age of 6 years old in the household

-1.207 (-1.54)*

-0.993 (-1.48)*

-1.359 (-1.87)**

Late license indicator (1 if driver was first licensed at age 17 or greater, 0 otherwise)

-3.851 (-4.38)**

-2.148 (-3.00)**

-1.919 (-2.50)**

Driver opinions

Good pavement indicator (1 if driver believes pavement quality on Indiana interstates is good or very good, 0 otherwise)

1.160 (1.46)*

1.067 (1.62)*

1.006 (1.43)*

German prestige indicator (1 if driver believes if German-brand vehicles are the most prestigious, 0 otherwise)

1.137 (1.85)**

–

0.582 (1.35)*

Japanese prestige indicator (1 if driver believes if Japanese-brand vehicles are the most prestigious, 0 otherwise)

-1.348 (-1.25)

-0.814 (-1.35)*

–

R2 0.226 0.200 0.170 Number of observations 195 Equation system R2 0.202

One-tailed t-test results: ** significantly different from zero at more than 95% confidence, * significantly different from zero at more than 90% confidence

Findings (70 mph roads)

Drive faster: Higher income, Believe Indiana has good pavement quality, Believe Germany makes most prestigious

cars

Drive slower: Older, Number of children under 6 years old, First licensed at 17 years old or older

Findings (65 mph roads) Drive faster:

Male Higher income, Believe Indiana has good pavement quality

Drive slower: Older, Number of children under 6 years old, First licensed at 17 years old or older, Believe Japan makes most prestigious cars

Findings (55 mph roads) Drive faster:

Higher income, Believe Indiana has good pavement quality, Believe Germany makes most prestigious

cars

Drive slower: Lower income, Number of children under 6 years old, First licensed at 17 years old or older, Believe Japan makes most prestigious cars

Conclusions

Wide range of socioeconomic factors influence speed choice and the effect of speed limits on it

An expanded sample to take into account the effect of traffic congestion on speed choice could reveal important speed/flow relationships