1. Improve signage
Every motorist is trying to get somewhere, and many of them aren't sure how to get there. While interstate signage is more or less uniformly good in that it is more or less uniform, rural highway and suburban signage is often quite poor.
Foot-long street signs were fine for city streets where traffic moved at 25 MPH, but 35-45 MPH suburban roads and 55 MPH country roads need bigger signs and more of them: one to announce the next street, one to mark the street at the corner, and one to mark the street beside the traffic light, if there is one. In areas where big trucks are common, extra signage is doubly necessary; trucks have gotten much bigger in the last 20 years and obscure signs to an equally greater degree. This suggestion isn't likely to reduce accidents by much, but it's so cheap it's worth doing anyway.
2. Raise speed limits on safe roads
This would be cheap and effective. By and large, major interstates are broad, well-maintained, smooth-flowing, and well-marked. Raising the speed limit on these roads for cars in daytime and good weather, would encourage motorists to leave dangerous back roads where they know they can drive fast because of limited police patrols. Moving traffic from back roads to major highways was a factor in the decrease of traffic accidents since the 1995 repeal of the national 55 MPH speed limit. Altho many states now mandate lower speeds for trucks than for cars, only Texas makes the sensible leap to mandating lower speeds for night driving than for day.
3. Get drunk drivers off the road
Similarly cheap and similarly effective, discouraging people from driving drunk or otherwise impaired is a proven method of reducing traffic accidents (about half of motor vehicle accidents involve intoxicants). I don't support roadblock checks for impaired drivers—that's a case of surrendering too many liberties for too little gain—but public awareness and messages targeted at bartenders are effective. Just a campaign to ask people not to drive distracted—eating, reading a map, talking on a cell phone, arguing with passengers—would be helpful at little cost. Lower blood-alcohol limits are helping on this front; making more people aware that even a little alcohol impairs their driving.
But don't lose sight of the fact that the main thing is to get really drunk drivers off the road, not slightly tipsy ones.
4. Implement better roadway lighting
One major factor in motor accidents is poor visibility (half of all motor vehicle accidents are at night, even tho the great majority of driving is done during the day), especially at intersections, where most accidents occur. If more rural highway intersections were lit, accidents at those intersections would go down. Target intersections with a history of accidents first for best effect and least cost.
5. Create more turn-only lanes
Every car that is stopped in the road to make a turn is an accident waiting to happen. An impaired or inattentive driver colliding with a car preparing for a turn is a major percentage of traffic accidents. Turn-only lanes require little extra roadway but can reduce accidents significantly, especially at intersections with poor visibility for oncoming traffic (around a curve or in a depression).
6. Improve driving conditions
Bad weather always causes a spike in traffic accidents and the cause often gets labeled as "driving too fast for conditions." State transportation departments could greatly reduce accidents by improving crumbling and pot-holed roads and clearing roads of debris, snow, and ice more efficiently (and closing roads or mandating special low speed limits in especially bad conditions). Intersections where gravel has accumulated are especially dangerous, since cars can easily slide into the intersection when trying to stop.
7. Eliminate stops
Highways are for driving. Any feature that brings all traffic from 70 MPH to 0 MPH is a 10-car pileup waiting to happen as well as a woefully inefficient use of roadway. Moving toll booths to exits is a good start; eliminating them entirely and paying for roads with ordinary taxes is better (you could still make long-haul trucks pull off to pay, as with weigh stations). Creating frontage roads can reduce or eliminate stop lights; so can funneling traffic from two or three crossroads into a single new overpass. On urban and suburban roads, creating better crosswalks with warning lights that pedestrians can activate can reduce pedestrian traffic accidents significantly.
8. Create more divided highways
Any road in which a median separates oncoming lanes of traffic is far safer than ordinary roads. It creates a barrier or buffer that goes a long way toward keeping inattentive and impaired drivers from drifting across the center line and creating a head-on collision, which is nearly always fatal.
They don't have to be four-lane behemoths with clover-leaf junctions; just extra space between lanes with a rumble strip would reduce drifting across lanes and still allow for passing on two-lane rural highways (head-on collisions are almost never the result of passing maneuvers). Altho still a new idea, more than a dozen states have begun to use centerline rumble strips, especially Pennsylvania, and report substantial reductions in crossover accidents.