Common HOT lane access/egress unsafe - study of accident rates
By Peter Samuel
Research in California shows that a common slip lane treatment for access/egress for HOV and HOT lanes is unsafe. The study, results of which are published in the Intellimotion newsletter, show significantly higher collision rates with limited access/egress in designated zones as opposed to continuous access/egress or lane changing at will.
The study focused on the accident history of HOV lanes in California 1999 to 2003 along 450km (279 miles) of expressway with continuous access/egress or simple lane changing between HOV and general purpose lanes and 877km (545 miles) of expressway with specified zones for access-egress and double striped buffer in between. (SEMANTIC NOTE: we use the term 'expressway' in the eastern, midwestern, southern US and international english sense of a fully grade-separated controlled-access highway commonly referred to in California as a 'freeway.') Expressways in the Los Angeles area tend to have HOV lanes with limited access whereas those in the San Francisco Bay Area are continuous access.
A number of toll lanes projects, often called HOT lanes (High Occupancy vehicles free, others Tolled) have the limited access/egress format including HOT lanes projects in Minneapolis (I-394), Seattle area WA167, Denver I-15. Limited access/egress has also been chosen recently for a massive "Green Lanes" project on the Illinois Tollway for some 129km (80 miles) of tollway. The Green Lanes plan was announced by the politically embattled state governor Rod Blagojevich without any feasibility study.
Fatality and injury collision rates are almost three times as high (490 vs 160/billion vehicle-miles traveled) with limited as opposed to continuous access/egress and property damage collisions are over twice as high (940 vs 430/bvmt) in the managed lanes themselves.
The survey also looked at accidents in the lane immediately alongside the HOV lane, the most leftward lane of general purpose lanes (called Left) as well. There the differences are not as large.
Accident rates in left and HOV lanes combined are 1,100/bvmt for limited vs 820/bvmt for continous looking at fatal and injury crashes and 3,390/bvmt for limited and 2,200/bvmt for continuous with property damage accidents. (see graphs nearby)
Special problems with limited access/egress zones
There were concentrations of crashes on the limited access HOT lanes:
- when access/egress zone was within 480m (0.3 miles) of the nearest on or off-ramp of the expressway
- the crossover zone was 400m (0.25 miles) or shorter in length
- volume in the HOV lane was high (1000 to 1200 veh/hr) vs the average of 700 to 800 veh/hr in peak hours.
This is a specially serious finding for HOT lane design because HOT lanes are intended to be run at high volume - 1400+/hr.
The research was conducted by Kitae Jong UC Berkeley, Koohong Chung of Caltrans, David Ragland UC Berkley and Ching Yao Chang, PATH.
ADDITIONAL THOUGHT: One traffic guy makes the good point that we need to wary of extrapolating too much from the California study. Both California metro areas studied typically have four general purpose lanes running alongside the HOV/HOT lane for a total of five lanes each direction. It may be that this makes for a dfferent maneuvering environment compared with more usual situations outside California of HOV/HOT lanes alongside just two or three general purpose lanes.
The comparison may also boil down to Bay Area traffic versus Los Angeles area traffic.
It could also be an artifact of the detailed designs of access/egress points adopted by Caltrans.
One conclusion remains: it is unwise now to simply assume that defined access/egress zones will improve safety. Skepticism is in order.
TOLLROADSnews 2008-12-04 1400